36 files changed, 22891 insertions, 0 deletions

diff --git a/crates/hir_ty/Cargo.toml b/crates/hir_ty/Cargo.toml
new file mode 100644
index 000000000..83b5013a9
--- /dev/null
+++ b/crates/hir_ty/Cargo.toml
@@ -0,0 +1,37 @@
+[package]
+name = "hir_ty"
+version = "0.0.0"
+license = "MIT OR Apache-2.0"
+authors = ["rust-analyzer developers"]
+edition = "2018"
+
+[lib]
+doctest = false
+
+[dependencies]
+itertools = "0.9.0"
+arrayvec = "0.5.1"
+smallvec = "1.2.0"
+ena = "0.14.0"
+log = "0.4.8"
+rustc-hash = "1.1.0"
+scoped-tls = "1"
+chalk-solve = { version = "0.21.0" }
+chalk-ir = { version = "0.21.0" }
+chalk-recursive = { version = "0.21.0" }
+
+stdx = { path = "../stdx" }
+hir_def = { path = "../hir_def" }
+hir_expand = { path = "../hir_expand" }
+arena = { path = "../arena" }
+base_db = { path = "../base_db" }
+profile = { path = "../profile" }
+syntax = { path = "../syntax" }
+test_utils = { path = "../test_utils" }
+
+[dev-dependencies]
+tracing = "0.1"
+tracing-subscriber = { version = "0.2", default-features = false, features = ["env-filter", "registry"] }
+tracing-tree = { version = "0.1.4" }
+
+expect = { path = "../expect" }
diff --git a/crates/hir_ty/src/autoderef.rs b/crates/hir_ty/src/autoderef.rs
new file mode 100644
index 000000000..ece68183e
--- /dev/null
+++ b/crates/hir_ty/src/autoderef.rs
@@ -0,0 +1,131 @@
+//! In certain situations, rust automatically inserts derefs as necessary: for
+//! example, field accesses `foo.bar` still work when `foo` is actually a
+//! reference to a type with the field `bar`. This is an approximation of the
+//! logic in rustc (which lives in librustc_typeck/check/autoderef.rs).
+
+use std::iter::successors;
+
+use base_db::CrateId;
+use hir_def::lang_item::LangItemTarget;
+use hir_expand::name::name;
+use log::{info, warn};
+
+use crate::{
+    db::HirDatabase,
+    traits::{InEnvironment, Solution},
+    utils::generics,
+    BoundVar, Canonical, DebruijnIndex, Obligation, Substs, TraitRef, Ty,
+};
+
+const AUTODEREF_RECURSION_LIMIT: usize = 10;
+
+pub fn autoderef<'a>(
+    db: &'a dyn HirDatabase,
+    krate: Option<CrateId>,
+    ty: InEnvironment<Canonical<Ty>>,
+) -> impl Iterator<Item = Canonical<Ty>> + 'a {
+    let InEnvironment { value: ty, environment } = ty;
+    successors(Some(ty), move |ty| {
+        deref(db, krate?, InEnvironment { value: ty, environment: environment.clone() })
+    })
+    .take(AUTODEREF_RECURSION_LIMIT)
+}
+
+pub(crate) fn deref(
+    db: &dyn HirDatabase,
+    krate: CrateId,
+    ty: InEnvironment<&Canonical<Ty>>,
+) -> Option<Canonical<Ty>> {
+    if let Some(derefed) = ty.value.value.builtin_deref() {
+        Some(Canonical { value: derefed, kinds: ty.value.kinds.clone() })
+    } else {
+        deref_by_trait(db, krate, ty)
+    }
+}
+
+fn deref_by_trait(
+    db: &dyn HirDatabase,
+    krate: CrateId,
+    ty: InEnvironment<&Canonical<Ty>>,
+) -> Option<Canonical<Ty>> {
+    let deref_trait = match db.lang_item(krate, "deref".into())? {
+        LangItemTarget::TraitId(it) => it,
+        _ => return None,
+    };
+    let target = db.trait_data(deref_trait).associated_type_by_name(&name![Target])?;
+
+    let generic_params = generics(db.upcast(), target.into());
+    if generic_params.len() != 1 {
+        // the Target type + Deref trait should only have one generic parameter,
+        // namely Deref's Self type
+        return None;
+    }
+
+    // FIXME make the Canonical / bound var handling nicer
+
+    let parameters =
+        Substs::build_for_generics(&generic_params).push(ty.value.value.clone()).build();
+
+    // Check that the type implements Deref at all
+    let trait_ref = TraitRef { trait_: deref_trait, substs: parameters.clone() };
+    let implements_goal = Canonical {
+        kinds: ty.value.kinds.clone(),
+        value: InEnvironment {
+            value: Obligation::Trait(trait_ref),
+            environment: ty.environment.clone(),
+        },
+    };
+    if db.trait_solve(krate, implements_goal).is_none() {
+        return None;
+    }
+
+    // Now do the assoc type projection
+    let projection = super::traits::ProjectionPredicate {
+        ty: Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, ty.value.kinds.len())),
+        projection_ty: super::ProjectionTy { associated_ty: target, parameters },
+    };
+
+    let obligation = super::Obligation::Projection(projection);
+
+    let in_env = InEnvironment { value: obligation, environment: ty.environment };
+
+    let canonical =
+        Canonical::new(in_env, ty.value.kinds.iter().copied().chain(Some(super::TyKind::General)));
+
+    let solution = db.trait_solve(krate, canonical)?;
+
+    match &solution {
+        Solution::Unique(vars) => {
+            // FIXME: vars may contain solutions for any inference variables
+            // that happened to be inside ty. To correctly handle these, we
+            // would have to pass the solution up to the inference context, but
+            // that requires a larger refactoring (especially if the deref
+            // happens during method resolution). So for the moment, we just
+            // check that we're not in the situation we're we would actually
+            // need to handle the values of the additional variables, i.e.
+            // they're just being 'passed through'. In the 'standard' case where
+            // we have `impl<T> Deref for Foo<T> { Target = T }`, that should be
+            // the case.
+
+            // FIXME: if the trait solver decides to truncate the type, these
+            // assumptions will be broken. We would need to properly introduce
+            // new variables in that case
+
+            for i in 1..vars.0.kinds.len() {
+                if vars.0.value[i - 1] != Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, i - 1))
+                {
+                    warn!("complex solution for derefing {:?}: {:?}, ignoring", ty.value, solution);
+                    return None;
+                }
+            }
+            Some(Canonical {
+                value: vars.0.value[vars.0.value.len() - 1].clone(),
+                kinds: vars.0.kinds.clone(),
+            })
+        }
+        Solution::Ambig(_) => {
+            info!("Ambiguous solution for derefing {:?}: {:?}", ty.value, solution);
+            None
+        }
+    }
+}
diff --git a/crates/hir_ty/src/db.rs b/crates/hir_ty/src/db.rs
new file mode 100644
index 000000000..25cf9eb7f
--- /dev/null
+++ b/crates/hir_ty/src/db.rs
@@ -0,0 +1,158 @@
+//! FIXME: write short doc here
+
+use std::sync::Arc;
+
+use arena::map::ArenaMap;
+use base_db::{impl_intern_key, salsa, CrateId, Upcast};
+use hir_def::{
+    db::DefDatabase, expr::ExprId, DefWithBodyId, FunctionId, GenericDefId, ImplId, LocalFieldId,
+    TypeParamId, VariantId,
+};
+
+use crate::{
+    method_resolution::{InherentImpls, TraitImpls},
+    traits::chalk,
+    Binders, CallableDefId, GenericPredicate, InferenceResult, OpaqueTyId, PolyFnSig,
+    ReturnTypeImplTraits, TraitRef, Ty, TyDefId, ValueTyDefId,
+};
+use hir_expand::name::Name;
+
+#[salsa::query_group(HirDatabaseStorage)]
+pub trait HirDatabase: DefDatabase + Upcast<dyn DefDatabase> {
+    #[salsa::invoke(infer_wait)]
+    #[salsa::transparent]
+    fn infer(&self, def: DefWithBodyId) -> Arc<InferenceResult>;
+
+    #[salsa::invoke(crate::infer::infer_query)]
+    fn infer_query(&self, def: DefWithBodyId) -> Arc<InferenceResult>;
+
+    #[salsa::invoke(crate::lower::ty_query)]
+    #[salsa::cycle(crate::lower::ty_recover)]
+    fn ty(&self, def: TyDefId) -> Binders<Ty>;
+
+    #[salsa::invoke(crate::lower::value_ty_query)]
+    fn value_ty(&self, def: ValueTyDefId) -> Binders<Ty>;
+
+    #[salsa::invoke(crate::lower::impl_self_ty_query)]
+    #[salsa::cycle(crate::lower::impl_self_ty_recover)]
+    fn impl_self_ty(&self, def: ImplId) -> Binders<Ty>;
+
+    #[salsa::invoke(crate::lower::impl_trait_query)]
+    fn impl_trait(&self, def: ImplId) -> Option<Binders<TraitRef>>;
+
+    #[salsa::invoke(crate::lower::field_types_query)]
+    fn field_types(&self, var: VariantId) -> Arc<ArenaMap<LocalFieldId, Binders<Ty>>>;
+
+    #[salsa::invoke(crate::callable_item_sig)]
+    fn callable_item_signature(&self, def: CallableDefId) -> PolyFnSig;
+
+    #[salsa::invoke(crate::lower::return_type_impl_traits)]
+    fn return_type_impl_traits(
+        &self,
+        def: FunctionId,
+    ) -> Option<Arc<Binders<ReturnTypeImplTraits>>>;
+
+    #[salsa::invoke(crate::lower::generic_predicates_for_param_query)]
+    #[salsa::cycle(crate::lower::generic_predicates_for_param_recover)]
+    fn generic_predicates_for_param(
+        &self,
+        param_id: TypeParamId,
+    ) -> Arc<[Binders<GenericPredicate>]>;
+
+    #[salsa::invoke(crate::lower::generic_predicates_query)]
+    fn generic_predicates(&self, def: GenericDefId) -> Arc<[Binders<GenericPredicate>]>;
+
+    #[salsa::invoke(crate::lower::generic_defaults_query)]
+    fn generic_defaults(&self, def: GenericDefId) -> Arc<[Binders<Ty>]>;
+
+    #[salsa::invoke(InherentImpls::inherent_impls_in_crate_query)]
+    fn inherent_impls_in_crate(&self, krate: CrateId) -> Arc<InherentImpls>;
+
+    #[salsa::invoke(TraitImpls::trait_impls_in_crate_query)]
+    fn trait_impls_in_crate(&self, krate: CrateId) -> Arc<TraitImpls>;
+
+    #[salsa::invoke(TraitImpls::trait_impls_in_deps_query)]
+    fn trait_impls_in_deps(&self, krate: CrateId) -> Arc<TraitImpls>;
+
+    // Interned IDs for Chalk integration
+    #[salsa::interned]
+    fn intern_callable_def(&self, callable_def: CallableDefId) -> InternedCallableDefId;
+    #[salsa::interned]
+    fn intern_type_param_id(&self, param_id: TypeParamId) -> GlobalTypeParamId;
+    #[salsa::interned]
+    fn intern_impl_trait_id(&self, id: OpaqueTyId) -> InternedOpaqueTyId;
+    #[salsa::interned]
+    fn intern_closure(&self, id: (DefWithBodyId, ExprId)) -> ClosureId;
+
+    #[salsa::invoke(chalk::associated_ty_data_query)]
+    fn associated_ty_data(&self, id: chalk::AssocTypeId) -> Arc<chalk::AssociatedTyDatum>;
+
+    #[salsa::invoke(chalk::trait_datum_query)]
+    fn trait_datum(&self, krate: CrateId, trait_id: chalk::TraitId) -> Arc<chalk::TraitDatum>;
+
+    #[salsa::invoke(chalk::struct_datum_query)]
+    fn struct_datum(&self, krate: CrateId, struct_id: chalk::AdtId) -> Arc<chalk::StructDatum>;
+
+    #[salsa::invoke(crate::traits::chalk::impl_datum_query)]
+    fn impl_datum(&self, krate: CrateId, impl_id: chalk::ImplId) -> Arc<chalk::ImplDatum>;
+
+    #[salsa::invoke(crate::traits::chalk::fn_def_datum_query)]
+    fn fn_def_datum(&self, krate: CrateId, fn_def_id: chalk::FnDefId) -> Arc<chalk::FnDefDatum>;
+
+    #[salsa::invoke(crate::traits::chalk::associated_ty_value_query)]
+    fn associated_ty_value(
+        &self,
+        krate: CrateId,
+        id: chalk::AssociatedTyValueId,
+    ) -> Arc<chalk::AssociatedTyValue>;
+
+    #[salsa::invoke(crate::traits::trait_solve_query)]
+    fn trait_solve(
+        &self,
+        krate: CrateId,
+        goal: crate::Canonical<crate::InEnvironment<crate::Obligation>>,
+    ) -> Option<crate::traits::Solution>;
+
+    #[salsa::invoke(crate::traits::chalk::program_clauses_for_chalk_env_query)]
+    fn program_clauses_for_chalk_env(
+        &self,
+        krate: CrateId,
+        env: chalk_ir::Environment<chalk::Interner>,
+    ) -> chalk_ir::ProgramClauses<chalk::Interner>;
+}
+
+fn infer_wait(db: &impl HirDatabase, def: DefWithBodyId) -> Arc<InferenceResult> {
+    let _p = profile::span("infer:wait").detail(|| match def {
+        DefWithBodyId::FunctionId(it) => db.function_data(it).name.to_string(),
+        DefWithBodyId::StaticId(it) => {
+            db.static_data(it).name.clone().unwrap_or_else(Name::missing).to_string()
+        }
+        DefWithBodyId::ConstId(it) => {
+            db.const_data(it).name.clone().unwrap_or_else(Name::missing).to_string()
+        }
+    });
+    db.infer_query(def)
+}
+
+#[test]
+fn hir_database_is_object_safe() {
+    fn _assert_object_safe(_: &dyn HirDatabase) {}
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct GlobalTypeParamId(salsa::InternId);
+impl_intern_key!(GlobalTypeParamId);
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct InternedOpaqueTyId(salsa::InternId);
+impl_intern_key!(InternedOpaqueTyId);
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct ClosureId(salsa::InternId);
+impl_intern_key!(ClosureId);
+
+/// This exists just for Chalk, because Chalk just has a single `FnDefId` where
+/// we have different IDs for struct and enum variant constructors.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Ord, PartialOrd)]
+pub struct InternedCallableDefId(salsa::InternId);
+impl_intern_key!(InternedCallableDefId);
diff --git a/crates/hir_ty/src/diagnostics.rs b/crates/hir_ty/src/diagnostics.rs
new file mode 100644
index 000000000..ae0cf8d09
--- /dev/null
+++ b/crates/hir_ty/src/diagnostics.rs
@@ -0,0 +1,444 @@
+//! FIXME: write short doc here
+mod expr;
+mod match_check;
+mod unsafe_check;
+
+use std::any::Any;
+
+use hir_def::DefWithBodyId;
+use hir_expand::diagnostics::{Diagnostic, DiagnosticSink};
+use hir_expand::{name::Name, HirFileId, InFile};
+use stdx::format_to;
+use syntax::{ast, AstPtr, SyntaxNodePtr};
+
+use crate::db::HirDatabase;
+
+pub use crate::diagnostics::expr::{record_literal_missing_fields, record_pattern_missing_fields};
+
+pub fn validate_body(db: &dyn HirDatabase, owner: DefWithBodyId, sink: &mut DiagnosticSink<'_>) {
+    let _p = profile::span("validate_body");
+    let infer = db.infer(owner);
+    infer.add_diagnostics(db, owner, sink);
+    let mut validator = expr::ExprValidator::new(owner, infer.clone(), sink);
+    validator.validate_body(db);
+    let mut validator = unsafe_check::UnsafeValidator::new(owner, infer, sink);
+    validator.validate_body(db);
+}
+
+#[derive(Debug)]
+pub struct NoSuchField {
+    pub file: HirFileId,
+    pub field: AstPtr<ast::RecordExprField>,
+}
+
+impl Diagnostic for NoSuchField {
+    fn message(&self) -> String {
+        "no such field".to_string()
+    }
+
+    fn display_source(&self) -> InFile<SyntaxNodePtr> {
+        InFile::new(self.file, self.field.clone().into())
+    }
+
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
+#[derive(Debug)]
+pub struct MissingFields {
+    pub file: HirFileId,
+    pub field_list_parent: AstPtr<ast::RecordExpr>,
+    pub field_list_parent_path: Option<AstPtr<ast::Path>>,
+    pub missed_fields: Vec<Name>,
+}
+
+impl Diagnostic for MissingFields {
+    fn message(&self) -> String {
+        let mut buf = String::from("Missing structure fields:\n");
+        for field in &self.missed_fields {
+            format_to!(buf, "- {}\n", field);
+        }
+        buf
+    }
+
+    fn display_source(&self) -> InFile<SyntaxNodePtr> {
+        InFile {
+            file_id: self.file,
+            value: self
+                .field_list_parent_path
+                .clone()
+                .map(SyntaxNodePtr::from)
+                .unwrap_or_else(|| self.field_list_parent.clone().into()),
+        }
+    }
+
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
+#[derive(Debug)]
+pub struct MissingPatFields {
+    pub file: HirFileId,
+    pub field_list_parent: AstPtr<ast::RecordPat>,
+    pub field_list_parent_path: Option<AstPtr<ast::Path>>,
+    pub missed_fields: Vec<Name>,
+}
+
+impl Diagnostic for MissingPatFields {
+    fn message(&self) -> String {
+        let mut buf = String::from("Missing structure fields:\n");
+        for field in &self.missed_fields {
+            format_to!(buf, "- {}\n", field);
+        }
+        buf
+    }
+    fn display_source(&self) -> InFile<SyntaxNodePtr> {
+        InFile {
+            file_id: self.file,
+            value: self
+                .field_list_parent_path
+                .clone()
+                .map(SyntaxNodePtr::from)
+                .unwrap_or_else(|| self.field_list_parent.clone().into()),
+        }
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
+#[derive(Debug)]
+pub struct MissingMatchArms {
+    pub file: HirFileId,
+    pub match_expr: AstPtr<ast::Expr>,
+    pub arms: AstPtr<ast::MatchArmList>,
+}
+
+impl Diagnostic for MissingMatchArms {
+    fn message(&self) -> String {
+        String::from("Missing match arm")
+    }
+    fn display_source(&self) -> InFile<SyntaxNodePtr> {
+        InFile { file_id: self.file, value: self.match_expr.clone().into() }
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
+#[derive(Debug)]
+pub struct MissingOkInTailExpr {
+    pub file: HirFileId,
+    pub expr: AstPtr<ast::Expr>,
+}
+
+impl Diagnostic for MissingOkInTailExpr {
+    fn message(&self) -> String {
+        "wrap return expression in Ok".to_string()
+    }
+    fn display_source(&self) -> InFile<SyntaxNodePtr> {
+        InFile { file_id: self.file, value: self.expr.clone().into() }
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
+#[derive(Debug)]
+pub struct BreakOutsideOfLoop {
+    pub file: HirFileId,
+    pub expr: AstPtr<ast::Expr>,
+}
+
+impl Diagnostic for BreakOutsideOfLoop {
+    fn message(&self) -> String {
+        "break outside of loop".to_string()
+    }
+    fn display_source(&self) -> InFile<SyntaxNodePtr> {
+        InFile { file_id: self.file, value: self.expr.clone().into() }
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
+#[derive(Debug)]
+pub struct MissingUnsafe {
+    pub file: HirFileId,
+    pub expr: AstPtr<ast::Expr>,
+}
+
+impl Diagnostic for MissingUnsafe {
+    fn message(&self) -> String {
+        format!("This operation is unsafe and requires an unsafe function or block")
+    }
+    fn display_source(&self) -> InFile<SyntaxNodePtr> {
+        InFile { file_id: self.file, value: self.expr.clone().into() }
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
+#[derive(Debug)]
+pub struct MismatchedArgCount {
+    pub file: HirFileId,
+    pub call_expr: AstPtr<ast::Expr>,
+    pub expected: usize,
+    pub found: usize,
+}
+
+impl Diagnostic for MismatchedArgCount {
+    fn message(&self) -> String {
+        let s = if self.expected == 1 { "" } else { "s" };
+        format!("Expected {} argument{}, found {}", self.expected, s, self.found)
+    }
+    fn display_source(&self) -> InFile<SyntaxNodePtr> {
+        InFile { file_id: self.file, value: self.call_expr.clone().into() }
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+    fn is_experimental(&self) -> bool {
+        true
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use base_db::{fixture::WithFixture, FileId, SourceDatabase, SourceDatabaseExt};
+    use hir_def::{db::DefDatabase, AssocItemId, ModuleDefId};
+    use hir_expand::{
+        db::AstDatabase,
+        diagnostics::{Diagnostic, DiagnosticSinkBuilder},
+    };
+    use rustc_hash::FxHashMap;
+    use syntax::{TextRange, TextSize};
+
+    use crate::{diagnostics::validate_body, test_db::TestDB};
+
+    impl TestDB {
+        fn diagnostics<F: FnMut(&dyn Diagnostic)>(&self, mut cb: F) {
+            let crate_graph = self.crate_graph();
+            for krate in crate_graph.iter() {
+                let crate_def_map = self.crate_def_map(krate);
+
+                let mut fns = Vec::new();
+                for (module_id, _) in crate_def_map.modules.iter() {
+                    for decl in crate_def_map[module_id].scope.declarations() {
+                        if let ModuleDefId::FunctionId(f) = decl {
+                            fns.push(f)
+                        }
+                    }
+
+                    for impl_id in crate_def_map[module_id].scope.impls() {
+                        let impl_data = self.impl_data(impl_id);
+                        for item in impl_data.items.iter() {
+                            if let AssocItemId::FunctionId(f) = item {
+                                fns.push(*f)
+                            }
+                        }
+                    }
+                }
+
+                for f in fns {
+                    let mut sink = DiagnosticSinkBuilder::new().build(&mut cb);
+                    validate_body(self, f.into(), &mut sink);
+                }
+            }
+        }
+    }
+
+    pub(crate) fn check_diagnostics(ra_fixture: &str) {
+        let db = TestDB::with_files(ra_fixture);
+        let annotations = db.extract_annotations();
+
+        let mut actual: FxHashMap<FileId, Vec<(TextRange, String)>> = FxHashMap::default();
+        db.diagnostics(|d| {
+            let src = d.display_source();
+            let root = db.parse_or_expand(src.file_id).unwrap();
+            // FIXME: macros...
+            let file_id = src.file_id.original_file(&db);
+            let range = src.value.to_node(&root).text_range();
+            let message = d.message().to_owned();
+            actual.entry(file_id).or_default().push((range, message));
+        });
+
+        for (file_id, diags) in actual.iter_mut() {
+            diags.sort_by_key(|it| it.0.start());
+            let text = db.file_text(*file_id);
+            // For multiline spans, place them on line start
+            for (range, content) in diags {
+                if text[*range].contains('\n') {
+                    *range = TextRange::new(range.start(), range.start() + TextSize::from(1));
+                    *content = format!("... {}", content);
+                }
+            }
+        }
+
+        assert_eq!(annotations, actual);
+    }
+
+    #[test]
+    fn no_such_field_diagnostics() {
+        check_diagnostics(
+            r#"
+struct S { foo: i32, bar: () }
+impl S {
+    fn new() -> S {
+        S {
+      //^ Missing structure fields:
+      //|    - bar
+            foo: 92,
+            baz: 62,
+          //^^^^^^^ no such field
+        }
+    }
+}
+"#,
+        );
+    }
+    #[test]
+    fn no_such_field_with_feature_flag_diagnostics() {
+        check_diagnostics(
+            r#"
+//- /lib.rs crate:foo cfg:feature=foo
+struct MyStruct {
+    my_val: usize,
+    #[cfg(feature = "foo")]
+    bar: bool,
+}
+
+impl MyStruct {
+    #[cfg(feature = "foo")]
+    pub(crate) fn new(my_val: usize, bar: bool) -> Self {
+        Self { my_val, bar }
+    }
+    #[cfg(not(feature = "foo"))]
+    pub(crate) fn new(my_val: usize, _bar: bool) -> Self {
+        Self { my_val }
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn no_such_field_enum_with_feature_flag_diagnostics() {
+        check_diagnostics(
+            r#"
+//- /lib.rs crate:foo cfg:feature=foo
+enum Foo {
+    #[cfg(not(feature = "foo"))]
+    Buz,
+    #[cfg(feature = "foo")]
+    Bar,
+    Baz
+}
+
+fn test_fn(f: Foo) {
+    match f {
+        Foo::Bar => {},
+        Foo::Baz => {},
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn no_such_field_with_feature_flag_diagnostics_on_struct_lit() {
+        check_diagnostics(
+            r#"
+//- /lib.rs crate:foo cfg:feature=foo
+struct S {
+    #[cfg(feature = "foo")]
+    foo: u32,
+    #[cfg(not(feature = "foo"))]
+    bar: u32,
+}
+
+impl S {
+    #[cfg(feature = "foo")]
+    fn new(foo: u32) -> Self {
+        Self { foo }
+    }
+    #[cfg(not(feature = "foo"))]
+    fn new(bar: u32) -> Self {
+        Self { bar }
+    }
+    fn new2(bar: u32) -> Self {
+        #[cfg(feature = "foo")]
+        { Self { foo: bar } }
+        #[cfg(not(feature = "foo"))]
+        { Self { bar } }
+    }
+    fn new2(val: u32) -> Self {
+        Self {
+            #[cfg(feature = "foo")]
+            foo: val,
+            #[cfg(not(feature = "foo"))]
+            bar: val,
+        }
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn no_such_field_with_type_macro() {
+        check_diagnostics(
+            r#"
+macro_rules! Type { () => { u32 }; }
+struct Foo { bar: Type![] }
+
+impl Foo {
+    fn new() -> Self {
+        Foo { bar: 0 }
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn missing_record_pat_field_diagnostic() {
+        check_diagnostics(
+            r#"
+struct S { foo: i32, bar: () }
+fn baz(s: S) {
+    let S { foo: _ } = s;
+      //^ Missing structure fields:
+      //| - bar
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn missing_record_pat_field_no_diagnostic_if_not_exhaustive() {
+        check_diagnostics(
+            r"
+struct S { foo: i32, bar: () }
+fn baz(s: S) -> i32 {
+    match s {
+        S { foo, .. } => foo,
+    }
+}
+",
+        )
+    }
+
+    #[test]
+    fn break_outside_of_loop() {
+        check_diagnostics(
+            r#"
+fn foo() { break; }
+         //^^^^^ break outside of loop
+"#,
+        );
+    }
+}
diff --git a/crates/hir_ty/src/diagnostics/expr.rs b/crates/hir_ty/src/diagnostics/expr.rs
new file mode 100644
index 000000000..fb76e2e4e
--- /dev/null
+++ b/crates/hir_ty/src/diagnostics/expr.rs
@@ -0,0 +1,569 @@
+//! FIXME: write short doc here
+
+use std::sync::Arc;
+
+use hir_def::{path::path, resolver::HasResolver, AdtId, DefWithBodyId};
+use hir_expand::diagnostics::DiagnosticSink;
+use rustc_hash::FxHashSet;
+use syntax::{ast, AstPtr};
+
+use crate::{
+    db::HirDatabase,
+    diagnostics::{
+        match_check::{is_useful, MatchCheckCtx, Matrix, PatStack, Usefulness},
+        MismatchedArgCount, MissingFields, MissingMatchArms, MissingOkInTailExpr, MissingPatFields,
+    },
+    utils::variant_data,
+    ApplicationTy, InferenceResult, Ty, TypeCtor,
+};
+
+pub use hir_def::{
+    body::{
+        scope::{ExprScopes, ScopeEntry, ScopeId},
+        Body, BodySourceMap, ExprPtr, ExprSource, PatPtr, PatSource,
+    },
+    expr::{
+        ArithOp, Array, BinaryOp, BindingAnnotation, CmpOp, Expr, ExprId, Literal, LogicOp,
+        MatchArm, Ordering, Pat, PatId, RecordFieldPat, RecordLitField, Statement, UnaryOp,
+    },
+    src::HasSource,
+    LocalFieldId, Lookup, VariantId,
+};
+
+pub(super) struct ExprValidator<'a, 'b: 'a> {
+    owner: DefWithBodyId,
+    infer: Arc<InferenceResult>,
+    sink: &'a mut DiagnosticSink<'b>,
+}
+
+impl<'a, 'b> ExprValidator<'a, 'b> {
+    pub(super) fn new(
+        owner: DefWithBodyId,
+        infer: Arc<InferenceResult>,
+        sink: &'a mut DiagnosticSink<'b>,
+    ) -> ExprValidator<'a, 'b> {
+        ExprValidator { owner, infer, sink }
+    }
+
+    pub(super) fn validate_body(&mut self, db: &dyn HirDatabase) {
+        let body = db.body(self.owner.into());
+
+        for (id, expr) in body.exprs.iter() {
+            if let Some((variant_def, missed_fields, true)) =
+                record_literal_missing_fields(db, &self.infer, id, expr)
+            {
+                self.create_record_literal_missing_fields_diagnostic(
+                    id,
+                    db,
+                    variant_def,
+                    missed_fields,
+                );
+            }
+
+            match expr {
+                Expr::Match { expr, arms } => {
+                    self.validate_match(id, *expr, arms, db, self.infer.clone());
+                }
+                Expr::Call { .. } | Expr::MethodCall { .. } => {
+                    self.validate_call(db, id, expr);
+                }
+                _ => {}
+            }
+        }
+        for (id, pat) in body.pats.iter() {
+            if let Some((variant_def, missed_fields, true)) =
+                record_pattern_missing_fields(db, &self.infer, id, pat)
+            {
+                self.create_record_pattern_missing_fields_diagnostic(
+                    id,
+                    db,
+                    variant_def,
+                    missed_fields,
+                );
+            }
+        }
+        let body_expr = &body[body.body_expr];
+        if let Expr::Block { tail: Some(t), .. } = body_expr {
+            self.validate_results_in_tail_expr(body.body_expr, *t, db);
+        }
+    }
+
+    fn create_record_literal_missing_fields_diagnostic(
+        &mut self,
+        id: ExprId,
+        db: &dyn HirDatabase,
+        variant_def: VariantId,
+        missed_fields: Vec<LocalFieldId>,
+    ) {
+        // XXX: only look at source_map if we do have missing fields
+        let (_, source_map) = db.body_with_source_map(self.owner.into());
+
+        if let Ok(source_ptr) = source_map.expr_syntax(id) {
+            let root = source_ptr.file_syntax(db.upcast());
+            if let ast::Expr::RecordExpr(record_expr) = &source_ptr.value.to_node(&root) {
+                if let Some(_) = record_expr.record_expr_field_list() {
+                    let variant_data = variant_data(db.upcast(), variant_def);
+                    let missed_fields = missed_fields
+                        .into_iter()
+                        .map(|idx| variant_data.fields()[idx].name.clone())
+                        .collect();
+                    self.sink.push(MissingFields {
+                        file: source_ptr.file_id,
+                        field_list_parent: AstPtr::new(&record_expr),
+                        field_list_parent_path: record_expr.path().map(|path| AstPtr::new(&path)),
+                        missed_fields,
+                    })
+                }
+            }
+        }
+    }
+
+    fn create_record_pattern_missing_fields_diagnostic(
+        &mut self,
+        id: PatId,
+        db: &dyn HirDatabase,
+        variant_def: VariantId,
+        missed_fields: Vec<LocalFieldId>,
+    ) {
+        // XXX: only look at source_map if we do have missing fields
+        let (_, source_map) = db.body_with_source_map(self.owner.into());
+
+        if let Ok(source_ptr) = source_map.pat_syntax(id) {
+            if let Some(expr) = source_ptr.value.as_ref().left() {
+                let root = source_ptr.file_syntax(db.upcast());
+                if let ast::Pat::RecordPat(record_pat) = expr.to_node(&root) {
+                    if let Some(_) = record_pat.record_pat_field_list() {
+                        let variant_data = variant_data(db.upcast(), variant_def);
+                        let missed_fields = missed_fields
+                            .into_iter()
+                            .map(|idx| variant_data.fields()[idx].name.clone())
+                            .collect();
+                        self.sink.push(MissingPatFields {
+                            file: source_ptr.file_id,
+                            field_list_parent: AstPtr::new(&record_pat),
+                            field_list_parent_path: record_pat
+                                .path()
+                                .map(|path| AstPtr::new(&path)),
+                            missed_fields,
+                        })
+                    }
+                }
+            }
+        }
+    }
+
+    fn validate_call(&mut self, db: &dyn HirDatabase, call_id: ExprId, expr: &Expr) -> Option<()> {
+        // Check that the number of arguments matches the number of parameters.
+
+        // FIXME: Due to shortcomings in the current type system implementation, only emit this
+        // diagnostic if there are no type mismatches in the containing function.
+        if self.infer.type_mismatches.iter().next().is_some() {
+            return Some(());
+        }
+
+        let is_method_call = matches!(expr, Expr::MethodCall { .. });
+        let (sig, args) = match expr {
+            Expr::Call { callee, args } => {
+                let callee = &self.infer.type_of_expr[*callee];
+                let sig = callee.callable_sig(db)?;
+                (sig, args.clone())
+            }
+            Expr::MethodCall { receiver, args, .. } => {
+                let mut args = args.clone();
+                args.insert(0, *receiver);
+
+                // FIXME: note that we erase information about substs here. This
+                // is not right, but, luckily, doesn't matter as we care only
+                // about the number of params
+                let callee = self.infer.method_resolution(call_id)?;
+                let sig = db.callable_item_signature(callee.into()).value;
+
+                (sig, args)
+            }
+            _ => return None,
+        };
+
+        if sig.is_varargs {
+            return None;
+        }
+
+        let params = sig.params();
+
+        let mut param_count = params.len();
+        let mut arg_count = args.len();
+
+        if arg_count != param_count {
+            let (_, source_map) = db.body_with_source_map(self.owner.into());
+            if let Ok(source_ptr) = source_map.expr_syntax(call_id) {
+                if is_method_call {
+                    param_count -= 1;
+                    arg_count -= 1;
+                }
+                self.sink.push(MismatchedArgCount {
+                    file: source_ptr.file_id,
+                    call_expr: source_ptr.value,
+                    expected: param_count,
+                    found: arg_count,
+                });
+            }
+        }
+
+        None
+    }
+
+    fn validate_match(
+        &mut self,
+        id: ExprId,
+        match_expr: ExprId,
+        arms: &[MatchArm],
+        db: &dyn HirDatabase,
+        infer: Arc<InferenceResult>,
+    ) {
+        let (body, source_map): (Arc<Body>, Arc<BodySourceMap>) =
+            db.body_with_source_map(self.owner.into());
+
+        let match_expr_ty = match infer.type_of_expr.get(match_expr) {
+            Some(ty) => ty,
+            // If we can't resolve the type of the match expression
+            // we cannot perform exhaustiveness checks.
+            None => return,
+        };
+
+        let cx = MatchCheckCtx { match_expr, body, infer: infer.clone(), db };
+        let pats = arms.iter().map(|arm| arm.pat);
+
+        let mut seen = Matrix::empty();
+        for pat in pats {
+            if let Some(pat_ty) = infer.type_of_pat.get(pat) {
+                // We only include patterns whose type matches the type
+                // of the match expression. If we had a InvalidMatchArmPattern
+                // diagnostic or similar we could raise that in an else
+                // block here.
+                //
+                // When comparing the types, we also have to consider that rustc
+                // will automatically de-reference the match expression type if
+                // necessary.
+                //
+                // FIXME we should use the type checker for this.
+                if pat_ty == match_expr_ty
+                    || match_expr_ty
+                        .as_reference()
+                        .map(|(match_expr_ty, _)| match_expr_ty == pat_ty)
+                        .unwrap_or(false)
+                {
+                    // If we had a NotUsefulMatchArm diagnostic, we could
+                    // check the usefulness of each pattern as we added it
+                    // to the matrix here.
+                    let v = PatStack::from_pattern(pat);
+                    seen.push(&cx, v);
+                    continue;
+                }
+            }
+
+            // If we can't resolve the type of a pattern, or the pattern type doesn't
+            // fit the match expression, we skip this diagnostic. Skipping the entire
+            // diagnostic rather than just not including this match arm is preferred
+            // to avoid the chance of false positives.
+            return;
+        }
+
+        match is_useful(&cx, &seen, &PatStack::from_wild()) {
+            Ok(Usefulness::Useful) => (),
+            // if a wildcard pattern is not useful, then all patterns are covered
+            Ok(Usefulness::NotUseful) => return,
+            // this path is for unimplemented checks, so we err on the side of not
+            // reporting any errors
+            _ => return,
+        }
+
+        if let Ok(source_ptr) = source_map.expr_syntax(id) {
+            let root = source_ptr.file_syntax(db.upcast());
+            if let ast::Expr::MatchExpr(match_expr) = &source_ptr.value.to_node(&root) {
+                if let (Some(match_expr), Some(arms)) =
+                    (match_expr.expr(), match_expr.match_arm_list())
+                {
+                    self.sink.push(MissingMatchArms {
+                        file: source_ptr.file_id,
+                        match_expr: AstPtr::new(&match_expr),
+                        arms: AstPtr::new(&arms),
+                    })
+                }
+            }
+        }
+    }
+
+    fn validate_results_in_tail_expr(&mut self, body_id: ExprId, id: ExprId, db: &dyn HirDatabase) {
+        // the mismatch will be on the whole block currently
+        let mismatch = match self.infer.type_mismatch_for_expr(body_id) {
+            Some(m) => m,
+            None => return,
+        };
+
+        let core_result_path = path![core::result::Result];
+
+        let resolver = self.owner.resolver(db.upcast());
+        let core_result_enum = match resolver.resolve_known_enum(db.upcast(), &core_result_path) {
+            Some(it) => it,
+            _ => return,
+        };
+
+        let core_result_ctor = TypeCtor::Adt(AdtId::EnumId(core_result_enum));
+        let params = match &mismatch.expected {
+            Ty::Apply(ApplicationTy { ctor, parameters }) if ctor == &core_result_ctor => {
+                parameters
+            }
+            _ => return,
+        };
+
+        if params.len() == 2 && params[0] == mismatch.actual {
+            let (_, source_map) = db.body_with_source_map(self.owner.into());
+
+            if let Ok(source_ptr) = source_map.expr_syntax(id) {
+                self.sink
+                    .push(MissingOkInTailExpr { file: source_ptr.file_id, expr: source_ptr.value });
+            }
+        }
+    }
+}
+
+pub fn record_literal_missing_fields(
+    db: &dyn HirDatabase,
+    infer: &InferenceResult,
+    id: ExprId,
+    expr: &Expr,
+) -> Option<(VariantId, Vec<LocalFieldId>, /*exhaustive*/ bool)> {
+    let (fields, exhausitve) = match expr {
+        Expr::RecordLit { path: _, fields, spread } => (fields, spread.is_none()),
+        _ => return None,
+    };
+
+    let variant_def = infer.variant_resolution_for_expr(id)?;
+    if let VariantId::UnionId(_) = variant_def {
+        return None;
+    }
+
+    let variant_data = variant_data(db.upcast(), variant_def);
+
+    let specified_fields: FxHashSet<_> = fields.iter().map(|f| &f.name).collect();
+    let missed_fields: Vec<LocalFieldId> = variant_data
+        .fields()
+        .iter()
+        .filter_map(|(f, d)| if specified_fields.contains(&d.name) { None } else { Some(f) })
+        .collect();
+    if missed_fields.is_empty() {
+        return None;
+    }
+    Some((variant_def, missed_fields, exhausitve))
+}
+
+pub fn record_pattern_missing_fields(
+    db: &dyn HirDatabase,
+    infer: &InferenceResult,
+    id: PatId,
+    pat: &Pat,
+) -> Option<(VariantId, Vec<LocalFieldId>, /*exhaustive*/ bool)> {
+    let (fields, exhaustive) = match pat {
+        Pat::Record { path: _, args, ellipsis } => (args, !ellipsis),
+        _ => return None,
+    };
+
+    let variant_def = infer.variant_resolution_for_pat(id)?;
+    if let VariantId::UnionId(_) = variant_def {
+        return None;
+    }
+
+    let variant_data = variant_data(db.upcast(), variant_def);
+
+    let specified_fields: FxHashSet<_> = fields.iter().map(|f| &f.name).collect();
+    let missed_fields: Vec<LocalFieldId> = variant_data
+        .fields()
+        .iter()
+        .filter_map(|(f, d)| if specified_fields.contains(&d.name) { None } else { Some(f) })
+        .collect();
+    if missed_fields.is_empty() {
+        return None;
+    }
+    Some((variant_def, missed_fields, exhaustive))
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::diagnostics::tests::check_diagnostics;
+
+    #[test]
+    fn simple_free_fn_zero() {
+        check_diagnostics(
+            r#"
+fn zero() {}
+fn f() { zero(1); }
+       //^^^^^^^ Expected 0 arguments, found 1
+"#,
+        );
+
+        check_diagnostics(
+            r#"
+fn zero() {}
+fn f() { zero(); }
+"#,
+        );
+    }
+
+    #[test]
+    fn simple_free_fn_one() {
+        check_diagnostics(
+            r#"
+fn one(arg: u8) {}
+fn f() { one(); }
+       //^^^^^ Expected 1 argument, found 0
+"#,
+        );
+
+        check_diagnostics(
+            r#"
+fn one(arg: u8) {}
+fn f() { one(1); }
+"#,
+        );
+    }
+
+    #[test]
+    fn method_as_fn() {
+        check_diagnostics(
+            r#"
+struct S;
+impl S { fn method(&self) {} }
+
+fn f() {
+    S::method();
+} //^^^^^^^^^^^ Expected 1 argument, found 0
+"#,
+        );
+
+        check_diagnostics(
+            r#"
+struct S;
+impl S { fn method(&self) {} }
+
+fn f() {
+    S::method(&S);
+    S.method();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn method_with_arg() {
+        check_diagnostics(
+            r#"
+struct S;
+impl S { fn method(&self, arg: u8) {} }
+
+            fn f() {
+                S.method();
+            } //^^^^^^^^^^ Expected 1 argument, found 0
+            "#,
+        );
+
+        check_diagnostics(
+            r#"
+struct S;
+impl S { fn method(&self, arg: u8) {} }
+
+fn f() {
+    S::method(&S, 0);
+    S.method(1);
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn tuple_struct() {
+        check_diagnostics(
+            r#"
+struct Tup(u8, u16);
+fn f() {
+    Tup(0);
+} //^^^^^^ Expected 2 arguments, found 1
+"#,
+        )
+    }
+
+    #[test]
+    fn enum_variant() {
+        check_diagnostics(
+            r#"
+enum En { Variant(u8, u16), }
+fn f() {
+    En::Variant(0);
+} //^^^^^^^^^^^^^^ Expected 2 arguments, found 1
+"#,
+        )
+    }
+
+    #[test]
+    fn enum_variant_type_macro() {
+        check_diagnostics(
+            r#"
+macro_rules! Type {
+    () => { u32 };
+}
+enum Foo {
+    Bar(Type![])
+}
+impl Foo {
+    fn new() {
+        Foo::Bar(0);
+        Foo::Bar(0, 1);
+      //^^^^^^^^^^^^^^ Expected 1 argument, found 2
+        Foo::Bar();
+      //^^^^^^^^^^ Expected 1 argument, found 0
+    }
+}
+        "#,
+        );
+    }
+
+    #[test]
+    fn varargs() {
+        check_diagnostics(
+            r#"
+extern "C" {
+    fn fixed(fixed: u8);
+    fn varargs(fixed: u8, ...);
+    fn varargs2(...);
+}
+
+fn f() {
+    unsafe {
+        fixed(0);
+        fixed(0, 1);
+      //^^^^^^^^^^^ Expected 1 argument, found 2
+        varargs(0);
+        varargs(0, 1);
+        varargs2();
+        varargs2(0);
+        varargs2(0, 1);
+    }
+}
+        "#,
+        )
+    }
+
+    #[test]
+    fn arg_count_lambda() {
+        check_diagnostics(
+            r#"
+fn main() {
+    let f = |()| ();
+    f();
+  //^^^ Expected 1 argument, found 0
+    f(());
+    f((), ());
+  //^^^^^^^^^ Expected 1 argument, found 2
+}
+"#,
+        )
+    }
+}
diff --git a/crates/hir_ty/src/diagnostics/match_check.rs b/crates/hir_ty/src/diagnostics/match_check.rs
new file mode 100644
index 000000000..7f007f1d6
--- /dev/null
+++ b/crates/hir_ty/src/diagnostics/match_check.rs
@@ -0,0 +1,1421 @@
+//! This module implements match statement exhaustiveness checking and usefulness checking
+//! for match arms.
+//!
+//! It is modeled on the rustc module `librustc_mir_build::hair::pattern::_match`, which
+//! contains very detailed documentation about the algorithms used here. I've duplicated
+//! most of that documentation below.
+//!
+//! This file includes the logic for exhaustiveness and usefulness checking for
+//! pattern-matching. Specifically, given a list of patterns for a type, we can
+//! tell whether:
+//! - (a) the patterns cover every possible constructor for the type (exhaustiveness).
+//! - (b) each pattern is necessary (usefulness).
+//!
+//! The algorithm implemented here is a modified version of the one described in
+//! <http://moscova.inria.fr/~maranget/papers/warn/index.html>.
+//! However, to save future implementors from reading the original paper, we
+//! summarise the algorithm here to hopefully save time and be a little clearer
+//! (without being so rigorous).
+//!
+//! The core of the algorithm revolves about a "usefulness" check. In particular, we
+//! are trying to compute a predicate `U(P, p)` where `P` is a list of patterns (we refer to this as
+//! a matrix). `U(P, p)` represents whether, given an existing list of patterns
+//! `P_1 ..= P_m`, adding a new pattern `p` will be "useful" (that is, cover previously-
+//! uncovered values of the type).
+//!
+//! If we have this predicate, then we can easily compute both exhaustiveness of an
+//! entire set of patterns and the individual usefulness of each one.
+//! (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e., adding a wildcard
+//! match doesn't increase the number of values we're matching)
+//! (b) a pattern `P_i` is not useful if `U(P[0..=(i-1), P_i)` is false (i.e., adding a
+//! pattern to those that have come before it doesn't increase the number of values
+//! we're matching).
+//!
+//! During the course of the algorithm, the rows of the matrix won't just be individual patterns,
+//! but rather partially-deconstructed patterns in the form of a list of patterns. The paper
+//! calls those pattern-vectors, and we will call them pattern-stacks. The same holds for the
+//! new pattern `p`.
+//!
+//! For example, say we have the following:
+//!
+//! ```ignore
+//! // x: (Option<bool>, Result<()>)
+//! match x {
+//!     (Some(true), _) => (),
+//!     (None, Err(())) => (),
+//!     (None, Err(_)) => (),
+//! }
+//! ```
+//!
+//! Here, the matrix `P` starts as:
+//!
+//! ```text
+//! [
+//!     [(Some(true), _)],
+//!     [(None, Err(()))],
+//!     [(None, Err(_))],
+//! ]
+//! ```
+//!
+//! We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering
+//! `[(Some(false), _)]`, for instance). In addition, row 3 is not useful, because
+//! all the values it covers are already covered by row 2.
+//!
+//! A list of patterns can be thought of as a stack, because we are mainly interested in the top of
+//! the stack at any given point, and we can pop or apply constructors to get new pattern-stacks.
+//! To match the paper, the top of the stack is at the beginning / on the left.
+//!
+//! There are two important operations on pattern-stacks necessary to understand the algorithm:
+//!
+//! 1. We can pop a given constructor off the top of a stack. This operation is called
+//!    `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or
+//!    `None`) and `p` a pattern-stack.
+//!    If the pattern on top of the stack can cover `c`, this removes the constructor and
+//!    pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns.
+//!    Otherwise the pattern-stack is discarded.
+//!    This essentially filters those pattern-stacks whose top covers the constructor `c` and
+//!    discards the others.
+//!
+//!    For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we
+//!    pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the
+//!    `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get
+//!    nothing back.
+//!
+//!    This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1`
+//!    on top of the stack, and we have four cases:
+//!
+//!    * 1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We push onto
+//!           the stack the arguments of this constructor, and return the result:
+//!
+//!          r_1, .., r_a, p_2, .., p_n
+//!
+//!    * 1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and return
+//!           nothing.
+//!    * 1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has
+//!           arguments (its arity), and return the resulting stack:
+//!
+//!          _, .., _, p_2, .., p_n
+//!
+//!    * 1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting stack:
+//!
+//!          S(c, (r_1, p_2, .., p_n))
+//!          S(c, (r_2, p_2, .., p_n))
+//!
+//! 2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is
+//!    a pattern-stack.
+//!    This is used when we know there are missing constructor cases, but there might be
+//!    existing wildcard patterns, so to check the usefulness of the matrix, we have to check
+//!    all its *other* components.
+//!
+//!    It is computed as follows. We look at the pattern `p_1` on top of the stack,
+//!    and we have three cases:
+//!    * 1.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing.
+//!    * 1.2. `p_1 = _`. We return the rest of the stack:
+//!
+//!          p_2, .., p_n
+//!
+//!    * 1.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting stack:
+//!
+//!          D((r_1, p_2, .., p_n))
+//!          D((r_2, p_2, .., p_n))
+//!
+//!    Note that the OR-patterns are not always used directly in Rust, but are used to derive the
+//!    exhaustive integer matching rules, so they're written here for posterity.
+//!
+//! Both those operations extend straightforwardly to a list or pattern-stacks, i.e. a matrix, by
+//! working row-by-row. Popping a constructor ends up keeping only the matrix rows that start with
+//! the given constructor, and popping a wildcard keeps those rows that start with a wildcard.
+//!
+//!
+//! The algorithm for computing `U`
+//! -------------------------------
+//! The algorithm is inductive (on the number of columns: i.e., components of tuple patterns).
+//! That means we're going to check the components from left-to-right, so the algorithm
+//! operates principally on the first component of the matrix and new pattern-stack `p`.
+//! This algorithm is realised in the `is_useful` function.
+//!
+//! Base case (`n = 0`, i.e., an empty tuple pattern):
+//! - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`), then
+//!   `U(P, p)` is false.
+//! - Otherwise, `P` must be empty, so `U(P, p)` is true.
+//!
+//! Inductive step (`n > 0`, i.e., whether there's at least one column [which may then be expanded
+//! into further columns later]). We're going to match on the top of the new pattern-stack, `p_1`:
+//!
+//! - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern.
+//!   Then, the usefulness of `p_1` can be reduced to whether it is useful when
+//!   we ignore all the patterns in the first column of `P` that involve other constructors.
+//!   This is where `S(c, P)` comes in:
+//!
+//!   ```text
+//!   U(P, p) := U(S(c, P), S(c, p))
+//!   ```
+//!
+//!   This special case is handled in `is_useful_specialized`.
+//!
+//!   For example, if `P` is:
+//!
+//!   ```text
+//!   [
+//!       [Some(true), _],
+//!       [None, 0],
+//!   ]
+//!   ```
+//!
+//!   and `p` is `[Some(false), 0]`, then we don't care about row 2 since we know `p` only
+//!   matches values that row 2 doesn't. For row 1 however, we need to dig into the
+//!   arguments of `Some` to know whether some new value is covered. So we compute
+//!   `U([[true, _]], [false, 0])`.
+//!
+//! - If `p_1 == _`, then we look at the list of constructors that appear in the first component of
+//!   the rows of `P`:
+//!     - If there are some constructors that aren't present, then we might think that the
+//!       wildcard `_` is useful, since it covers those constructors that weren't covered
+//!       before.
+//!       That's almost correct, but only works if there were no wildcards in those first
+//!       components. So we need to check that `p` is useful with respect to the rows that
+//!       start with a wildcard, if there are any. This is where `D` comes in:
+//!       `U(P, p) := U(D(P), D(p))`
+//!
+//!       For example, if `P` is:
+//!       ```text
+//!       [
+//!           [_, true, _],
+//!           [None, false, 1],
+//!       ]
+//!       ```
+//!       and `p` is `[_, false, _]`, the `Some` constructor doesn't appear in `P`. So if we
+//!       only had row 2, we'd know that `p` is useful. However row 1 starts with a
+//!       wildcard, so we need to check whether `U([[true, _]], [false, 1])`.
+//!
+//!     - Otherwise, all possible constructors (for the relevant type) are present. In this
+//!       case we must check whether the wildcard pattern covers any unmatched value. For
+//!       that, we can think of the `_` pattern as a big OR-pattern that covers all
+//!       possible constructors. For `Option`, that would mean `_ = None | Some(_)` for
+//!       example. The wildcard pattern is useful in this case if it is useful when
+//!       specialized to one of the possible constructors. So we compute:
+//!       `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))`
+//!
+//!       For example, if `P` is:
+//!       ```text
+//!       [
+//!           [Some(true), _],
+//!           [None, false],
+//!       ]
+//!       ```
+//!       and `p` is `[_, false]`, both `None` and `Some` constructors appear in the first
+//!       components of `P`. We will therefore try popping both constructors in turn: we
+//!       compute `U([[true, _]], [_, false])` for the `Some` constructor, and `U([[false]],
+//!       [false])` for the `None` constructor. The first case returns true, so we know that
+//!       `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched
+//!       before.
+//!
+//! - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately:
+//!
+//!   ```text
+//!   U(P, p) := U(P, (r_1, p_2, .., p_n))
+//!            || U(P, (r_2, p_2, .., p_n))
+//!   ```
+use std::sync::Arc;
+
+use arena::Idx;
+use hir_def::{
+    adt::VariantData,
+    body::Body,
+    expr::{Expr, Literal, Pat, PatId},
+    AdtId, EnumVariantId, VariantId,
+};
+use smallvec::{smallvec, SmallVec};
+
+use crate::{db::HirDatabase, ApplicationTy, InferenceResult, Ty, TypeCtor};
+
+#[derive(Debug, Clone, Copy)]
+/// Either a pattern from the source code being analyzed, represented as
+/// as `PatId`, or a `Wild` pattern which is created as an intermediate
+/// step in the match checking algorithm and thus is not backed by a
+/// real `PatId`.
+///
+/// Note that it is totally valid for the `PatId` variant to contain
+/// a `PatId` which resolves to a `Wild` pattern, if that wild pattern
+/// exists in the source code being analyzed.
+enum PatIdOrWild {
+    PatId(PatId),
+    Wild,
+}
+
+impl PatIdOrWild {
+    fn as_pat(self, cx: &MatchCheckCtx) -> Pat {
+        match self {
+            PatIdOrWild::PatId(id) => cx.body.pats[id].clone(),
+            PatIdOrWild::Wild => Pat::Wild,
+        }
+    }
+
+    fn as_id(self) -> Option<PatId> {
+        match self {
+            PatIdOrWild::PatId(id) => Some(id),
+            PatIdOrWild::Wild => None,
+        }
+    }
+}
+
+impl From<PatId> for PatIdOrWild {
+    fn from(pat_id: PatId) -> Self {
+        Self::PatId(pat_id)
+    }
+}
+
+impl From<&PatId> for PatIdOrWild {
+    fn from(pat_id: &PatId) -> Self {
+        Self::PatId(*pat_id)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub(super) enum MatchCheckErr {
+    NotImplemented,
+    MalformedMatchArm,
+    /// Used when type inference cannot resolve the type of
+    /// a pattern or expression.
+    Unknown,
+}
+
+/// The return type of `is_useful` is either an indication of usefulness
+/// of the match arm, or an error in the case the match statement
+/// is made up of types for which exhaustiveness checking is currently
+/// not completely implemented.
+///
+/// The `std::result::Result` type is used here rather than a custom enum
+/// to allow the use of `?`.
+pub(super) type MatchCheckResult<T> = Result<T, MatchCheckErr>;
+
+#[derive(Debug)]
+/// A row in a Matrix.
+///
+/// This type is modeled from the struct of the same name in `rustc`.
+pub(super) struct PatStack(PatStackInner);
+type PatStackInner = SmallVec<[PatIdOrWild; 2]>;
+
+impl PatStack {
+    pub(super) fn from_pattern(pat_id: PatId) -> PatStack {
+        Self(smallvec!(pat_id.into()))
+    }
+
+    pub(super) fn from_wild() -> PatStack {
+        Self(smallvec!(PatIdOrWild::Wild))
+    }
+
+    fn from_slice(slice: &[PatIdOrWild]) -> PatStack {
+        Self(SmallVec::from_slice(slice))
+    }
+
+    fn from_vec(v: PatStackInner) -> PatStack {
+        Self(v)
+    }
+
+    fn get_head(&self) -> Option<PatIdOrWild> {
+        self.0.first().copied()
+    }
+
+    fn tail(&self) -> &[PatIdOrWild] {
+        self.0.get(1..).unwrap_or(&[])
+    }
+
+    fn to_tail(&self) -> PatStack {
+        Self::from_slice(self.tail())
+    }
+
+    fn replace_head_with<I, T>(&self, pats: I) -> PatStack
+    where
+        I: Iterator<Item = T>,
+        T: Into<PatIdOrWild>,
+    {
+        let mut patterns: PatStackInner = smallvec![];
+        for pat in pats {
+            patterns.push(pat.into());
+        }
+        for pat in &self.0[1..] {
+            patterns.push(*pat);
+        }
+        PatStack::from_vec(patterns)
+    }
+
+    /// Computes `D(self)`.
+    ///
+    /// See the module docs and the associated documentation in rustc for details.
+    fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Option<PatStack> {
+        if matches!(self.get_head()?.as_pat(cx), Pat::Wild) {
+            Some(self.to_tail())
+        } else {
+            None
+        }
+    }
+
+    /// Computes `S(constructor, self)`.
+    ///
+    /// See the module docs and the associated documentation in rustc for details.
+    fn specialize_constructor(
+        &self,
+        cx: &MatchCheckCtx,
+        constructor: &Constructor,
+    ) -> MatchCheckResult<Option<PatStack>> {
+        let head = match self.get_head() {
+            Some(head) => head,
+            None => return Ok(None),
+        };
+
+        let head_pat = head.as_pat(cx);
+        let result = match (head_pat, constructor) {
+            (Pat::Tuple { args: ref pat_ids, ellipsis }, Constructor::Tuple { arity: _ }) => {
+                if ellipsis.is_some() {
+                    // If there are ellipsis here, we should add the correct number of
+                    // Pat::Wild patterns to `pat_ids`. We should be able to use the
+                    // constructors arity for this, but at the time of writing we aren't
+                    // correctly calculating this arity when ellipsis are present.
+                    return Err(MatchCheckErr::NotImplemented);
+                }
+
+                Some(self.replace_head_with(pat_ids.iter()))
+            }
+            (Pat::Lit(lit_expr), Constructor::Bool(constructor_val)) => {
+                match cx.body.exprs[lit_expr] {
+                    Expr::Literal(Literal::Bool(pat_val)) if *constructor_val == pat_val => {
+                        Some(self.to_tail())
+                    }
+                    // it was a bool but the value doesn't match
+                    Expr::Literal(Literal::Bool(_)) => None,
+                    // perhaps this is actually unreachable given we have
+                    // already checked that these match arms have the appropriate type?
+                    _ => return Err(MatchCheckErr::NotImplemented),
+                }
+            }
+            (Pat::Wild, constructor) => Some(self.expand_wildcard(cx, constructor)?),
+            (Pat::Path(_), Constructor::Enum(constructor)) => {
+                // unit enum variants become `Pat::Path`
+                let pat_id = head.as_id().expect("we know this isn't a wild");
+                if !enum_variant_matches(cx, pat_id, *constructor) {
+                    None
+                } else {
+                    Some(self.to_tail())
+                }
+            }
+            (
+                Pat::TupleStruct { args: ref pat_ids, ellipsis, .. },
+                Constructor::Enum(enum_constructor),
+            ) => {
+                let pat_id = head.as_id().expect("we know this isn't a wild");
+                if !enum_variant_matches(cx, pat_id, *enum_constructor) {
+                    None
+                } else {
+                    let constructor_arity = constructor.arity(cx)?;
+                    if let Some(ellipsis_position) = ellipsis {
+                        // If there are ellipsis in the pattern, the ellipsis must take the place
+                        // of at least one sub-pattern, so `pat_ids` should be smaller than the
+                        // constructor arity.
+                        if pat_ids.len() < constructor_arity {
+                            let mut new_patterns: Vec<PatIdOrWild> = vec![];
+
+                            for pat_id in &pat_ids[0..ellipsis_position] {
+                                new_patterns.push((*pat_id).into());
+                            }
+
+                            for _ in 0..(constructor_arity - pat_ids.len()) {
+                                new_patterns.push(PatIdOrWild::Wild);
+                            }
+
+                            for pat_id in &pat_ids[ellipsis_position..pat_ids.len()] {
+                                new_patterns.push((*pat_id).into());
+                            }
+
+                            Some(self.replace_head_with(new_patterns.into_iter()))
+                        } else {
+                            return Err(MatchCheckErr::MalformedMatchArm);
+                        }
+                    } else {
+                        // If there is no ellipsis in the tuple pattern, the number
+                        // of patterns must equal the constructor arity.
+                        if pat_ids.len() == constructor_arity {
+                            Some(self.replace_head_with(pat_ids.into_iter()))
+                        } else {
+                            return Err(MatchCheckErr::MalformedMatchArm);
+                        }
+                    }
+                }
+            }
+            (Pat::Record { args: ref arg_patterns, .. }, Constructor::Enum(e)) => {
+                let pat_id = head.as_id().expect("we know this isn't a wild");
+                if !enum_variant_matches(cx, pat_id, *e) {
+                    None
+                } else {
+                    match cx.db.enum_data(e.parent).variants[e.local_id].variant_data.as_ref() {
+                        VariantData::Record(struct_field_arena) => {
+                            // Here we treat any missing fields in the record as the wild pattern, as
+                            // if the record has ellipsis. We want to do this here even if the
+                            // record does not contain ellipsis, because it allows us to continue
+                            // enforcing exhaustiveness for the rest of the match statement.
+                            //
+                            // Creating the diagnostic for the missing field in the pattern
+                            // should be done in a different diagnostic.
+                            let patterns = struct_field_arena.iter().map(|(_, struct_field)| {
+                                arg_patterns
+                                    .iter()
+                                    .find(|pat| pat.name == struct_field.name)
+                                    .map(|pat| PatIdOrWild::from(pat.pat))
+                                    .unwrap_or(PatIdOrWild::Wild)
+                            });
+
+                            Some(self.replace_head_with(patterns))
+                        }
+                        _ => return Err(MatchCheckErr::Unknown),
+                    }
+                }
+            }
+            (Pat::Or(_), _) => return Err(MatchCheckErr::NotImplemented),
+            (_, _) => return Err(MatchCheckErr::NotImplemented),
+        };
+
+        Ok(result)
+    }
+
+    /// A special case of `specialize_constructor` where the head of the pattern stack
+    /// is a Wild pattern.
+    ///
+    /// Replaces the Wild pattern at the head of the pattern stack with N Wild patterns
+    /// (N >= 0), where N is the arity of the given constructor.
+    fn expand_wildcard(
+        &self,
+        cx: &MatchCheckCtx,
+        constructor: &Constructor,
+    ) -> MatchCheckResult<PatStack> {
+        assert_eq!(
+            Pat::Wild,
+            self.get_head().expect("expand_wildcard called on empty PatStack").as_pat(cx),
+            "expand_wildcard must only be called on PatStack with wild at head",
+        );
+
+        let mut patterns: PatStackInner = smallvec![];
+
+        for _ in 0..constructor.arity(cx)? {
+            patterns.push(PatIdOrWild::Wild);
+        }
+
+        for pat in &self.0[1..] {
+            patterns.push(*pat);
+        }
+
+        Ok(PatStack::from_vec(patterns))
+    }
+}
+
+/// A collection of PatStack.
+///
+/// This type is modeled from the struct of the same name in `rustc`.
+pub(super) struct Matrix(Vec<PatStack>);
+
+impl Matrix {
+    pub(super) fn empty() -> Self {
+        Self(vec![])
+    }
+
+    pub(super) fn push(&mut self, cx: &MatchCheckCtx, row: PatStack) {
+        if let Some(Pat::Or(pat_ids)) = row.get_head().map(|pat_id| pat_id.as_pat(cx)) {
+            // Or patterns are expanded here
+            for pat_id in pat_ids {
+                self.0.push(PatStack::from_pattern(pat_id));
+            }
+        } else {
+            self.0.push(row);
+        }
+    }
+
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    fn heads(&self) -> Vec<PatIdOrWild> {
+        self.0.iter().flat_map(|p| p.get_head()).collect()
+    }
+
+    /// Computes `D(self)` for each contained PatStack.
+    ///
+    /// See the module docs and the associated documentation in rustc for details.
+    fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Self {
+        Self::collect(cx, self.0.iter().filter_map(|r| r.specialize_wildcard(cx)))
+    }
+
+    /// Computes `S(constructor, self)` for each contained PatStack.
+    ///
+    /// See the module docs and the associated documentation in rustc for details.
+    fn specialize_constructor(
+        &self,
+        cx: &MatchCheckCtx,
+        constructor: &Constructor,
+    ) -> MatchCheckResult<Self> {
+        let mut new_matrix = Matrix::empty();
+        for pat in &self.0 {
+            if let Some(pat) = pat.specialize_constructor(cx, constructor)? {
+                new_matrix.push(cx, pat);
+            }
+        }
+
+        Ok(new_matrix)
+    }
+
+    fn collect<T: IntoIterator<Item = PatStack>>(cx: &MatchCheckCtx, iter: T) -> Self {
+        let mut matrix = Matrix::empty();
+
+        for pat in iter {
+            // using push ensures we expand or-patterns
+            matrix.push(cx, pat);
+        }
+
+        matrix
+    }
+}
+
+#[derive(Clone, Debug, PartialEq)]
+/// An indication of the usefulness of a given match arm, where
+/// usefulness is defined as matching some patterns which were
+/// not matched by an prior match arms.
+///
+/// We may eventually need an `Unknown` variant here.
+pub(super) enum Usefulness {
+    Useful,
+    NotUseful,
+}
+
+pub(super) struct MatchCheckCtx<'a> {
+    pub(super) match_expr: Idx<Expr>,
+    pub(super) body: Arc<Body>,
+    pub(super) infer: Arc<InferenceResult>,
+    pub(super) db: &'a dyn HirDatabase,
+}
+
+/// Given a set of patterns `matrix`, and pattern to consider `v`, determines
+/// whether `v` is useful. A pattern is useful if it covers cases which were
+/// not previously covered.
+///
+/// When calling this function externally (that is, not the recursive calls) it
+/// expected that you have already type checked the match arms. All patterns in
+/// matrix should be the same type as v, as well as they should all be the same
+/// type as the match expression.
+pub(super) fn is_useful(
+    cx: &MatchCheckCtx,
+    matrix: &Matrix,
+    v: &PatStack,
+) -> MatchCheckResult<Usefulness> {
+    // Handle two special cases:
+    // - enum with no variants
+    // - `!` type
+    // In those cases, no match arm is useful.
+    match cx.infer[cx.match_expr].strip_references() {
+        Ty::Apply(ApplicationTy { ctor: TypeCtor::Adt(AdtId::EnumId(enum_id)), .. }) => {
+            if cx.db.enum_data(*enum_id).variants.is_empty() {
+                return Ok(Usefulness::NotUseful);
+            }
+        }
+        Ty::Apply(ApplicationTy { ctor: TypeCtor::Never, .. }) => {
+            return Ok(Usefulness::NotUseful);
+        }
+        _ => (),
+    }
+
+    let head = match v.get_head() {
+        Some(head) => head,
+        None => {
+            let result = if matrix.is_empty() { Usefulness::Useful } else { Usefulness::NotUseful };
+
+            return Ok(result);
+        }
+    };
+
+    if let Pat::Or(pat_ids) = head.as_pat(cx) {
+        let mut found_unimplemented = false;
+        let any_useful = pat_ids.iter().any(|&pat_id| {
+            let v = PatStack::from_pattern(pat_id);
+
+            match is_useful(cx, matrix, &v) {
+                Ok(Usefulness::Useful) => true,
+                Ok(Usefulness::NotUseful) => false,
+                _ => {
+                    found_unimplemented = true;
+                    false
+                }
+            }
+        });
+
+        return if any_useful {
+            Ok(Usefulness::Useful)
+        } else if found_unimplemented {
+            Err(MatchCheckErr::NotImplemented)
+        } else {
+            Ok(Usefulness::NotUseful)
+        };
+    }
+
+    if let Some(constructor) = pat_constructor(cx, head)? {
+        let matrix = matrix.specialize_constructor(&cx, &constructor)?;
+        let v = v
+            .specialize_constructor(&cx, &constructor)?
+            .expect("we know this can't fail because we get the constructor from `v.head()` above");
+
+        is_useful(&cx, &matrix, &v)
+    } else {
+        // expanding wildcard
+        let mut used_constructors: Vec<Constructor> = vec![];
+        for pat in matrix.heads() {
+            if let Some(constructor) = pat_constructor(cx, pat)? {
+                used_constructors.push(constructor);
+            }
+        }
+
+        // We assume here that the first constructor is the "correct" type. Since we
+        // only care about the "type" of the constructor (i.e. if it is a bool we
+        // don't care about the value), this assumption should be valid as long as
+        // the match statement is well formed. We currently uphold this invariant by
+        // filtering match arms before calling `is_useful`, only passing in match arms
+        // whose type matches the type of the match expression.
+        match &used_constructors.first() {
+            Some(constructor) if all_constructors_covered(&cx, constructor, &used_constructors) => {
+                // If all constructors are covered, then we need to consider whether
+                // any values are covered by this wildcard.
+                //
+                // For example, with matrix '[[Some(true)], [None]]', all
+                // constructors are covered (`Some`/`None`), so we need
+                // to perform specialization to see that our wildcard will cover
+                // the `Some(false)` case.
+                //
+                // Here we create a constructor for each variant and then check
+                // usefulness after specializing for that constructor.
+                let mut found_unimplemented = false;
+                for constructor in constructor.all_constructors(cx) {
+                    let matrix = matrix.specialize_constructor(&cx, &constructor)?;
+                    let v = v.expand_wildcard(&cx, &constructor)?;
+
+                    match is_useful(&cx, &matrix, &v) {
+                        Ok(Usefulness::Useful) => return Ok(Usefulness::Useful),
+                        Ok(Usefulness::NotUseful) => continue,
+                        _ => found_unimplemented = true,
+                    };
+                }
+
+                if found_unimplemented {
+                    Err(MatchCheckErr::NotImplemented)
+                } else {
+                    Ok(Usefulness::NotUseful)
+                }
+            }
+            _ => {
+                // Either not all constructors are covered, or the only other arms
+                // are wildcards. Either way, this pattern is useful if it is useful
+                // when compared to those arms with wildcards.
+                let matrix = matrix.specialize_wildcard(&cx);
+                let v = v.to_tail();
+
+                is_useful(&cx, &matrix, &v)
+            }
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+/// Similar to TypeCtor, but includes additional information about the specific
+/// value being instantiated. For example, TypeCtor::Bool doesn't contain the
+/// boolean value.
+enum Constructor {
+    Bool(bool),
+    Tuple { arity: usize },
+    Enum(EnumVariantId),
+}
+
+impl Constructor {
+    fn arity(&self, cx: &MatchCheckCtx) -> MatchCheckResult<usize> {
+        let arity = match self {
+            Constructor::Bool(_) => 0,
+            Constructor::Tuple { arity } => *arity,
+            Constructor::Enum(e) => {
+                match cx.db.enum_data(e.parent).variants[e.local_id].variant_data.as_ref() {
+                    VariantData::Tuple(struct_field_data) => struct_field_data.len(),
+                    VariantData::Record(struct_field_data) => struct_field_data.len(),
+                    VariantData::Unit => 0,
+                }
+            }
+        };
+
+        Ok(arity)
+    }
+
+    fn all_constructors(&self, cx: &MatchCheckCtx) -> Vec<Constructor> {
+        match self {
+            Constructor::Bool(_) => vec![Constructor::Bool(true), Constructor::Bool(false)],
+            Constructor::Tuple { .. } => vec![*self],
+            Constructor::Enum(e) => cx
+                .db
+                .enum_data(e.parent)
+                .variants
+                .iter()
+                .map(|(local_id, _)| {
+                    Constructor::Enum(EnumVariantId { parent: e.parent, local_id })
+                })
+                .collect(),
+        }
+    }
+}
+
+/// Returns the constructor for the given pattern. Should only return None
+/// in the case of a Wild pattern.
+fn pat_constructor(cx: &MatchCheckCtx, pat: PatIdOrWild) -> MatchCheckResult<Option<Constructor>> {
+    let res = match pat.as_pat(cx) {
+        Pat::Wild => None,
+        // FIXME somehow create the Tuple constructor with the proper arity. If there are
+        // ellipsis, the arity is not equal to the number of patterns.
+        Pat::Tuple { args: pats, ellipsis } if ellipsis.is_none() => {
+            Some(Constructor::Tuple { arity: pats.len() })
+        }
+        Pat::Lit(lit_expr) => match cx.body.exprs[lit_expr] {
+            Expr::Literal(Literal::Bool(val)) => Some(Constructor::Bool(val)),
+            _ => return Err(MatchCheckErr::NotImplemented),
+        },
+        Pat::TupleStruct { .. } | Pat::Path(_) | Pat::Record { .. } => {
+            let pat_id = pat.as_id().expect("we already know this pattern is not a wild");
+            let variant_id =
+                cx.infer.variant_resolution_for_pat(pat_id).ok_or(MatchCheckErr::Unknown)?;
+            match variant_id {
+                VariantId::EnumVariantId(enum_variant_id) => {
+                    Some(Constructor::Enum(enum_variant_id))
+                }
+                _ => return Err(MatchCheckErr::NotImplemented),
+            }
+        }
+        _ => return Err(MatchCheckErr::NotImplemented),
+    };
+
+    Ok(res)
+}
+
+fn all_constructors_covered(
+    cx: &MatchCheckCtx,
+    constructor: &Constructor,
+    used_constructors: &[Constructor],
+) -> bool {
+    match constructor {
+        Constructor::Tuple { arity } => {
+            used_constructors.iter().any(|constructor| match constructor {
+                Constructor::Tuple { arity: used_arity } => arity == used_arity,
+                _ => false,
+            })
+        }
+        Constructor::Bool(_) => {
+            if used_constructors.is_empty() {
+                return false;
+            }
+
+            let covers_true =
+                used_constructors.iter().any(|c| matches!(c, Constructor::Bool(true)));
+            let covers_false =
+                used_constructors.iter().any(|c| matches!(c, Constructor::Bool(false)));
+
+            covers_true && covers_false
+        }
+        Constructor::Enum(e) => cx.db.enum_data(e.parent).variants.iter().all(|(id, _)| {
+            for constructor in used_constructors {
+                if let Constructor::Enum(e) = constructor {
+                    if id == e.local_id {
+                        return true;
+                    }
+                }
+            }
+
+            false
+        }),
+    }
+}
+
+fn enum_variant_matches(cx: &MatchCheckCtx, pat_id: PatId, enum_variant_id: EnumVariantId) -> bool {
+    Some(enum_variant_id.into()) == cx.infer.variant_resolution_for_pat(pat_id)
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::diagnostics::tests::check_diagnostics;
+
+    #[test]
+    fn empty_tuple() {
+        check_diagnostics(
+            r#"
+fn main() {
+    match () { }
+        //^^ Missing match arm
+   match (()) { }
+       //^^^^ Missing match arm
+
+    match () { _ => (), }
+    match () { () => (), }
+    match (()) { (()) => (), }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn tuple_of_two_empty_tuple() {
+        check_diagnostics(
+            r#"
+fn main() {
+    match ((), ()) { }
+        //^^^^^^^^ Missing match arm
+
+    match ((), ()) { ((), ()) => (), }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn boolean() {
+        check_diagnostics(
+            r#"
+fn test_main() {
+    match false { }
+        //^^^^^ Missing match arm
+    match false { true => (), }
+        //^^^^^ Missing match arm
+    match (false, true) {}
+        //^^^^^^^^^^^^^ Missing match arm
+    match (false, true) { (true, true) => (), }
+        //^^^^^^^^^^^^^ Missing match arm
+    match (false, true) {
+        //^^^^^^^^^^^^^ Missing match arm
+        (false, true) => (),
+        (false, false) => (),
+        (true, false) => (),
+    }
+    match (false, true) { (true, _x) => (), }
+        //^^^^^^^^^^^^^ Missing match arm
+
+    match false { true => (), false => (), }
+    match (false, true) {
+        (false, _) => (),
+        (true, false) => (),
+        (_, true) => (),
+    }
+    match (false, true) {
+        (true, true) => (),
+        (true, false) => (),
+        (false, true) => (),
+        (false, false) => (),
+    }
+    match (false, true) {
+        (true, _x) => (),
+        (false, true) => (),
+        (false, false) => (),
+    }
+    match (false, true, false) {
+        (false, ..) => (),
+        (true, ..) => (),
+    }
+    match (false, true, false) {
+        (.., false) => (),
+        (.., true) => (),
+    }
+    match (false, true, false) { (..) => (), }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools() {
+        check_diagnostics(
+            r#"
+fn main() {
+    match (false, ((), false)) {}
+        //^^^^^^^^^^^^^^^^^^^^ Missing match arm
+    match (false, ((), false)) { (true, ((), true)) => (), }
+        //^^^^^^^^^^^^^^^^^^^^ Missing match arm
+    match (false, ((), false)) { (true, _) => (), }
+        //^^^^^^^^^^^^^^^^^^^^ Missing match arm
+
+    match (false, ((), false)) {
+        (true, ((), true)) => (),
+        (true, ((), false)) => (),
+        (false, ((), true)) => (),
+        (false, ((), false)) => (),
+    }
+    match (false, ((), false)) {
+        (true, ((), true)) => (),
+        (true, ((), false)) => (),
+        (false, _) => (),
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn enums() {
+        check_diagnostics(
+            r#"
+enum Either { A, B, }
+
+fn main() {
+    match Either::A { }
+        //^^^^^^^^^ Missing match arm
+    match Either::B { Either::A => (), }
+        //^^^^^^^^^ Missing match arm
+
+    match &Either::B {
+        //^^^^^^^^^^ Missing match arm
+        Either::A => (),
+    }
+
+    match Either::B {
+        Either::A => (), Either::B => (),
+    }
+    match &Either::B {
+        Either::A => (), Either::B => (),
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn enum_containing_bool() {
+        check_diagnostics(
+            r#"
+enum Either { A(bool), B }
+
+fn main() {
+    match Either::B { }
+        //^^^^^^^^^ Missing match arm
+    match Either::B {
+        //^^^^^^^^^ Missing match arm
+        Either::A(true) => (), Either::B => ()
+    }
+
+    match Either::B {
+        Either::A(true) => (),
+        Either::A(false) => (),
+        Either::B => (),
+    }
+    match Either::B {
+        Either::B => (),
+        _ => (),
+    }
+    match Either::B {
+        Either::A(_) => (),
+        Either::B => (),
+    }
+
+}
+        "#,
+        );
+    }
+
+    #[test]
+    fn enum_different_sizes() {
+        check_diagnostics(
+            r#"
+enum Either { A(bool), B(bool, bool) }
+
+fn main() {
+    match Either::A(false) {
+        //^^^^^^^^^^^^^^^^ Missing match arm
+        Either::A(_) => (),
+        Either::B(false, _) => (),
+    }
+
+    match Either::A(false) {
+        Either::A(_) => (),
+        Either::B(true, _) => (),
+        Either::B(false, _) => (),
+    }
+    match Either::A(false) {
+        Either::A(true) | Either::A(false) => (),
+        Either::B(true, _) => (),
+        Either::B(false, _) => (),
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn tuple_of_enum_no_diagnostic() {
+        check_diagnostics(
+            r#"
+enum Either { A(bool), B(bool, bool) }
+enum Either2 { C, D }
+
+fn main() {
+    match (Either::A(false), Either2::C) {
+        (Either::A(true), _) | (Either::A(false), _) => (),
+        (Either::B(true, _), Either2::C) => (),
+        (Either::B(false, _), Either2::C) => (),
+        (Either::B(_, _), Either2::D) => (),
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn mismatched_types() {
+        // Match statements with arms that don't match the
+        // expression pattern do not fire this diagnostic.
+        check_diagnostics(
+            r#"
+enum Either { A, B }
+enum Either2 { C, D }
+
+fn main() {
+    match Either::A {
+        Either2::C => (),
+        Either2::D => (),
+    }
+    match (true, false) {
+        (true, false, true) => (),
+        (true) => (),
+    }
+    match (0) { () => () }
+    match Unresolved::Bar { Unresolved::Baz => () }
+}
+        "#,
+        );
+    }
+
+    #[test]
+    fn malformed_match_arm_tuple_enum_missing_pattern() {
+        // We are testing to be sure we don't panic here when the match
+        // arm `Either::B` is missing its pattern.
+        check_diagnostics(
+            r#"
+enum Either { A, B(u32) }
+
+fn main() {
+    match Either::A {
+        Either::A => (),
+        Either::B() => (),
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn expr_diverges() {
+        check_diagnostics(
+            r#"
+enum Either { A, B }
+
+fn main() {
+    match loop {} {
+        Either::A => (),
+        Either::B => (),
+    }
+    match loop {} {
+        Either::A => (),
+    }
+    match loop { break Foo::A } {
+        //^^^^^^^^^^^^^^^^^^^^^ Missing match arm
+        Either::A => (),
+    }
+    match loop { break Foo::A } {
+        Either::A => (),
+        Either::B => (),
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn expr_partially_diverges() {
+        check_diagnostics(
+            r#"
+enum Either<T> { A(T), B }
+
+fn foo() -> Either<!> { Either::B }
+fn main() -> u32 {
+    match foo() {
+        Either::A(val) => val,
+        Either::B => 0,
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn enum_record() {
+        check_diagnostics(
+            r#"
+enum Either { A { foo: bool }, B }
+
+fn main() {
+    let a = Either::A { foo: true };
+    match a { }
+        //^ Missing match arm
+    match a { Either::A { foo: true } => () }
+        //^ Missing match arm
+    match a {
+        Either::A { } => (),
+      //^^^^^^^^^ Missing structure fields:
+      //        | - foo
+        Either::B => (),
+    }
+    match a {
+        //^ Missing match arm
+        Either::A { } => (),
+    } //^^^^^^^^^ Missing structure fields:
+      //        | - foo
+
+    match a {
+        Either::A { foo: true } => (),
+        Either::A { foo: false } => (),
+        Either::B => (),
+    }
+    match a {
+        Either::A { foo: _ } => (),
+        Either::B => (),
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn enum_record_fields_out_of_order() {
+        check_diagnostics(
+            r#"
+enum Either {
+    A { foo: bool, bar: () },
+    B,
+}
+
+fn main() {
+    let a = Either::A { foo: true, bar: () };
+    match a {
+        //^ Missing match arm
+        Either::A { bar: (), foo: false } => (),
+        Either::A { foo: true, bar: () } => (),
+    }
+
+    match a {
+        Either::A { bar: (), foo: false } => (),
+        Either::A { foo: true, bar: () } => (),
+        Either::B => (),
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn enum_record_ellipsis() {
+        check_diagnostics(
+            r#"
+enum Either {
+    A { foo: bool, bar: bool },
+    B,
+}
+
+fn main() {
+    let a = Either::B;
+    match a {
+        //^ Missing match arm
+        Either::A { foo: true, .. } => (),
+        Either::B => (),
+    }
+    match a {
+        //^ Missing match arm
+        Either::A { .. } => (),
+    }
+
+    match a {
+        Either::A { foo: true, .. } => (),
+        Either::A { foo: false, .. } => (),
+        Either::B => (),
+    }
+
+    match a {
+        Either::A { .. } => (),
+        Either::B => (),
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn enum_tuple_partial_ellipsis() {
+        check_diagnostics(
+            r#"
+enum Either {
+    A(bool, bool, bool, bool),
+    B,
+}
+
+fn main() {
+    match Either::B {
+        //^^^^^^^^^ Missing match arm
+        Either::A(true, .., true) => (),
+        Either::A(true, .., false) => (),
+        Either::A(false, .., false) => (),
+        Either::B => (),
+    }
+    match Either::B {
+        //^^^^^^^^^ Missing match arm
+        Either::A(true, .., true) => (),
+        Either::A(true, .., false) => (),
+        Either::A(.., true) => (),
+        Either::B => (),
+    }
+
+    match Either::B {
+        Either::A(true, .., true) => (),
+        Either::A(true, .., false) => (),
+        Either::A(false, .., true) => (),
+        Either::A(false, .., false) => (),
+        Either::B => (),
+    }
+    match Either::B {
+        Either::A(true, .., true) => (),
+        Either::A(true, .., false) => (),
+        Either::A(.., true) => (),
+        Either::A(.., false) => (),
+        Either::B => (),
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn never() {
+        check_diagnostics(
+            r#"
+enum Never {}
+
+fn enum_(never: Never) {
+    match never {}
+}
+fn enum_ref(never: &Never) {
+    match never {}
+}
+fn bang(never: !) {
+    match never {}
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn or_pattern_panic() {
+        check_diagnostics(
+            r#"
+pub enum Category { Infinity, Zero }
+
+fn panic(a: Category, b: Category) {
+    match (a, b) {
+        (Category::Zero | Category::Infinity, _) => (),
+        (_, Category::Zero | Category::Infinity) => (),
+    }
+
+    // FIXME: This is a false positive, but the code used to cause a panic in the match checker,
+    // so this acts as a regression test for that.
+    match (a, b) {
+        //^^^^^^ Missing match arm
+        (Category::Infinity, Category::Infinity) | (Category::Zero, Category::Zero) => (),
+        (Category::Infinity | Category::Zero, _) => (),
+    }
+}
+"#,
+        );
+    }
+
+    mod false_negatives {
+        //! The implementation of match checking here is a work in progress. As we roll this out, we
+        //! prefer false negatives to false positives (ideally there would be no false positives). This
+        //! test module should document known false negatives. Eventually we will have a complete
+        //! implementation of match checking and this module will be empty.
+        //!
+        //! The reasons for documenting known false negatives:
+        //!
+        //!   1. It acts as a backlog of work that can be done to improve the behavior of the system.
+        //!   2. It ensures the code doesn't panic when handling these cases.
+        use super::*;
+
+        #[test]
+        fn integers() {
+            // We don't currently check integer exhaustiveness.
+            check_diagnostics(
+                r#"
+fn main() {
+    match 5 {
+        10 => (),
+        11..20 => (),
+    }
+}
+"#,
+            );
+        }
+
+        #[test]
+        fn internal_or() {
+            // We do not currently handle patterns with internal `or`s.
+            check_diagnostics(
+                r#"
+fn main() {
+    enum Either { A(bool), B }
+    match Either::B {
+        Either::A(true | false) => (),
+    }
+}
+"#,
+            );
+        }
+
+        #[test]
+        fn tuple_of_bools_with_ellipsis_at_end_missing_arm() {
+            // We don't currently handle tuple patterns with ellipsis.
+            check_diagnostics(
+                r#"
+fn main() {
+    match (false, true, false) {
+        (false, ..) => (),
+    }
+}
+"#,
+            );
+        }
+
+        #[test]
+        fn tuple_of_bools_with_ellipsis_at_beginning_missing_arm() {
+            // We don't currently handle tuple patterns with ellipsis.
+            check_diagnostics(
+                r#"
+fn main() {
+    match (false, true, false) {
+        (.., false) => (),
+    }
+}
+"#,
+            );
+        }
+
+        #[test]
+        fn struct_missing_arm() {
+            // We don't currently handle structs.
+            check_diagnostics(
+                r#"
+struct Foo { a: bool }
+fn main(f: Foo) {
+    match f { Foo { a: true } => () }
+}
+"#,
+            );
+        }
+    }
+}
diff --git a/crates/hir_ty/src/diagnostics/unsafe_check.rs b/crates/hir_ty/src/diagnostics/unsafe_check.rs
new file mode 100644
index 000000000..61ffbf5d1
--- /dev/null
+++ b/crates/hir_ty/src/diagnostics/unsafe_check.rs
@@ -0,0 +1,205 @@
+//! Provides validations for unsafe code. Currently checks if unsafe functions are missing
+//! unsafe blocks.
+
+use std::sync::Arc;
+
+use hir_def::{
+    body::Body,
+    expr::{Expr, ExprId, UnaryOp},
+    resolver::{resolver_for_expr, ResolveValueResult, ValueNs},
+    DefWithBodyId,
+};
+use hir_expand::diagnostics::DiagnosticSink;
+
+use crate::{
+    db::HirDatabase, diagnostics::MissingUnsafe, lower::CallableDefId, ApplicationTy,
+    InferenceResult, Ty, TypeCtor,
+};
+
+pub(super) struct UnsafeValidator<'a, 'b: 'a> {
+    owner: DefWithBodyId,
+    infer: Arc<InferenceResult>,
+    sink: &'a mut DiagnosticSink<'b>,
+}
+
+impl<'a, 'b> UnsafeValidator<'a, 'b> {
+    pub(super) fn new(
+        owner: DefWithBodyId,
+        infer: Arc<InferenceResult>,
+        sink: &'a mut DiagnosticSink<'b>,
+    ) -> UnsafeValidator<'a, 'b> {
+        UnsafeValidator { owner, infer, sink }
+    }
+
+    pub(super) fn validate_body(&mut self, db: &dyn HirDatabase) {
+        let def = self.owner.into();
+        let unsafe_expressions = unsafe_expressions(db, self.infer.as_ref(), def);
+        let is_unsafe = match self.owner {
+            DefWithBodyId::FunctionId(it) => db.function_data(it).is_unsafe,
+            DefWithBodyId::StaticId(_) | DefWithBodyId::ConstId(_) => false,
+        };
+        if is_unsafe
+            || unsafe_expressions
+                .iter()
+                .filter(|unsafe_expr| !unsafe_expr.inside_unsafe_block)
+                .count()
+                == 0
+        {
+            return;
+        }
+
+        let (_, body_source) = db.body_with_source_map(def);
+        for unsafe_expr in unsafe_expressions {
+            if !unsafe_expr.inside_unsafe_block {
+                if let Ok(in_file) = body_source.as_ref().expr_syntax(unsafe_expr.expr) {
+                    self.sink.push(MissingUnsafe { file: in_file.file_id, expr: in_file.value })
+                }
+            }
+        }
+    }
+}
+
+pub struct UnsafeExpr {
+    pub expr: ExprId,
+    pub inside_unsafe_block: bool,
+}
+
+pub fn unsafe_expressions(
+    db: &dyn HirDatabase,
+    infer: &InferenceResult,
+    def: DefWithBodyId,
+) -> Vec<UnsafeExpr> {
+    let mut unsafe_exprs = vec![];
+    let body = db.body(def);
+    walk_unsafe(&mut unsafe_exprs, db, infer, def, &body, body.body_expr, false);
+
+    unsafe_exprs
+}
+
+fn walk_unsafe(
+    unsafe_exprs: &mut Vec<UnsafeExpr>,
+    db: &dyn HirDatabase,
+    infer: &InferenceResult,
+    def: DefWithBodyId,
+    body: &Body,
+    current: ExprId,
+    inside_unsafe_block: bool,
+) {
+    let expr = &body.exprs[current];
+    match expr {
+        Expr::Call { callee, .. } => {
+            let ty = &infer[*callee];
+            if let &Ty::Apply(ApplicationTy {
+                ctor: TypeCtor::FnDef(CallableDefId::FunctionId(func)),
+                ..
+            }) = ty
+            {
+                if db.function_data(func).is_unsafe {
+                    unsafe_exprs.push(UnsafeExpr { expr: current, inside_unsafe_block });
+                }
+            }
+        }
+        Expr::Path(path) => {
+            let resolver = resolver_for_expr(db.upcast(), def, current);
+            let value_or_partial = resolver.resolve_path_in_value_ns(db.upcast(), path.mod_path());
+            if let Some(ResolveValueResult::ValueNs(ValueNs::StaticId(id))) = value_or_partial {
+                if db.static_data(id).mutable {
+                    unsafe_exprs.push(UnsafeExpr { expr: current, inside_unsafe_block });
+                }
+            }
+        }
+        Expr::MethodCall { .. } => {
+            if infer
+                .method_resolution(current)
+                .map(|func| db.function_data(func).is_unsafe)
+                .unwrap_or(false)
+            {
+                unsafe_exprs.push(UnsafeExpr { expr: current, inside_unsafe_block });
+            }
+        }
+        Expr::UnaryOp { expr, op: UnaryOp::Deref } => {
+            if let Ty::Apply(ApplicationTy { ctor: TypeCtor::RawPtr(..), .. }) = &infer[*expr] {
+                unsafe_exprs.push(UnsafeExpr { expr: current, inside_unsafe_block });
+            }
+        }
+        Expr::Unsafe { body: child } => {
+            return walk_unsafe(unsafe_exprs, db, infer, def, body, *child, true);
+        }
+        _ => {}
+    }
+
+    expr.walk_child_exprs(|child| {
+        walk_unsafe(unsafe_exprs, db, infer, def, body, child, inside_unsafe_block);
+    });
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::diagnostics::tests::check_diagnostics;
+
+    #[test]
+    fn missing_unsafe_diagnostic_with_raw_ptr() {
+        check_diagnostics(
+            r#"
+fn main() {
+    let x = &5 as *const usize;
+    unsafe { let y = *x; }
+    let z = *x;
+}         //^^ This operation is unsafe and requires an unsafe function or block
+"#,
+        )
+    }
+
+    #[test]
+    fn missing_unsafe_diagnostic_with_unsafe_call() {
+        check_diagnostics(
+            r#"
+struct HasUnsafe;
+
+impl HasUnsafe {
+    unsafe fn unsafe_fn(&self) {
+        let x = &5 as *const usize;
+        let y = *x;
+    }
+}
+
+unsafe fn unsafe_fn() {
+    let x = &5 as *const usize;
+    let y = *x;
+}
+
+fn main() {
+    unsafe_fn();
+  //^^^^^^^^^^^ This operation is unsafe and requires an unsafe function or block
+    HasUnsafe.unsafe_fn();
+  //^^^^^^^^^^^^^^^^^^^^^ This operation is unsafe and requires an unsafe function or block
+    unsafe {
+        unsafe_fn();
+        HasUnsafe.unsafe_fn();
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn missing_unsafe_diagnostic_with_static_mut() {
+        check_diagnostics(
+            r#"
+struct Ty {
+    a: u8,
+}
+
+static mut static_mut: Ty = Ty { a: 0 };
+
+fn main() {
+    let x = static_mut.a;
+          //^^^^^^^^^^ This operation is unsafe and requires an unsafe function or block
+    unsafe {
+        let x = static_mut.a;
+    }
+}
+"#,
+        );
+    }
+}
diff --git a/crates/hir_ty/src/display.rs b/crates/hir_ty/src/display.rs
new file mode 100644
index 000000000..64b68014d
--- /dev/null
+++ b/crates/hir_ty/src/display.rs
@@ -0,0 +1,632 @@
+//! FIXME: write short doc here
+
+use std::fmt;
+
+use crate::{
+    db::HirDatabase, utils::generics, ApplicationTy, CallableDefId, FnSig, GenericPredicate,
+    Obligation, OpaqueTyId, ProjectionTy, Substs, TraitRef, Ty, TypeCtor,
+};
+use hir_def::{
+    find_path, generics::TypeParamProvenance, item_scope::ItemInNs, AdtId, AssocContainerId,
+    Lookup, ModuleId,
+};
+use hir_expand::name::Name;
+
+pub struct HirFormatter<'a> {
+    pub db: &'a dyn HirDatabase,
+    fmt: &'a mut dyn fmt::Write,
+    buf: String,
+    curr_size: usize,
+    pub(crate) max_size: Option<usize>,
+    omit_verbose_types: bool,
+    display_target: DisplayTarget,
+}
+
+pub trait HirDisplay {
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError>;
+
+    /// Returns a `Display`able type that is human-readable.
+    /// Use this for showing types to the user (e.g. diagnostics)
+    fn display<'a>(&'a self, db: &'a dyn HirDatabase) -> HirDisplayWrapper<'a, Self>
+    where
+        Self: Sized,
+    {
+        HirDisplayWrapper {
+            db,
+            t: self,
+            max_size: None,
+            omit_verbose_types: false,
+            display_target: DisplayTarget::Diagnostics,
+        }
+    }
+
+    /// Returns a `Display`able type that is human-readable and tries to be succinct.
+    /// Use this for showing types to the user where space is constrained (e.g. doc popups)
+    fn display_truncated<'a>(
+        &'a self,
+        db: &'a dyn HirDatabase,
+        max_size: Option<usize>,
+    ) -> HirDisplayWrapper<'a, Self>
+    where
+        Self: Sized,
+    {
+        HirDisplayWrapper {
+            db,
+            t: self,
+            max_size,
+            omit_verbose_types: true,
+            display_target: DisplayTarget::Diagnostics,
+        }
+    }
+
+    /// Returns a String representation of `self` that can be inserted into the given module.
+    /// Use this when generating code (e.g. assists)
+    fn display_source_code<'a>(
+        &'a self,
+        db: &'a dyn HirDatabase,
+        module_id: ModuleId,
+    ) -> Result<String, DisplaySourceCodeError> {
+        let mut result = String::new();
+        match self.hir_fmt(&mut HirFormatter {
+            db,
+            fmt: &mut result,
+            buf: String::with_capacity(20),
+            curr_size: 0,
+            max_size: None,
+            omit_verbose_types: false,
+            display_target: DisplayTarget::SourceCode { module_id },
+        }) {
+            Ok(()) => {}
+            Err(HirDisplayError::FmtError) => panic!("Writing to String can't fail!"),
+            Err(HirDisplayError::DisplaySourceCodeError(e)) => return Err(e),
+        };
+        Ok(result)
+    }
+}
+
+impl<'a> HirFormatter<'a> {
+    pub fn write_joined<T: HirDisplay>(
+        &mut self,
+        iter: impl IntoIterator<Item = T>,
+        sep: &str,
+    ) -> Result<(), HirDisplayError> {
+        let mut first = true;
+        for e in iter {
+            if !first {
+                write!(self, "{}", sep)?;
+            }
+            first = false;
+            e.hir_fmt(self)?;
+        }
+        Ok(())
+    }
+
+    /// This allows using the `write!` macro directly with a `HirFormatter`.
+    pub fn write_fmt(&mut self, args: fmt::Arguments) -> Result<(), HirDisplayError> {
+        // We write to a buffer first to track output size
+        self.buf.clear();
+        fmt::write(&mut self.buf, args)?;
+        self.curr_size += self.buf.len();
+
+        // Then we write to the internal formatter from the buffer
+        self.fmt.write_str(&self.buf).map_err(HirDisplayError::from)
+    }
+
+    pub fn should_truncate(&self) -> bool {
+        if let Some(max_size) = self.max_size {
+            self.curr_size >= max_size
+        } else {
+            false
+        }
+    }
+
+    pub fn omit_verbose_types(&self) -> bool {
+        self.omit_verbose_types
+    }
+}
+
+#[derive(Clone, Copy)]
+enum DisplayTarget {
+    /// Display types for inlays, doc popups, autocompletion, etc...
+    /// Showing `{unknown}` or not qualifying paths is fine here.
+    /// There's no reason for this to fail.
+    Diagnostics,
+    /// Display types for inserting them in source files.
+    /// The generated code should compile, so paths need to be qualified.
+    SourceCode { module_id: ModuleId },
+}
+
+impl DisplayTarget {
+    fn is_source_code(&self) -> bool {
+        matches!(self, Self::SourceCode {..})
+    }
+}
+
+#[derive(Debug)]
+pub enum DisplaySourceCodeError {
+    PathNotFound,
+}
+
+pub enum HirDisplayError {
+    /// Errors that can occur when generating source code
+    DisplaySourceCodeError(DisplaySourceCodeError),
+    /// `FmtError` is required to be compatible with std::fmt::Display
+    FmtError,
+}
+impl From<fmt::Error> for HirDisplayError {
+    fn from(_: fmt::Error) -> Self {
+        Self::FmtError
+    }
+}
+
+pub struct HirDisplayWrapper<'a, T> {
+    db: &'a dyn HirDatabase,
+    t: &'a T,
+    max_size: Option<usize>,
+    omit_verbose_types: bool,
+    display_target: DisplayTarget,
+}
+
+impl<'a, T> fmt::Display for HirDisplayWrapper<'a, T>
+where
+    T: HirDisplay,
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self.t.hir_fmt(&mut HirFormatter {
+            db: self.db,
+            fmt: f,
+            buf: String::with_capacity(20),
+            curr_size: 0,
+            max_size: self.max_size,
+            omit_verbose_types: self.omit_verbose_types,
+            display_target: self.display_target,
+        }) {
+            Ok(()) => Ok(()),
+            Err(HirDisplayError::FmtError) => Err(fmt::Error),
+            Err(HirDisplayError::DisplaySourceCodeError(_)) => {
+                // This should never happen
+                panic!("HirDisplay failed when calling Display::fmt!")
+            }
+        }
+    }
+}
+
+const TYPE_HINT_TRUNCATION: &str = "…";
+
+impl HirDisplay for &Ty {
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
+        HirDisplay::hir_fmt(*self, f)
+    }
+}
+
+impl HirDisplay for ApplicationTy {
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
+        if f.should_truncate() {
+            return write!(f, "{}", TYPE_HINT_TRUNCATION);
+        }
+
+        match self.ctor {
+            TypeCtor::Bool => write!(f, "bool")?,
+            TypeCtor::Char => write!(f, "char")?,
+            TypeCtor::Int(t) => write!(f, "{}", t)?,
+            TypeCtor::Float(t) => write!(f, "{}", t)?,
+            TypeCtor::Str => write!(f, "str")?,
+            TypeCtor::Slice => {
+                let t = self.parameters.as_single();
+                write!(f, "[{}]", t.display(f.db))?;
+            }
+            TypeCtor::Array => {
+                let t = self.parameters.as_single();
+                write!(f, "[{}; _]", t.display(f.db))?;
+            }
+            TypeCtor::RawPtr(m) => {
+                let t = self.parameters.as_single();
+                write!(f, "*{}{}", m.as_keyword_for_ptr(), t.display(f.db))?;
+            }
+            TypeCtor::Ref(m) => {
+                let t = self.parameters.as_single();
+                let ty_display = if f.omit_verbose_types() {
+                    t.display_truncated(f.db, f.max_size)
+                } else {
+                    t.display(f.db)
+                };
+                write!(f, "&{}{}", m.as_keyword_for_ref(), ty_display)?;
+            }
+            TypeCtor::Never => write!(f, "!")?,
+            TypeCtor::Tuple { .. } => {
+                let ts = &self.parameters;
+                if ts.len() == 1 {
+                    write!(f, "({},)", ts[0].display(f.db))?;
+                } else {
+                    write!(f, "(")?;
+                    f.write_joined(&*ts.0, ", ")?;
+                    write!(f, ")")?;
+                }
+            }
+            TypeCtor::FnPtr { is_varargs, .. } => {
+                let sig = FnSig::from_fn_ptr_substs(&self.parameters, is_varargs);
+                write!(f, "fn(")?;
+                f.write_joined(sig.params(), ", ")?;
+                if is_varargs {
+                    if sig.params().is_empty() {
+                        write!(f, "...")?;
+                    } else {
+                        write!(f, ", ...")?;
+                    }
+                }
+                write!(f, ")")?;
+                let ret = sig.ret();
+                if *ret != Ty::unit() {
+                    let ret_display = if f.omit_verbose_types() {
+                        ret.display_truncated(f.db, f.max_size)
+                    } else {
+                        ret.display(f.db)
+                    };
+                    write!(f, " -> {}", ret_display)?;
+                }
+            }
+            TypeCtor::FnDef(def) => {
+                let sig = f.db.callable_item_signature(def).subst(&self.parameters);
+                match def {
+                    CallableDefId::FunctionId(ff) => {
+                        write!(f, "fn {}", f.db.function_data(ff).name)?
+                    }
+                    CallableDefId::StructId(s) => write!(f, "{}", f.db.struct_data(s).name)?,
+                    CallableDefId::EnumVariantId(e) => {
+                        write!(f, "{}", f.db.enum_data(e.parent).variants[e.local_id].name)?
+                    }
+                };
+                if self.parameters.len() > 0 {
+                    let generics = generics(f.db.upcast(), def.into());
+                    let (parent_params, self_param, type_params, _impl_trait_params) =
+                        generics.provenance_split();
+                    let total_len = parent_params + self_param + type_params;
+                    // We print all params except implicit impl Trait params. Still a bit weird; should we leave out parent and self?
+                    if total_len > 0 {
+                        write!(f, "<")?;
+                        f.write_joined(&self.parameters.0[..total_len], ", ")?;
+                        write!(f, ">")?;
+                    }
+                }
+                write!(f, "(")?;
+                f.write_joined(sig.params(), ", ")?;
+                write!(f, ")")?;
+                let ret = sig.ret();
+                if *ret != Ty::unit() {
+                    let ret_display = if f.omit_verbose_types() {
+                        ret.display_truncated(f.db, f.max_size)
+                    } else {
+                        ret.display(f.db)
+                    };
+                    write!(f, " -> {}", ret_display)?;
+                }
+            }
+            TypeCtor::Adt(def_id) => {
+                match f.display_target {
+                    DisplayTarget::Diagnostics => {
+                        let name = match def_id {
+                            AdtId::StructId(it) => f.db.struct_data(it).name.clone(),
+                            AdtId::UnionId(it) => f.db.union_data(it).name.clone(),
+                            AdtId::EnumId(it) => f.db.enum_data(it).name.clone(),
+                        };
+                        write!(f, "{}", name)?;
+                    }
+                    DisplayTarget::SourceCode { module_id } => {
+                        if let Some(path) = find_path::find_path(
+                            f.db.upcast(),
+                            ItemInNs::Types(def_id.into()),
+                            module_id,
+                        ) {
+                            write!(f, "{}", path)?;
+                        } else {
+                            return Err(HirDisplayError::DisplaySourceCodeError(
+                                DisplaySourceCodeError::PathNotFound,
+                            ));
+                        }
+                    }
+                }
+
+                if self.parameters.len() > 0 {
+                    let parameters_to_write =
+                        if f.display_target.is_source_code() || f.omit_verbose_types() {
+                            match self
+                                .ctor
+                                .as_generic_def()
+                                .map(|generic_def_id| f.db.generic_defaults(generic_def_id))
+                                .filter(|defaults| !defaults.is_empty())
+                            {
+                                None => self.parameters.0.as_ref(),
+                                Some(default_parameters) => {
+                                    let mut default_from = 0;
+                                    for (i, parameter) in self.parameters.iter().enumerate() {
+                                        match (parameter, default_parameters.get(i)) {
+                                            (&Ty::Unknown, _) | (_, None) => {
+                                                default_from = i + 1;
+                                            }
+                                            (_, Some(default_parameter)) => {
+                                                let actual_default = default_parameter
+                                                    .clone()
+                                                    .subst(&self.parameters.prefix(i));
+                                                if parameter != &actual_default {
+                                                    default_from = i + 1;
+                                                }
+                                            }
+                                        }
+                                    }
+                                    &self.parameters.0[0..default_from]
+                                }
+                            }
+                        } else {
+                            self.parameters.0.as_ref()
+                        };
+                    if !parameters_to_write.is_empty() {
+                        write!(f, "<")?;
+                        f.write_joined(parameters_to_write, ", ")?;
+                        write!(f, ">")?;
+                    }
+                }
+            }
+            TypeCtor::AssociatedType(type_alias) => {
+                let trait_ = match type_alias.lookup(f.db.upcast()).container {
+                    AssocContainerId::TraitId(it) => it,
+                    _ => panic!("not an associated type"),
+                };
+                let trait_ = f.db.trait_data(trait_);
+                let type_alias = f.db.type_alias_data(type_alias);
+                write!(f, "{}::{}", trait_.name, type_alias.name)?;
+                if self.parameters.len() > 0 {
+                    write!(f, "<")?;
+                    f.write_joined(&*self.parameters.0, ", ")?;
+                    write!(f, ">")?;
+                }
+            }
+            TypeCtor::OpaqueType(opaque_ty_id) => {
+                let bounds = match opaque_ty_id {
+                    OpaqueTyId::ReturnTypeImplTrait(func, idx) => {
+                        let datas =
+                            f.db.return_type_impl_traits(func).expect("impl trait id without data");
+                        let data = (*datas)
+                            .as_ref()
+                            .map(|rpit| rpit.impl_traits[idx as usize].bounds.clone());
+                        data.subst(&self.parameters)
+                    }
+                };
+                write!(f, "impl ")?;
+                write_bounds_like_dyn_trait(&bounds.value, f)?;
+                // FIXME: it would maybe be good to distinguish this from the alias type (when debug printing), and to show the substitution
+            }
+            TypeCtor::Closure { .. } => {
+                let sig = self.parameters[0].callable_sig(f.db);
+                if let Some(sig) = sig {
+                    if sig.params().is_empty() {
+                        write!(f, "||")?;
+                    } else if f.omit_verbose_types() {
+                        write!(f, "|{}|", TYPE_HINT_TRUNCATION)?;
+                    } else {
+                        write!(f, "|")?;
+                        f.write_joined(sig.params(), ", ")?;
+                        write!(f, "|")?;
+                    };
+
+                    let ret_display = if f.omit_verbose_types() {
+                        sig.ret().display_truncated(f.db, f.max_size)
+                    } else {
+                        sig.ret().display(f.db)
+                    };
+                    write!(f, " -> {}", ret_display)?;
+                } else {
+                    write!(f, "{{closure}}")?;
+                }
+            }
+        }
+        Ok(())
+    }
+}
+
+impl HirDisplay for ProjectionTy {
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
+        if f.should_truncate() {
+            return write!(f, "{}", TYPE_HINT_TRUNCATION);
+        }
+
+        let trait_ = f.db.trait_data(self.trait_(f.db));
+        write!(f, "<{} as {}", self.parameters[0].display(f.db), trait_.name)?;
+        if self.parameters.len() > 1 {
+            write!(f, "<")?;
+            f.write_joined(&self.parameters[1..], ", ")?;
+            write!(f, ">")?;
+        }
+        write!(f, ">::{}", f.db.type_alias_data(self.associated_ty).name)?;
+        Ok(())
+    }
+}
+
+impl HirDisplay for Ty {
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
+        if f.should_truncate() {
+            return write!(f, "{}", TYPE_HINT_TRUNCATION);
+        }
+
+        match self {
+            Ty::Apply(a_ty) => a_ty.hir_fmt(f)?,
+            Ty::Projection(p_ty) => p_ty.hir_fmt(f)?,
+            Ty::Placeholder(id) => {
+                let generics = generics(f.db.upcast(), id.parent);
+                let param_data = &generics.params.types[id.local_id];
+                match param_data.provenance {
+                    TypeParamProvenance::TypeParamList | TypeParamProvenance::TraitSelf => {
+                        write!(f, "{}", param_data.name.clone().unwrap_or_else(Name::missing))?
+                    }
+                    TypeParamProvenance::ArgumentImplTrait => {
+                        write!(f, "impl ")?;
+                        let bounds = f.db.generic_predicates_for_param(*id);
+                        let substs = Substs::type_params_for_generics(&generics);
+                        write_bounds_like_dyn_trait(
+                            &bounds.iter().map(|b| b.clone().subst(&substs)).collect::<Vec<_>>(),
+                            f,
+                        )?;
+                    }
+                }
+            }
+            Ty::Bound(idx) => write!(f, "?{}.{}", idx.debruijn.depth(), idx.index)?,
+            Ty::Dyn(predicates) => {
+                write!(f, "dyn ")?;
+                write_bounds_like_dyn_trait(predicates, f)?;
+            }
+            Ty::Opaque(opaque_ty) => {
+                let bounds = match opaque_ty.opaque_ty_id {
+                    OpaqueTyId::ReturnTypeImplTrait(func, idx) => {
+                        let datas =
+                            f.db.return_type_impl_traits(func).expect("impl trait id without data");
+                        let data = (*datas)
+                            .as_ref()
+                            .map(|rpit| rpit.impl_traits[idx as usize].bounds.clone());
+                        data.subst(&opaque_ty.parameters)
+                    }
+                };
+                write!(f, "impl ")?;
+                write_bounds_like_dyn_trait(&bounds.value, f)?;
+            }
+            Ty::Unknown => write!(f, "{{unknown}}")?,
+            Ty::Infer(..) => write!(f, "_")?,
+        }
+        Ok(())
+    }
+}
+
+fn write_bounds_like_dyn_trait(
+    predicates: &[GenericPredicate],
+    f: &mut HirFormatter,
+) -> Result<(), HirDisplayError> {
+    // Note: This code is written to produce nice results (i.e.
+    // corresponding to surface Rust) for types that can occur in
+    // actual Rust. It will have weird results if the predicates
+    // aren't as expected (i.e. self types = $0, projection
+    // predicates for a certain trait come after the Implemented
+    // predicate for that trait).
+    let mut first = true;
+    let mut angle_open = false;
+    for p in predicates.iter() {
+        match p {
+            GenericPredicate::Implemented(trait_ref) => {
+                if angle_open {
+                    write!(f, ">")?;
+                    angle_open = false;
+                }
+                if !first {
+                    write!(f, " + ")?;
+                }
+                // We assume that the self type is $0 (i.e. the
+                // existential) here, which is the only thing that's
+                // possible in actual Rust, and hence don't print it
+                write!(f, "{}", f.db.trait_data(trait_ref.trait_).name)?;
+                if trait_ref.substs.len() > 1 {
+                    write!(f, "<")?;
+                    f.write_joined(&trait_ref.substs[1..], ", ")?;
+                    // there might be assoc type bindings, so we leave the angle brackets open
+                    angle_open = true;
+                }
+            }
+            GenericPredicate::Projection(projection_pred) => {
+                // in types in actual Rust, these will always come
+                // after the corresponding Implemented predicate
+                if angle_open {
+                    write!(f, ", ")?;
+                } else {
+                    write!(f, "<")?;
+                    angle_open = true;
+                }
+                let type_alias = f.db.type_alias_data(projection_pred.projection_ty.associated_ty);
+                write!(f, "{} = ", type_alias.name)?;
+                projection_pred.ty.hir_fmt(f)?;
+            }
+            GenericPredicate::Error => {
+                if angle_open {
+                    // impl Trait<X, {error}>
+                    write!(f, ", ")?;
+                } else if !first {
+                    // impl Trait + {error}
+                    write!(f, " + ")?;
+                }
+                p.hir_fmt(f)?;
+            }
+        }
+        first = false;
+    }
+    if angle_open {
+        write!(f, ">")?;
+    }
+    Ok(())
+}
+
+impl TraitRef {
+    fn hir_fmt_ext(&self, f: &mut HirFormatter, use_as: bool) -> Result<(), HirDisplayError> {
+        if f.should_truncate() {
+            return write!(f, "{}", TYPE_HINT_TRUNCATION);
+        }
+
+        self.substs[0].hir_fmt(f)?;
+        if use_as {
+            write!(f, " as ")?;
+        } else {
+            write!(f, ": ")?;
+        }
+        write!(f, "{}", f.db.trait_data(self.trait_).name)?;
+        if self.substs.len() > 1 {
+            write!(f, "<")?;
+            f.write_joined(&self.substs[1..], ", ")?;
+            write!(f, ">")?;
+        }
+        Ok(())
+    }
+}
+
+impl HirDisplay for TraitRef {
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
+        self.hir_fmt_ext(f, false)
+    }
+}
+
+impl HirDisplay for &GenericPredicate {
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
+        HirDisplay::hir_fmt(*self, f)
+    }
+}
+
+impl HirDisplay for GenericPredicate {
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
+        if f.should_truncate() {
+            return write!(f, "{}", TYPE_HINT_TRUNCATION);
+        }
+
+        match self {
+            GenericPredicate::Implemented(trait_ref) => trait_ref.hir_fmt(f)?,
+            GenericPredicate::Projection(projection_pred) => {
+                write!(f, "<")?;
+                projection_pred.projection_ty.trait_ref(f.db).hir_fmt_ext(f, true)?;
+                write!(
+                    f,
+                    ">::{} = {}",
+                    f.db.type_alias_data(projection_pred.projection_ty.associated_ty).name,
+                    projection_pred.ty.display(f.db)
+                )?;
+            }
+            GenericPredicate::Error => write!(f, "{{error}}")?,
+        }
+        Ok(())
+    }
+}
+
+impl HirDisplay for Obligation {
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
+        Ok(match self {
+            Obligation::Trait(tr) => write!(f, "Implements({})", tr.display(f.db))?,
+            Obligation::Projection(proj) => write!(
+                f,
+                "Normalize({} => {})",
+                proj.projection_ty.display(f.db),
+                proj.ty.display(f.db)
+            )?,
+        })
+    }
+}
diff --git a/crates/hir_ty/src/infer.rs b/crates/hir_ty/src/infer.rs
new file mode 100644
index 000000000..03b00b101
--- /dev/null
+++ b/crates/hir_ty/src/infer.rs
@@ -0,0 +1,802 @@
+//! Type inference, i.e. the process of walking through the code and determining
+//! the type of each expression and pattern.
+//!
+//! For type inference, compare the implementations in rustc (the various
+//! check_* methods in librustc_typeck/check/mod.rs are a good entry point) and
+//! IntelliJ-Rust (org.rust.lang.core.types.infer). Our entry point for
+//! inference here is the `infer` function, which infers the types of all
+//! expressions in a given function.
+//!
+//! During inference, types (i.e. the `Ty` struct) can contain type 'variables'
+//! which represent currently unknown types; as we walk through the expressions,
+//! we might determine that certain variables need to be equal to each other, or
+//! to certain types. To record this, we use the union-find implementation from
+//! the `ena` crate, which is extracted from rustc.
+
+use std::borrow::Cow;
+use std::mem;
+use std::ops::Index;
+use std::sync::Arc;
+
+use arena::map::ArenaMap;
+use hir_def::{
+    body::Body,
+    data::{ConstData, FunctionData, StaticData},
+    expr::{BindingAnnotation, ExprId, PatId},
+    lang_item::LangItemTarget,
+    path::{path, Path},
+    resolver::{HasResolver, Resolver, TypeNs},
+    type_ref::{Mutability, TypeRef},
+    AdtId, AssocItemId, DefWithBodyId, EnumVariantId, FieldId, FunctionId, Lookup, TraitId,
+    TypeAliasId, VariantId,
+};
+use hir_expand::{diagnostics::DiagnosticSink, name::name};
+use rustc_hash::FxHashMap;
+use stdx::impl_from;
+use syntax::SmolStr;
+
+use super::{
+    primitive::{FloatTy, IntTy},
+    traits::{Guidance, Obligation, ProjectionPredicate, Solution},
+    InEnvironment, ProjectionTy, Substs, TraitEnvironment, TraitRef, Ty, TypeCtor, TypeWalk,
+};
+use crate::{
+    db::HirDatabase, infer::diagnostics::InferenceDiagnostic, lower::ImplTraitLoweringMode,
+};
+
+pub(crate) use unify::unify;
+
+macro_rules! ty_app {
+    ($ctor:pat, $param:pat) => {
+        crate::Ty::Apply(crate::ApplicationTy { ctor: $ctor, parameters: $param })
+    };
+    ($ctor:pat) => {
+        ty_app!($ctor, _)
+    };
+}
+
+mod unify;
+mod path;
+mod expr;
+mod pat;
+mod coerce;
+
+/// The entry point of type inference.
+pub(crate) fn infer_query(db: &dyn HirDatabase, def: DefWithBodyId) -> Arc<InferenceResult> {
+    let _p = profile::span("infer_query");
+    let resolver = def.resolver(db.upcast());
+    let mut ctx = InferenceContext::new(db, def, resolver);
+
+    match def {
+        DefWithBodyId::ConstId(c) => ctx.collect_const(&db.const_data(c)),
+        DefWithBodyId::FunctionId(f) => ctx.collect_fn(&db.function_data(f)),
+        DefWithBodyId::StaticId(s) => ctx.collect_static(&db.static_data(s)),
+    }
+
+    ctx.infer_body();
+
+    Arc::new(ctx.resolve_all())
+}
+
+#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
+enum ExprOrPatId {
+    ExprId(ExprId),
+    PatId(PatId),
+}
+impl_from!(ExprId, PatId for ExprOrPatId);
+
+/// Binding modes inferred for patterns.
+/// https://doc.rust-lang.org/reference/patterns.html#binding-modes
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+enum BindingMode {
+    Move,
+    Ref(Mutability),
+}
+
+impl BindingMode {
+    pub fn convert(annotation: BindingAnnotation) -> BindingMode {
+        match annotation {
+            BindingAnnotation::Unannotated | BindingAnnotation::Mutable => BindingMode::Move,
+            BindingAnnotation::Ref => BindingMode::Ref(Mutability::Shared),
+            BindingAnnotation::RefMut => BindingMode::Ref(Mutability::Mut),
+        }
+    }
+}
+
+impl Default for BindingMode {
+    fn default() -> Self {
+        BindingMode::Move
+    }
+}
+
+/// A mismatch between an expected and an inferred type.
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
+pub struct TypeMismatch {
+    pub expected: Ty,
+    pub actual: Ty,
+}
+
+/// The result of type inference: A mapping from expressions and patterns to types.
+#[derive(Clone, PartialEq, Eq, Debug, Default)]
+pub struct InferenceResult {
+    /// For each method call expr, records the function it resolves to.
+    method_resolutions: FxHashMap<ExprId, FunctionId>,
+    /// For each field access expr, records the field it resolves to.
+    field_resolutions: FxHashMap<ExprId, FieldId>,
+    /// For each field in record literal, records the field it resolves to.
+    record_field_resolutions: FxHashMap<ExprId, FieldId>,
+    record_field_pat_resolutions: FxHashMap<PatId, FieldId>,
+    /// For each struct literal, records the variant it resolves to.
+    variant_resolutions: FxHashMap<ExprOrPatId, VariantId>,
+    /// For each associated item record what it resolves to
+    assoc_resolutions: FxHashMap<ExprOrPatId, AssocItemId>,
+    diagnostics: Vec<InferenceDiagnostic>,
+    pub type_of_expr: ArenaMap<ExprId, Ty>,
+    pub type_of_pat: ArenaMap<PatId, Ty>,
+    pub(super) type_mismatches: ArenaMap<ExprId, TypeMismatch>,
+}
+
+impl InferenceResult {
+    pub fn method_resolution(&self, expr: ExprId) -> Option<FunctionId> {
+        self.method_resolutions.get(&expr).copied()
+    }
+    pub fn field_resolution(&self, expr: ExprId) -> Option<FieldId> {
+        self.field_resolutions.get(&expr).copied()
+    }
+    pub fn record_field_resolution(&self, expr: ExprId) -> Option<FieldId> {
+        self.record_field_resolutions.get(&expr).copied()
+    }
+    pub fn record_field_pat_resolution(&self, pat: PatId) -> Option<FieldId> {
+        self.record_field_pat_resolutions.get(&pat).copied()
+    }
+    pub fn variant_resolution_for_expr(&self, id: ExprId) -> Option<VariantId> {
+        self.variant_resolutions.get(&id.into()).copied()
+    }
+    pub fn variant_resolution_for_pat(&self, id: PatId) -> Option<VariantId> {
+        self.variant_resolutions.get(&id.into()).copied()
+    }
+    pub fn assoc_resolutions_for_expr(&self, id: ExprId) -> Option<AssocItemId> {
+        self.assoc_resolutions.get(&id.into()).copied()
+    }
+    pub fn assoc_resolutions_for_pat(&self, id: PatId) -> Option<AssocItemId> {
+        self.assoc_resolutions.get(&id.into()).copied()
+    }
+    pub fn type_mismatch_for_expr(&self, expr: ExprId) -> Option<&TypeMismatch> {
+        self.type_mismatches.get(expr)
+    }
+    pub fn add_diagnostics(
+        &self,
+        db: &dyn HirDatabase,
+        owner: DefWithBodyId,
+        sink: &mut DiagnosticSink,
+    ) {
+        self.diagnostics.iter().for_each(|it| it.add_to(db, owner, sink))
+    }
+}
+
+impl Index<ExprId> for InferenceResult {
+    type Output = Ty;
+
+    fn index(&self, expr: ExprId) -> &Ty {
+        self.type_of_expr.get(expr).unwrap_or(&Ty::Unknown)
+    }
+}
+
+impl Index<PatId> for InferenceResult {
+    type Output = Ty;
+
+    fn index(&self, pat: PatId) -> &Ty {
+        self.type_of_pat.get(pat).unwrap_or(&Ty::Unknown)
+    }
+}
+
+/// The inference context contains all information needed during type inference.
+#[derive(Clone, Debug)]
+struct InferenceContext<'a> {
+    db: &'a dyn HirDatabase,
+    owner: DefWithBodyId,
+    body: Arc<Body>,
+    resolver: Resolver,
+    table: unify::InferenceTable,
+    trait_env: Arc<TraitEnvironment>,
+    obligations: Vec<Obligation>,
+    result: InferenceResult,
+    /// The return type of the function being inferred, or the closure if we're
+    /// currently within one.
+    ///
+    /// We might consider using a nested inference context for checking
+    /// closures, but currently this is the only field that will change there,
+    /// so it doesn't make sense.
+    return_ty: Ty,
+    diverges: Diverges,
+    breakables: Vec<BreakableContext>,
+}
+
+#[derive(Clone, Debug)]
+struct BreakableContext {
+    pub may_break: bool,
+    pub break_ty: Ty,
+    pub label: Option<name::Name>,
+}
+
+fn find_breakable<'c>(
+    ctxs: &'c mut [BreakableContext],
+    label: Option<&name::Name>,
+) -> Option<&'c mut BreakableContext> {
+    match label {
+        Some(_) => ctxs.iter_mut().rev().find(|ctx| ctx.label.as_ref() == label),
+        None => ctxs.last_mut(),
+    }
+}
+
+impl<'a> InferenceContext<'a> {
+    fn new(db: &'a dyn HirDatabase, owner: DefWithBodyId, resolver: Resolver) -> Self {
+        InferenceContext {
+            result: InferenceResult::default(),
+            table: unify::InferenceTable::new(),
+            obligations: Vec::default(),
+            return_ty: Ty::Unknown, // set in collect_fn_signature
+            trait_env: TraitEnvironment::lower(db, &resolver),
+            db,
+            owner,
+            body: db.body(owner),
+            resolver,
+            diverges: Diverges::Maybe,
+            breakables: Vec::new(),
+        }
+    }
+
+    fn resolve_all(mut self) -> InferenceResult {
+        // FIXME resolve obligations as well (use Guidance if necessary)
+        let mut result = std::mem::take(&mut self.result);
+        for ty in result.type_of_expr.values_mut() {
+            let resolved = self.table.resolve_ty_completely(mem::replace(ty, Ty::Unknown));
+            *ty = resolved;
+        }
+        for ty in result.type_of_pat.values_mut() {
+            let resolved = self.table.resolve_ty_completely(mem::replace(ty, Ty::Unknown));
+            *ty = resolved;
+        }
+        result
+    }
+
+    fn write_expr_ty(&mut self, expr: ExprId, ty: Ty) {
+        self.result.type_of_expr.insert(expr, ty);
+    }
+
+    fn write_method_resolution(&mut self, expr: ExprId, func: FunctionId) {
+        self.result.method_resolutions.insert(expr, func);
+    }
+
+    fn write_field_resolution(&mut self, expr: ExprId, field: FieldId) {
+        self.result.field_resolutions.insert(expr, field);
+    }
+
+    fn write_variant_resolution(&mut self, id: ExprOrPatId, variant: VariantId) {
+        self.result.variant_resolutions.insert(id, variant);
+    }
+
+    fn write_assoc_resolution(&mut self, id: ExprOrPatId, item: AssocItemId) {
+        self.result.assoc_resolutions.insert(id, item);
+    }
+
+    fn write_pat_ty(&mut self, pat: PatId, ty: Ty) {
+        self.result.type_of_pat.insert(pat, ty);
+    }
+
+    fn push_diagnostic(&mut self, diagnostic: InferenceDiagnostic) {
+        self.result.diagnostics.push(diagnostic);
+    }
+
+    fn make_ty_with_mode(
+        &mut self,
+        type_ref: &TypeRef,
+        impl_trait_mode: ImplTraitLoweringMode,
+    ) -> Ty {
+        // FIXME use right resolver for block
+        let ctx = crate::lower::TyLoweringContext::new(self.db, &self.resolver)
+            .with_impl_trait_mode(impl_trait_mode);
+        let ty = Ty::from_hir(&ctx, type_ref);
+        let ty = self.insert_type_vars(ty);
+        self.normalize_associated_types_in(ty)
+    }
+