9 files changed, 1037 insertions, 9 deletions

diff --git a/crates/ra_hir_ty/Cargo.toml b/crates/ra_hir_ty/Cargo.toml
index 45be08430..9a4a7aa6f 100644
--- a/crates/ra_hir_ty/Cargo.toml
+++ b/crates/ra_hir_ty/Cargo.toml
@@ -9,6 +9,7 @@ doctest = false
 
 [dependencies]
 arrayvec = "0.5.1"
+smallvec = "1.2.0"
 ena = "0.13.1"
 log = "0.4.8"
 rustc-hash = "1.1.0"
diff --git a/crates/ra_hir_ty/src/_match.rs b/crates/ra_hir_ty/src/_match.rs
new file mode 100644
index 000000000..ac66dd415
--- /dev/null
+++ b/crates/ra_hir_ty/src/_match.rs
@@ -0,0 +1,944 @@
+//! 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 match checking algorithm.
+use std::sync::Arc;
+
+use smallvec::{smallvec, SmallVec};
+
+use crate::{
+    db::HirDatabase,
+    expr::{Body, Expr, Literal, Pat, PatId},
+    InferenceResult,
+};
+use hir_def::{adt::VariantData, EnumVariantId, VariantId};
+
+#[derive(Debug, Clone, Copy)]
+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)
+    }
+}
+
+type PatStackInner = SmallVec<[PatIdOrWild; 2]>;
+#[derive(Debug)]
+pub(crate) struct PatStack(PatStackInner);
+
+impl PatStack {
+    pub(crate) fn from_pattern(pat_id: PatId) -> PatStack {
+        Self(smallvec!(pat_id.into()))
+    }
+
+    pub(crate) 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 is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    fn head(&self) -> PatIdOrWild {
+        self.0[0]
+    }
+
+    fn get_head(&self) -> Option<PatIdOrWild> {
+        self.0.first().copied()
+    }
+
+    fn to_tail(&self) -> PatStack {
+        Self::from_slice(&self.0[1..])
+    }
+
+    fn replace_head_with(&self, pat_ids: &[PatId]) -> PatStack {
+        let mut patterns: PatStackInner = smallvec![];
+        for pat in pat_ids {
+            patterns.push((*pat).into());
+        }
+        for pat in &self.0[1..] {
+            patterns.push(*pat);
+        }
+        PatStack::from_vec(patterns)
+    }
+
+    // Computes `D(self)`.
+    fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Option<PatStack> {
+        if matches!(self.head().as_pat(cx), Pat::Wild) {
+            Some(self.to_tail())
+        } else {
+            None
+        }
+    }
+
+    // Computes `S(constructor, self)`.
+    fn specialize_constructor(
+        &self,
+        cx: &MatchCheckCtx,
+        constructor: &Constructor,
+    ) -> Option<PatStack> {
+        match (self.head().as_pat(cx), constructor) {
+            (Pat::Tuple(ref pat_ids), Constructor::Tuple { arity }) => {
+                if pat_ids.len() != *arity {
+                    return None;
+                }
+
+                Some(self.replace_head_with(pat_ids))
+            }
+            (Pat::Lit(_), Constructor::Bool(_)) => {
+                // for now we only support bool literals
+                Some(self.to_tail())
+            }
+            (Pat::Wild, constructor) => Some(self.expand_wildcard(cx, constructor)),
+            (Pat::Path(_), Constructor::Enum(constructor)) => {
+                let pat_id = self.head().as_id().expect("we know this isn't a wild");
+                if !enum_variant_matches(cx, pat_id, *constructor) {
+                    return None;
+                }
+                // enums with no associated data become `Pat::Path`
+                Some(self.to_tail())
+            }
+            (Pat::TupleStruct { args: ref pat_ids, .. }, Constructor::Enum(constructor)) => {
+                let pat_id = self.head().as_id().expect("we know this isn't a wild");
+                if !enum_variant_matches(cx, pat_id, *constructor) {
+                    return None;
+                }
+
+                Some(self.replace_head_with(pat_ids))
+            }
+            (Pat::Or(_), _) => unreachable!("we desugar or patterns so this should never happen"),
+            (a, b) => unimplemented!("{:?}, {:?}", a, b),
+        }
+    }
+
+    fn expand_wildcard(&self, cx: &MatchCheckCtx, constructor: &Constructor) -> PatStack {
+        assert_eq!(
+            Pat::Wild,
+            self.head().as_pat(cx),
+            "expand_wildcard must only be called on PatStack with wild at head",
+        );
+
+        let mut patterns: PatStackInner = smallvec![];
+        let arity = match constructor {
+            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::Unit => 0,
+                    x => unimplemented!("{:?}", x),
+                }
+            }
+        };
+
+        for _ in 0..arity {
+            patterns.push(PatIdOrWild::Wild);
+        }
+
+        for pat in &self.0[1..] {
+            patterns.push(*pat);
+        }
+
+        PatStack::from_vec(patterns)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct Matrix(Vec<PatStack>);
+
+impl Matrix {
+    pub(crate) fn empty() -> Self {
+        Self(vec![])
+    }
+
+    pub(crate) fn push(&mut self, cx: &MatchCheckCtx, row: PatStack) {
+        // if the pattern is an or pattern it should be expanded
+        if let Some(Pat::Or(pat_ids)) = row.get_head().map(|pat_id| pat_id.as_pat(cx)) {
+            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().map(|p| p.head()).collect()
+    }
+
+    // Computes `D(self)`.
+    fn specialize_wildcard(&self, cx: &MatchCheckCtx) -> Self {
+        Self::collect(cx, self.0.iter().filter_map(|r| r.specialize_wildcard(cx)))
+    }
+
+    // Computes `S(constructor, self)`.
+    fn specialize_constructor(&self, cx: &MatchCheckCtx, constructor: &Constructor) -> Self {
+        Self::collect(cx, self.0.iter().filter_map(|r| r.specialize_constructor(cx, constructor)))
+    }
+
+    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)]
+pub enum Usefulness {
+    Useful,
+    NotUseful,
+}
+
+pub struct MatchCheckCtx<'a> {
+    pub body: Arc<Body>,
+    pub match_expr: &'a Expr,
+    pub infer: Arc<InferenceResult>,
+    pub db: &'a dyn HirDatabase,
+}
+
+// see src/librustc_mir_build/hair/pattern/_match.rs
+// It seems the rustc version of this method is able to assume that all the match arm
+// patterns are valid (they are valid given a particular match expression), but I
+// don't think we can make that assumption here. How should that be handled?
+//
+// Perhaps check that validity before passing the patterns into this method?
+pub(crate) fn is_useful(cx: &MatchCheckCtx, matrix: &Matrix, v: &PatStack) -> Usefulness {
+    dbg!(matrix);
+    dbg!(v);
+    if v.is_empty() {
+        if matrix.is_empty() {
+            return Usefulness::Useful;
+        } else {
+            return Usefulness::NotUseful;
+        }
+    }
+
+    if let Pat::Or(pat_ids) = v.head().as_pat(cx) {
+        let any_useful = pat_ids.iter().any(|&pat_id| {
+            let v = PatStack::from_pattern(pat_id);
+
+            is_useful(cx, matrix, &v) == Usefulness::Useful
+        });
+
+        return if any_useful { Usefulness::Useful } else { Usefulness::NotUseful };
+    }
+
+    if let Some(constructor) = pat_constructor(cx, v.head()) {
+        let matrix = matrix.specialize_constructor(&cx, &constructor);
+        let v = v.specialize_constructor(&cx, &constructor).expect("todo handle this case");
+
+        is_useful(&cx, &matrix, &v)
+    } else {
+        dbg!("expanding wildcard");
+        // expanding wildcard
+        let used_constructors: Vec<Constructor> =
+            matrix.heads().iter().filter_map(|&p| pat_constructor(cx, p)).collect();
+
+        // 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. But potentially a better way to handle
+        // this is to use the match expressions type.
+        match &used_constructors.first() {
+            Some(constructor) if all_constructors_covered(&cx, constructor, &used_constructors) => {
+                dbg!("all constructors are covered");
+                // 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.
+                let constructor =
+                    matrix.heads().iter().filter_map(|&pat| pat_constructor(cx, pat)).next();
+
+                if let Some(constructor) = constructor {
+                    dbg!("found constructor {:?}, specializing..", &constructor);
+                    if let Constructor::Enum(e) = constructor {
+                        // For enums we handle each variant as a distinct constructor, so
+                        // here we create a constructor for each variant and then check
+                        // usefulness after specializing for that constructor.
+                        let any_useful = cx
+                            .db
+                            .enum_data(e.parent)
+                            .variants
+                            .iter()
+                            .map(|(local_id, _)| {
+                                Constructor::Enum(EnumVariantId { parent: e.parent, local_id })
+                            })
+                            .any(|constructor| {
+                                let matrix = matrix.specialize_constructor(&cx, &constructor);
+                                let v = v.expand_wildcard(&cx, &constructor);
+
+                                is_useful(&cx, &matrix, &v) == Usefulness::Useful
+                            });
+
+                        if any_useful {
+                            Usefulness::Useful
+                        } else {
+                            Usefulness::NotUseful
+                        }
+                    } else {
+                        let matrix = matrix.specialize_constructor(&cx, &constructor);
+                        let v = v.expand_wildcard(&cx, &constructor);
+
+                        is_useful(&cx, &matrix, &v)
+                    }
+                } else {
+                    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)]
+enum Constructor {
+    Bool(bool),
+    Tuple { arity: usize },
+    Enum(EnumVariantId),
+}
+
+fn pat_constructor(cx: &MatchCheckCtx, pat: PatIdOrWild) -> Option<Constructor> {
+    match pat.as_pat(cx) {
+        Pat::Wild => None,
+        Pat::Tuple(pats) => Some(Constructor::Tuple { arity: pats.len() }),
+        Pat::Lit(lit_expr) => {
+            // for now we only support bool literals
+            match cx.body.exprs[lit_expr] {
+                Expr::Literal(Literal::Bool(val)) => Some(Constructor::Bool(val)),
+                _ => unimplemented!(),
+            }
+        }
+        Pat::TupleStruct { .. } | Pat::Path(_) => {
+            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).unwrap_or_else(|| unimplemented!());
+            match variant_id {
+                VariantId::EnumVariantId(enum_variant_id) => {
+                    Some(Constructor::Enum(enum_variant_id))
+                }
+                _ => unimplemented!(),
+            }
+        }
+        x => unimplemented!("{:?} not yet implemented", x),
+    }
+}
+
+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 {
+    if let Some(VariantId::EnumVariantId(pat_variant_id)) =
+        cx.infer.variant_resolution_for_pat(pat_id)
+    {
+        if pat_variant_id.local_id == enum_variant_id.local_id {
+            return true;
+        }
+    }
+    false
+}
+
+#[cfg(test)]
+mod tests {
+    pub(super) use insta::assert_snapshot;
+    pub(super) use ra_db::fixture::WithFixture;
+
+    pub(super) use crate::test_db::TestDB;
+
+    pub(super) fn check_diagnostic_message(content: &str) -> String {
+        TestDB::with_single_file(content).0.diagnostics().0
+    }
+
+    pub(super) fn check_diagnostic_with_no_fix(content: &str) {
+        let diagnostic_count = TestDB::with_single_file(content).0.diagnostics().1;
+
+        assert_eq!(1, diagnostic_count, "no diagnotic reported");
+    }
+
+    pub(super) fn check_no_diagnostic(content: &str) {
+        let diagnostic_count = TestDB::with_single_file(content).0.diagnostics().1;
+
+        assert_eq!(0, diagnostic_count, "expected no diagnostic, found one");
+    }
+
+    #[test]
+    fn empty_tuple_no_arms_diagnostic_message() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                }
+            }
+        ";
+
+        assert_snapshot!(
+            check_diagnostic_message(content),
+            @"\"{\\n                }\": Missing match arm\n"
+        );
+    }
+
+    #[test]
+    fn empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match () {
+                    () => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (()) {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (()) {
+                    (()) => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_two_empty_tuple_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match ((), ()) {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_two_empty_tuple_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match ((), ()) {
+                    ((), ()) => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn bool_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn bool_missing_arm() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                    true => {}
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn bool_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match false {
+                    true => {}
+                    false => {}
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, true) => {},
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_bools_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, true) {
+                    (true, true) => {},
+                    (true, false) => {},
+                    (false, true) => {},
+                    (false, false) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_no_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                    (true, ((), false)) => {},
+                    (false, ((), true)) => {},
+                    (false, ((), false)) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_wildcard_missing_arms() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, _) => {},
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn tuple_of_tuple_and_bools_wildcard_no_diagnostic() {
+        let content = r"
+            fn test_fn() {
+                match (false, ((), false)) {
+                    (true, ((), true)) => {},
+                    (true, ((), false)) => {},
+                    (false, _) => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_no_arms() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::A {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn enum_missing_arms() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A => {},
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn enum_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A => {},
+                    Either::B => {},
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_no_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_missing_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(true) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(true) => (),
+                    Either::A(false) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_with_wild_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::B => (),
+                    _ => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_containing_bool_with_wild_2_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B,
+            }
+            fn test_fn() {
+                match Either::B {
+                    Either::A(_) => (),
+                    Either::B => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn enum_different_sizes_missing_arms() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(_) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_diagnostic_with_no_fix(content);
+    }
+
+    #[test]
+    fn enum_different_sizes_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(_) => (),
+                    Either::B(true, _) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn or_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            fn test_fn() {
+                match Either::A(false) {
+                    Either::A(true) | Either::A(false) => (),
+                    Either::B(true, _) => (),
+                    Either::B(false, _) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+
+    #[test]
+    fn tuple_of_enum_no_diagnostic() {
+        let content = r"
+            enum Either {
+                A(bool),
+                B(bool, bool),
+            }
+            enum Either2 {
+                C,
+                D,
+            }
+            fn test_fn() {
+                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) => (),
+                }
+            }
+        ";
+
+        check_no_diagnostic(content);
+    }
+}
+
+#[cfg(test)]
+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::tests::*;
+
+    #[test]
+    fn mismatched_types() {
+        let content = r"
+            enum Either {
+                A,
+                B,
+            }
+            enum Either2 {
+                C,
+                D,
+            }
+            fn test_fn() {
+                match Either::A {
+                    Either2::C => (),
+                    Either2::D => (),
+                }
+            }
+        ";
+
+        // This is a false negative.
+        // We don't currently check that the match arms actually
+        // match the type of the match expression.
+        check_no_diagnostic(content);
+    }
+}
diff --git a/crates/ra_hir_ty/src/diagnostics.rs b/crates/ra_hir_ty/src/diagnostics.rs
index 0f8522021..3457905e2 100644
--- a/crates/ra_hir_ty/src/diagnostics.rs
+++ b/crates/ra_hir_ty/src/diagnostics.rs
@@ -6,7 +6,7 @@ use hir_expand::{db::AstDatabase, name::Name, HirFileId, InFile};
 use ra_syntax::{ast, AstNode, AstPtr, SyntaxNodePtr};
 use stdx::format_to;
 
-pub use hir_def::diagnostics::UnresolvedModule;
+pub use hir_def::{diagnostics::UnresolvedModule, expr::MatchArm};
 pub use hir_expand::diagnostics::{AstDiagnostic, Diagnostic, DiagnosticSink};
 
 #[derive(Debug)]
@@ -63,6 +63,24 @@ impl AstDiagnostic for MissingFields {
 }
 
 #[derive(Debug)]
+pub struct MissingMatchArms {
+    pub file: HirFileId,
+    pub arms: AstPtr<ast::MatchArmList>,
+}
+
+impl Diagnostic for MissingMatchArms {
+    fn message(&self) -> String {
+        String::from("Missing match arm")
+    }
+    fn source(&self) -> InFile<SyntaxNodePtr> {
+        InFile { file_id: self.file, value: self.arms.into() }
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
+
+#[derive(Debug)]
 pub struct MissingOkInTailExpr {
     pub file: HirFileId,
     pub expr: AstPtr<ast::Expr>,
diff --git a/crates/ra_hir_ty/src/expr.rs b/crates/ra_hir_ty/src/expr.rs
index b7b476b4c..3caeeb394 100644
--- a/crates/ra_hir_ty/src/expr.rs
+++ b/crates/ra_hir_ty/src/expr.rs
@@ -14,9 +14,10 @@ use rustc_hash::FxHashSet;
 
 use crate::{
     db::HirDatabase,
-    diagnostics::{MissingFields, MissingOkInTailExpr},
+    diagnostics::{MissingFields, MissingMatchArms, MissingOkInTailExpr},
     utils::variant_data,
     ApplicationTy, InferenceResult, Ty, TypeCtor,
+    _match::{is_useful, MatchCheckCtx, Matrix, PatStack, Usefulness},
 };
 
 pub use hir_def::{
@@ -52,15 +53,63 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
         for e in body.exprs.iter() {
             if let (id, Expr::RecordLit { path, fields, spread }) = e {
                 self.validate_record_literal(id, path, fields, *spread, db);
+            } else if let (id, Expr::Match { expr, arms }) = e {
+                self.validate_match(id, *expr, arms, db, self.infer.clone());
             }
         }
 
         let body_expr = &body[body.body_expr];
-        if let Expr::Block { statements: _, tail: Some(t) } = body_expr {
+        if let Expr::Block { tail: Some(t), .. } = body_expr {
             self.validate_results_in_tail_expr(body.body_expr, *t, db);
         }
     }
 
+    fn validate_match(
+        &mut self,
+        id: ExprId,
+        expr: ExprId,
+        arms: &[MatchArm],
+        db: &dyn HirDatabase,
+        infer: Arc<InferenceResult>,
+    ) {
+        let (body, source_map): (Arc<Body>, Arc<BodySourceMap>) =
+            db.body_with_source_map(self.func.into());
+
+        let match_expr: &hir_def::expr::Expr = &body[expr];
+
+        let cx = MatchCheckCtx { body: body.clone(), match_expr, infer, db };
+        let pats = arms.iter().map(|arm| arm.pat);
+
+        let mut seen = Matrix::empty();
+        for pat in pats {
+            // 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);
+        }
+
+        match is_useful(&cx, &seen, &PatStack::from_wild()) {
+            Usefulness::Useful => (),
+            // if a wildcard pattern is not useful, then all patterns are covered
+            Usefulness::NotUseful => return,
+        }
+
+        if let Ok(source_ptr) = source_map.expr_syntax(id) {
+            if let Some(expr) = source_ptr.value.left() {
+                let root = source_ptr.file_syntax(db.upcast());
+                if let ast::Expr::MatchExpr(match_expr) = expr.to_node(&root) {
+                    if let Some(arms) = match_expr.match_arm_list() {
+                        self.sink.push(MissingMatchArms {
+                            file: source_ptr.file_id,
+                            arms: AstPtr::new(&arms),
+                        })
+                    }
+                }
+            }
+        }
+    }
+
     fn validate_record_literal(
         &mut self,
         id: ExprId,
diff --git a/crates/ra_hir_ty/src/infer/pat.rs b/crates/ra_hir_ty/src/infer/pat.rs
index 86acd27f8..69bbb4307 100644
--- a/crates/ra_hir_ty/src/infer/pat.rs
+++ b/crates/ra_hir_ty/src/infer/pat.rs
@@ -21,9 +21,13 @@ impl<'a> InferenceContext<'a> {
         subpats: &[PatId],
         expected: &Ty,
         default_bm: BindingMode,
+        id: PatId,
     ) -> Ty {
         let (ty, def) = self.resolve_variant(path);
         let var_data = def.map(|it| variant_data(self.db.upcast(), it));
+        if let Some(variant) = def {
+            self.write_variant_resolution(id.into(), variant);
+        }
         self.unify(&ty, expected);
 
         let substs = ty.substs().unwrap_or_else(Substs::empty);
@@ -152,7 +156,7 @@ impl<'a> InferenceContext<'a> {
                 Ty::apply_one(TypeCtor::Ref(*mutability), subty)
             }
             Pat::TupleStruct { path: p, args: subpats } => {
-                self.infer_tuple_struct_pat(p.as_ref(), subpats, expected, default_bm)
+                self.infer_tuple_struct_pat(p.as_ref(), subpats, expected, default_bm, pat)
             }
             Pat::Record { path: p, args: fields } => {
                 self.infer_record_pat(p.as_ref(), fields, expected, default_bm, pat)
diff --git a/crates/ra_hir_ty/src/infer/path.rs b/crates/ra_hir_ty/src/infer/path.rs
index 318652c61..2b6bc0f79 100644
--- a/crates/ra_hir_ty/src/infer/path.rs
+++ b/crates/ra_hir_ty/src/infer/path.rs
@@ -67,8 +67,16 @@ impl<'a> InferenceContext<'a> {
             ValueNs::FunctionId(it) => it.into(),
             ValueNs::ConstId(it) => it.into(),
             ValueNs::StaticId(it) => it.into(),
-            ValueNs::StructId(it) => it.into(),
-            ValueNs::EnumVariantId(it) => it.into(),
+            ValueNs::StructId(it) => {
+                self.write_variant_resolution(id, it.into());
+
+                it.into()
+            }
+            ValueNs::EnumVariantId(it) => {
+                self.write_variant_resolution(id, it.into());
+
+                it.into()
+            }
         };
 
         let ty = self.db.value_ty(typable);
diff --git a/crates/ra_hir_ty/src/lib.rs b/crates/ra_hir_ty/src/lib.rs
index 717399b6d..18f74d3b1 100644
--- a/crates/ra_hir_ty/src/lib.rs
+++ b/crates/ra_hir_ty/src/lib.rs
@@ -43,6 +43,7 @@ mod tests;
 #[cfg(test)]
 mod test_db;
 mod marks;
+mod _match;
 
 use std::ops::Deref;
 use std::sync::Arc;
diff --git a/crates/ra_hir_ty/src/test_db.rs b/crates/ra_hir_ty/src/test_db.rs
index 208096aab..3a4d58bf9 100644
--- a/crates/ra_hir_ty/src/test_db.rs
+++ b/crates/ra_hir_ty/src/test_db.rs
@@ -105,8 +105,9 @@ impl TestDB {
     }
 
     // FIXME: don't duplicate this
-    pub fn diagnostics(&self) -> String {
+    pub fn diagnostics(&self) -> (String, u32) {
         let mut buf = String::new();
+        let mut count = 0;
         let crate_graph = self.crate_graph();
         for krate in crate_graph.iter() {
             let crate_def_map = self.crate_def_map(krate);
@@ -133,13 +134,14 @@ impl TestDB {
                 let infer = self.infer(f.into());
                 let mut sink = DiagnosticSink::new(|d| {
                     format_to!(buf, "{:?}: {}\n", d.syntax_node(self).text(), d.message());
+                    count += 1;
                 });
                 infer.add_diagnostics(self, f, &mut sink);
                 let mut validator = ExprValidator::new(f, infer, &mut sink);
                 validator.validate_body(self);
             }
         }
-        buf
+        (buf, count)
     }
 }
 
diff --git a/crates/ra_hir_ty/src/tests.rs b/crates/ra_hir_ty/src/tests.rs
index 027e5a8f8..e4a103d1b 100644
--- a/crates/ra_hir_ty/src/tests.rs
+++ b/crates/ra_hir_ty/src/tests.rs
@@ -309,7 +309,8 @@ fn no_such_field_diagnostics() {
         }
         ",
     )
-    .diagnostics();
+    .diagnostics()
+    .0;
 
     assert_snapshot!(diagnostics, @r###"
     "baz: 62": no such field