9 files changed, 206 insertions, 33 deletions
diff --git a/crates/hir_ty/src/diagnostics.rs b/crates/hir_ty/src/diagnostics.rs
index 247da43f2..323c5f963 100644
--- a/crates/hir_ty/src/diagnostics.rs
+++ b/crates/hir_ty/src/diagnostics.rs
@@ -247,7 +247,7 @@ impl Diagnostic for RemoveThisSemicolon {
 // Diagnostic: break-outside-of-loop
 //
-// This diagnostic is triggered if `break` keyword is used outside of a loop.
+// This diagnostic is triggered if the `break` keyword is used outside of a loop.
 #[derive(Debug)]
 pub struct BreakOutsideOfLoop {
    pub file: HirFileId,
@@ -271,7 +271,7 @@ impl Diagnostic for BreakOutsideOfLoop {
 // Diagnostic: missing-unsafe
 //
-// This diagnostic is triggered if operation marked as `unsafe` is used outside of `unsafe` function or block.
+// This diagnostic is triggered if an operation marked as `unsafe` is used outside of an `unsafe` function or block.
 #[derive(Debug)]
 pub struct MissingUnsafe {
    pub file: HirFileId,
@@ -295,7 +295,7 @@ impl Diagnostic for MissingUnsafe {
 // Diagnostic: mismatched-arg-count
 //
-// This diagnostic is triggered if function is invoked with an incorrect amount of arguments.
+// This diagnostic is triggered if a function is invoked with an incorrect amount of arguments.
 #[derive(Debug)]
 pub struct MismatchedArgCount {
    pub file: HirFileId,
@@ -347,7 +347,7 @@ impl fmt::Display for CaseType {
 // Diagnostic: incorrect-ident-case
 //
-// This diagnostic is triggered if item name doesn't follow https://doc.rust-lang.org/1.0.0/style/style/naming/README.html[Rust naming convention].
+// This diagnostic is triggered if an item name doesn't follow https://doc.rust-lang.org/1.0.0/style/style/naming/README.html[Rust naming convention].
 #[derive(Debug)]
 pub struct IncorrectCase {
    pub file: HirFileId,
@@ -386,6 +386,31 @@ impl Diagnostic for IncorrectCase {
    }
 }
+// Diagnostic: replace-filter-map-next-with-find-map
+//
+// This diagnostic is triggered when `.filter_map(..).next()` is used, rather than the more concise `.find_map(..)`.
+#[derive(Debug)]
+pub struct ReplaceFilterMapNextWithFindMap {
+    pub file: HirFileId,
+    /// This expression is the whole method chain up to and including `.filter_map(..).next()`.
+    pub next_expr: AstPtr<ast::Expr>,
+}
+impl Diagnostic for ReplaceFilterMapNextWithFindMap {
+    fn code(&self) -> DiagnosticCode {
+        DiagnosticCode("replace-filter-map-next-with-find-map")
+    }
+    fn message(&self) -> String {
+        "replace filter_map(..).next() with find_map(..)".to_string()
+    }
+    fn display_source(&self) -> InFile<SyntaxNodePtr> {
+        InFile { file_id: self.file, value: self.next_expr.clone().into() }
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}
 #[cfg(test)]
 mod tests {
    use base_db::{fixture::WithFixture, FileId, SourceDatabase, SourceDatabaseExt};
@@ -644,4 +669,87 @@ fn foo() { break; }
            "#,
        );
    }
+    // Register the required standard library types to make the tests work
+    fn add_filter_map_with_find_next_boilerplate(body: &str) -> String {
+        let prefix = r#"
+        //- /main.rs crate:main deps:core
+        use core::iter::Iterator;
+        use core::option::Option::{self, Some, None};
+        "#;
+        let suffix = r#"
+        //- /core/lib.rs crate:core
+        pub mod option {
+            pub enum Option<T> { Some(T), None }
+        }
+        pub mod iter {
+            pub trait Iterator {
+                type Item;
+                fn filter_map<B, F>(self, f: F) -> FilterMap where F: FnMut(Self::Item) -> Option<B> { FilterMap }
+                fn next(&mut self) -> Option<Self::Item>;
+            }
+            pub struct FilterMap {}
+            impl Iterator for FilterMap {
+                type Item = i32;
+                fn next(&mut self) -> i32 { 7 }
+            }
+        }
+        "#;
+        format!("{}{}{}", prefix, body, suffix)
+    }
+    #[test]
+    fn replace_filter_map_next_with_find_map2() {
+        check_diagnostics(&add_filter_map_with_find_next_boilerplate(
+            r#"
+            fn foo() {
+                let m = [1, 2, 3].iter().filter_map(|x| if *x == 2 { Some (4) } else { None }).next();
+                      //^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ replace filter_map(..).next() with find_map(..)
+            }
+        "#,
+        ));
+    }
+    #[test]
+    fn replace_filter_map_next_with_find_map_no_diagnostic_without_next() {
+        check_diagnostics(&add_filter_map_with_find_next_boilerplate(
+            r#"
+            fn foo() {
+                let m = [1, 2, 3]
+                    .iter()
+                    .filter_map(|x| if *x == 2 { Some (4) } else { None })
+                    .len();
+            }
+            "#,
+        ));
+    }
+    #[test]
+    fn replace_filter_map_next_with_find_map_no_diagnostic_with_intervening_methods() {
+        check_diagnostics(&add_filter_map_with_find_next_boilerplate(
+            r#"
+            fn foo() {
+                let m = [1, 2, 3]
+                    .iter()
+                    .filter_map(|x| if *x == 2 { Some (4) } else { None })
+                    .map(|x| x + 2)
+                    .len();
+            }
+            "#,
+        ));
+    }
+    #[test]
+    fn replace_filter_map_next_with_find_map_no_diagnostic_if_not_in_chain() {
+        check_diagnostics(&add_filter_map_with_find_next_boilerplate(
+            r#"
+            fn foo() {
+                let m = [1, 2, 3]
+                    .iter()
+                    .filter_map(|x| if *x == 2 { Some (4) } else { None });
+                let n = m.next();
+            }
+            "#,
+        ));
+    }
 }
diff --git a/crates/hir_ty/src/diagnostics/expr.rs b/crates/hir_ty/src/diagnostics/expr.rs
index 107417c27..d740b7265 100644
--- a/crates/hir_ty/src/diagnostics/expr.rs
+++ b/crates/hir_ty/src/diagnostics/expr.rs
@@ -2,8 +2,10 @@
 use std::sync::Arc;
-use hir_def::{expr::Statement, path::path, resolver::HasResolver, AdtId, DefWithBodyId};
+use hir_def::{
-use hir_expand::diagnostics::DiagnosticSink;
+    expr::Statement, path::path, resolver::HasResolver, AdtId, AssocItemId, DefWithBodyId,
+};
+use hir_expand::{diagnostics::DiagnosticSink, name};
 use rustc_hash::FxHashSet;
 use syntax::{ast, AstPtr};
@@ -24,6 +26,8 @@ pub(crate) use hir_def::{
    LocalFieldId, VariantId,
 };
+use super::ReplaceFilterMapNextWithFindMap;
 pub(super) struct ExprValidator<'a, 'b: 'a> {
    owner: DefWithBodyId,
    infer: Arc<InferenceResult>,
@@ -40,6 +44,8 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
    }
    pub(super) fn validate_body(&mut self, db: &dyn HirDatabase) {
+        self.check_for_filter_map_next(db);
        let body = db.body(self.owner.into());
        for (id, expr) in body.exprs.iter() {
@@ -150,20 +156,76 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
        }
    }
-    fn validate_call(&mut self, db: &dyn HirDatabase, call_id: ExprId, expr: &Expr) -> Option<()> {
+    fn check_for_filter_map_next(&mut self, db: &dyn HirDatabase) {
+        // Find the FunctionIds for Iterator::filter_map and Iterator::next
+        let iterator_path = path![core::iter::Iterator];
+        let resolver = self.owner.resolver(db.upcast());
+        let iterator_trait_id = match resolver.resolve_known_trait(db.upcast(), &iterator_path) {
+            Some(id) => id,
+            None => return,
+        };
+        let iterator_trait_items = &db.trait_data(iterator_trait_id).items;
+        let filter_map_function_id =
+            match iterator_trait_items.iter().find(|item| item.0 == name![filter_map]) {
+                Some((_, AssocItemId::FunctionId(id))) => id,
+                _ => return,
+            };
+        let next_function_id = match iterator_trait_items.iter().find(|item| item.0 == name![next])
+        {
+            Some((_, AssocItemId::FunctionId(id))) => id,
+            _ => return,
+        };
+        // Search function body for instances of .filter_map(..).next()
+        let body = db.body(self.owner.into());
+        let mut prev = None;
+        for (id, expr) in body.exprs.iter() {
+            if let Expr::MethodCall { receiver, .. } = expr {
+                let function_id = match self.infer.method_resolution(id) {
+                    Some(id) => id,
+                    None => continue,
+                };
+                if function_id == *filter_map_function_id {
+                    prev = Some(id);
+                    continue;
+                }
+                if function_id == *next_function_id {
+                    if let Some(filter_map_id) = prev {
+                        if *receiver == filter_map_id {
+                            let (_, source_map) = db.body_with_source_map(self.owner.into());
+                            if let Ok(next_source_ptr) = source_map.expr_syntax(id) {
+                                self.sink.push(ReplaceFilterMapNextWithFindMap {
+                                    file: next_source_ptr.file_id,
+                                    next_expr: next_source_ptr.value,
+                                });
+                            }
+                        }
+                    }
+                }
+            }
+            prev = None;
+        }
+    }
+    fn validate_call(&mut self, db: &dyn HirDatabase, call_id: ExprId, expr: &Expr) {
        // 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 None;
+            return;
        }
        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)?;
+                let sig = match callee.callable_sig(db) {
+                    Some(sig) => sig,
+                    None => return,
+                };
                (sig, args.clone())
            }
            Expr::MethodCall { receiver, args, .. } => {
@@ -175,22 +237,25 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
                    // if the receiver is of unknown type, it's very likely we
                    // don't know enough to correctly resolve the method call.
                    // This is kind of a band-aid for #6975.
-                    return None;
+                    return;
                }
                // 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 callee = match self.infer.method_resolution(call_id) {
+                    Some(callee) => callee,
+                    None => return,
+                };
                let sig = db.callable_item_signature(callee.into()).value;
                (sig, args)
            }
-            _ => return None,
+            _ => return,
        };
        if sig.is_varargs {
-            return None;
+            return;
        }
        let params = sig.params();
@@ -213,8 +278,6 @@ impl<'a, 'b> ExprValidator<'a, 'b> {
                });
            }
        }
-        None
    }
    fn validate_match(
diff --git a/crates/hir_ty/src/display.rs b/crates/hir_ty/src/display.rs
index d2f1b4014..38a043c48 100644
--- a/crates/hir_ty/src/display.rs
+++ b/crates/hir_ty/src/display.rs
@@ -617,7 +617,7 @@ impl HirDisplay for FnSig {
 }
 fn fn_traits(db: &dyn DefDatabase, trait_: TraitId) -> impl Iterator<Item = TraitId> {
-    let krate = trait_.lookup(db).container.module(db).krate;
+    let krate = trait_.lookup(db).container.module(db).krate();
    let fn_traits = [
        db.lang_item(krate, "fn".into()),
        db.lang_item(krate, "fn_mut".into()),
diff --git a/crates/hir_ty/src/lib.rs b/crates/hir_ty/src/lib.rs
index e00c7e176..d47f975d2 100644
--- a/crates/hir_ty/src/lib.rs
+++ b/crates/hir_ty/src/lib.rs
@@ -211,19 +211,21 @@ impl TypeCtor {
            | TypeCtor::Tuple { .. } => None,
            // Closure's krate is irrelevant for coherence I would think?
            TypeCtor::Closure { .. } => None,
-            TypeCtor::Adt(adt) => Some(adt.module(db.upcast()).krate),
+            TypeCtor::Adt(adt) => Some(adt.module(db.upcast()).krate()),
            TypeCtor::FnDef(callable) => Some(callable.krate(db)),
            TypeCtor::AssociatedType(type_alias) => {
-                Some(type_alias.lookup(db.upcast()).module(db.upcast()).krate)
+                Some(type_alias.lookup(db.upcast()).module(db.upcast()).krate())
            }
            TypeCtor::ForeignType(type_alias) => {
-                Some(type_alias.lookup(db.upcast()).module(db.upcast()).krate)
+                Some(type_alias.lookup(db.upcast()).module(db.upcast()).krate())
            }
            TypeCtor::OpaqueType(opaque_ty_id) => match opaque_ty_id {
                OpaqueTyId::ReturnTypeImplTrait(func, _) => {
-                    Some(func.lookup(db.upcast()).module(db.upcast()).krate)
+                    Some(func.lookup(db.upcast()).module(db.upcast()).krate())
+                }
+                OpaqueTyId::AsyncBlockTypeImplTrait(def, _) => {
+                    Some(def.module(db.upcast()).krate())
                }
-                OpaqueTyId::AsyncBlockTypeImplTrait(def, _) => Some(def.module(db.upcast()).krate),
            },
        }
    }
@@ -870,7 +872,7 @@ impl Ty {
            Ty::Apply(ApplicationTy { ctor: TypeCtor::OpaqueType(opaque_ty_id), .. }) => {
                match opaque_ty_id {
                    OpaqueTyId::AsyncBlockTypeImplTrait(def, _expr) => {
-                        let krate = def.module(db.upcast()).krate;
+                        let krate = def.module(db.upcast()).krate();
                        if let Some(future_trait) = db
                            .lang_item(krate, "future_trait".into())
                            .and_then(|item| item.as_trait())
diff --git a/crates/hir_ty/src/lower.rs b/crates/hir_ty/src/lower.rs
index 7a734c8b9..dfb573ff3 100644
--- a/crates/hir_ty/src/lower.rs
+++ b/crates/hir_ty/src/lower.rs
@@ -1147,7 +1147,7 @@ impl CallableDefId {
            CallableDefId::StructId(s) => s.lookup(db).container.module(db),
            CallableDefId::EnumVariantId(e) => e.parent.lookup(db).container.module(db),
        }
-        .krate
+        .krate()
    }
 }
diff --git a/crates/hir_ty/src/method_resolution.rs b/crates/hir_ty/src/method_resolution.rs
index f06aeeb42..a302456b0 100644
--- a/crates/hir_ty/src/method_resolution.rs
+++ b/crates/hir_ty/src/method_resolution.rs
@@ -248,12 +248,12 @@ impl Ty {
        let lang_item_targets = match self {
            Ty::Apply(a_ty) => match a_ty.ctor {
                TypeCtor::Adt(def_id) => {
-                    return Some(std::iter::once(def_id.module(db.upcast()).krate).collect())
+                    return Some(std::iter::once(def_id.module(db.upcast()).krate()).collect())
                }
                TypeCtor::ForeignType(type_alias_id) => {
                    return Some(
                        std::iter::once(
-                            type_alias_id.lookup(db.upcast()).module(db.upcast()).krate,
+                            type_alias_id.lookup(db.upcast()).module(db.upcast()).krate(),
                        )
                        .collect(),
                    )
@@ -280,7 +280,7 @@ impl Ty {
                LangItemTarget::ImplDefId(it) => Some(it),
                _ => None,
            })
-            .map(|it| it.lookup(db.upcast()).container.module(db.upcast()).krate)
+            .map(|it| it.lookup(db.upcast()).container.module(db.upcast()).krate())
            .collect();
        Some(res)
    }
diff --git a/crates/hir_ty/src/test_db.rs b/crates/hir_ty/src/test_db.rs
index 3bbcbc242..381b98ba8 100644
--- a/crates/hir_ty/src/test_db.rs
+++ b/crates/hir_ty/src/test_db.rs
@@ -83,7 +83,7 @@ impl TestDB {
            let crate_def_map = self.crate_def_map(krate);
            for (local_id, data) in crate_def_map.modules() {
                if data.origin.file_id() == Some(file_id) {
-                    return ModuleId { krate, local_id };
+                    return crate_def_map.module_id(local_id);
                }
            }
        }
diff --git a/crates/hir_ty/src/tests.rs b/crates/hir_ty/src/tests.rs
index 4a3fcea8d..7386a4e7b 100644
--- a/crates/hir_ty/src/tests.rs
+++ b/crates/hir_ty/src/tests.rs
@@ -188,10 +188,10 @@ fn infer_with_mismatches(content: &str, include_mismatches: bool) -> String {
    };
    let module = db.module_for_file(file_id);
-    let crate_def_map = db.crate_def_map(module.krate);
+    let def_map = module.def_map(&db);
    let mut defs: Vec<DefWithBodyId> = Vec::new();
-    visit_module(&db, &crate_def_map, module.local_id, &mut |it| defs.push(it));
+    visit_module(&db, &def_map, module.local_id, &mut |it| defs.push(it));
    defs.sort_by_key(|def| match def {
        DefWithBodyId::FunctionId(it) => {
            let loc = it.lookup(&db);
@@ -321,7 +321,7 @@ fn typing_whitespace_inside_a_function_should_not_invalidate_types() {
    {
        let events = db.log_executed(|| {
            let module = db.module_for_file(pos.file_id);
-            let crate_def_map = db.crate_def_map(module.krate);
+            let crate_def_map = module.def_map(&db);
            visit_module(&db, &crate_def_map, module.local_id, &mut |def| {
                db.infer(def);
            });
@@ -343,7 +343,7 @@ fn typing_whitespace_inside_a_function_should_not_invalidate_types() {
    {
        let events = db.log_executed(|| {
            let module = db.module_for_file(pos.file_id);
-            let crate_def_map = db.crate_def_map(module.krate);
+            let crate_def_map = module.def_map(&db);
            visit_module(&db, &crate_def_map, module.local_id, &mut |def| {
                db.infer(def);
            });
diff --git a/crates/hir_ty/src/traits/chalk.rs b/crates/hir_ty/src/traits/chalk.rs
index 2196af677..cfb756158 100644
--- a/crates/hir_ty/src/traits/chalk.rs
+++ b/crates/hir_ty/src/traits/chalk.rs
@@ -426,7 +426,7 @@ pub(crate) fn trait_datum_query(
    let bound_vars = Substs::bound_vars(&generic_params, DebruijnIndex::INNERMOST);
    let flags = rust_ir::TraitFlags {
        auto: trait_data.auto,
-        upstream: trait_.lookup(db.upcast()).container.module(db.upcast()).krate != krate,
+        upstream: trait_.lookup(db.upcast()).container.module(db.upcast()).krate() != krate,
        non_enumerable: true,
        coinductive: false, // only relevant for Chalk testing
        // FIXME: set these flags correctly
@@ -549,7 +549,7 @@ fn impl_def_datum(
    let generic_params = generics(db.upcast(), impl_id.into());
    let bound_vars = Substs::bound_vars(&generic_params, DebruijnIndex::INNERMOST);
    let trait_ = trait_ref.trait_;
-    let impl_type = if impl_id.lookup(db.upcast()).container.module(db.upcast()).krate == krate {
+    let impl_type = if impl_id.lookup(db.upcast()).container.module(db.upcast()).krate() == krate {
        rust_ir::ImplType::Local
    } else {
        rust_ir::ImplType::External