Handle closure return types

Fixes #2547.

crates/ra_hir_def/src/body/lower.rs

@@ -372,8 +372,9 @@ where
                         arg_types.push(type_ref);
                     }
                 }
+                let ret_type = e.ret_type().and_then(|r| r.type_ref()).map(TypeRef::from_ast);
                 let body = self.collect_expr_opt(e.body());
-                self.alloc_expr(Expr::Lambda { args, arg_types, body }, syntax_ptr)
+                self.alloc_expr(Expr::Lambda { args, arg_types, ret_type, body }, syntax_ptr)
             }
             ast::Expr::BinExpr(e) => {
                 let lhs = self.collect_expr_opt(e.lhs());

crates/ra_hir_def/src/expr.rs

@@ -143,6 +143,7 @@ pub enum Expr {
     Lambda {
         args: Vec<PatId>,
         arg_types: Vec<Option<TypeRef>>,
+        ret_type: Option<TypeRef>,
         body: ExprId,
     },
     Tuple {

crates/ra_hir_ty/src/infer.rs

@@ -196,7 +196,12 @@ struct InferenceContext<'a, D: HirDatabase> {
     trait_env: Arc<TraitEnvironment>,
     obligations: Vec<Obligation>,
     result: InferenceResult,
-    /// The return type of the function being inferred.
+    /// 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,
 
     /// Impls of `CoerceUnsized` used in coercion.

crates/ra_hir_ty/src/infer/expr.rs

@@ -102,7 +102,7 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 self.infer_expr(*body, &Expectation::has_type(Ty::unit()));
                 Ty::unit()
             }
-            Expr::Lambda { body, args, arg_types } => {
+            Expr::Lambda { body, args, ret_type, arg_types } => {
                 assert_eq!(args.len(), arg_types.len());
 
                 let mut sig_tys = Vec::new();
@@ -118,7 +118,10 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 }
 
                 // add return type
-                let ret_ty = self.table.new_type_var();
+                let ret_ty = match ret_type {
+                    Some(type_ref) => self.make_ty(type_ref),
+                    None => self.table.new_type_var(),
+                };
                 sig_tys.push(ret_ty.clone());
                 let sig_ty = Ty::apply(
                     TypeCtor::FnPtr { num_args: sig_tys.len() as u16 - 1 },
@@ -134,7 +137,12 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 // infer the body.
                 self.coerce(&closure_ty, &expected.ty);
 
-                self.infer_expr(*body, &Expectation::has_type(ret_ty));
+                let prev_ret_ty = std::mem::replace(&mut self.return_ty, ret_ty.clone());
+
+                self.infer_expr_coerce(*body, &Expectation::has_type(ret_ty));
+
+                self.return_ty = prev_ret_ty;
+
                 closure_ty
             }
             Expr::Call { callee, args } => {
@@ -192,6 +200,9 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
             Expr::Return { expr } => {
                 if let Some(expr) = expr {
                     self.infer_expr_coerce(*expr, &Expectation::has_type(self.return_ty.clone()));
+                } else {
+                    let unit = Ty::unit();
+                    self.coerce(&unit, &self.return_ty.clone());
                 }
                 Ty::simple(TypeCtor::Never)
             }

crates/ra_hir_ty/src/tests/coercion.rs

@@ -440,3 +440,34 @@ fn test() {
     "###
     );
 }
+
+#[test]
+fn closure_return_coerce() {
+    assert_snapshot!(
+        infer_with_mismatches(r#"
+fn foo() {
+    let x = || {
+        if true {
+            return &1u32;
+        }
+        &&1u32
+    };
+}
+"#, true),
+        @r###"
+    [10; 106) '{     ...  }; }': ()
+    [20; 21) 'x': || -> &u32
+    [24; 103) '|| {  ...     }': || -> &u32
+    [27; 103) '{     ...     }': &u32
+    [37; 82) 'if tru...     }': ()
+    [40; 44) 'true': bool
+    [45; 82) '{     ...     }': !
+    [59; 71) 'return &1u32': !
+    [66; 71) '&1u32': &u32
+    [67; 71) '1u32': u32
+    [91; 97) '&&1u32': &&u32
+    [92; 97) '&1u32': &u32
+    [93; 97) '1u32': u32
+    "###
+    );
+}

crates/ra_hir_ty/src/tests/simple.rs

@@ -1606,3 +1606,58 @@ fn main() {
     );
     assert_eq!(t, "u32");
 }
+
+#[test]
+fn closure_return() {
+    assert_snapshot!(
+        infer(r#"
+fn foo() -> u32 {
+    let x = || -> usize { return 1; };
+}
+"#),
+        @r###"
+    [17; 59) '{     ...; }; }': ()
+    [27; 28) 'x': || -> usize
+    [31; 56) '|| -> ...n 1; }': || -> usize
+    [43; 56) '{ return 1; }': !
+    [45; 53) 'return 1': !
+    [52; 53) '1': usize
+    "###
+    );
+}
+
+#[test]
+fn closure_return_unit() {
+    assert_snapshot!(
+        infer(r#"
+fn foo() -> u32 {
+    let x = || { return; };
+}
+"#),
+        @r###"
+    [17; 48) '{     ...; }; }': ()
+    [27; 28) 'x': || -> ()
+    [31; 45) '|| { return; }': || -> ()
+    [34; 45) '{ return; }': !
+    [36; 42) 'return': !
+    "###
+    );
+}
+
+#[test]
+fn closure_return_inferred() {
+    assert_snapshot!(
+        infer(r#"
+fn foo() -> u32 {
+    let x = || { "test" };
+}
+"#),
+        @r###"
+    [17; 47) '{     ..." }; }': ()
+    [27; 28) 'x': || -> &str
+    [31; 44) '|| { "test" }': || -> &str
+    [34; 44) '{ "test" }': &str
+    [36; 42) '"test"': &str
+    "###
+    );
+}

crates/ra_syntax/src/ast/generated.rs

@@ -1426,6 +1426,9 @@ impl LambdaExpr {
     pub fn param_list(&self) -> Option<ParamList> {
         AstChildren::new(&self.syntax).next()
     }
+    pub fn ret_type(&self) -> Option<RetType> {
+        AstChildren::new(&self.syntax).next()
+    }
     pub fn body(&self) -> Option<Expr> {
         AstChildren::new(&self.syntax).next()
     }

crates/ra_syntax/src/grammar.ron

@@ -426,7 +426,7 @@ Grammar(
         "PathExpr": (options: ["Path"]),
         "LambdaExpr": (
             options: [
-                "ParamList",
+                "ParamList", "RetType",
                 ["body", "Expr"],
             ]
         ),