Suppress malformed closure return type suggestions

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -799,13 +799,24 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         can_suggest: bool,
         fn_id: LocalDefId,
     ) -> bool {
-        // Can't suggest `->` on a block-like coroutine
+        // Can't suggest `->` on a block-like coroutine.
         if let Some(hir::CoroutineKind::Desugared(_, hir::CoroutineSource::Block)) =
             self.tcx.coroutine_kind(fn_id)
         {
             return false;
         }
 
+        // FIXME: Add braces to a closure when suggesting a return type.
+        // A case where this may happen is `|| if { return x; }`, though we
+        // should first think hard about whether these suggestions are useful.
+        if let hir::Node::Expr(hir::Expr {
+            kind: ExprKind::Closure(hir::Closure { body, .. }), ..
+        }) = self.tcx.hir_node_by_def_id(fn_id)
+            && !matches!(self.tcx.hir().body(*body).value.kind, hir::ExprKind::Block(..))
+        {
+            return false;
+        }
+
         let found =
             self.resolve_numeric_literals_with_default(self.resolve_vars_if_possible(found));
         // Only suggest changing the return type for methods that

tests/ui/typeck/issue-81943.stderr

@@ -2,32 +2,36 @@ error[E0308]: mismatched types
   --> $DIR/issue-81943.rs:7:9
    |
 LL |   f(|x| lib::d!(x));
-   |        -^^^^^^^^^^ expected `()`, found `i32`
-   |        |
-   |        help: try adding a return type: `-> i32`
+   |         ^^^^^^^^^^ expected `()`, found `i32`
    |
    = note: this error originates in the macro `lib::d` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0308]: mismatched types
   --> $DIR/issue-81943.rs:8:28
    |
 LL |   f(|x| match x { tmp => { g(tmp) } });
-   |                            ^^^^^^ expected `()`, found `i32`
+   |         -------------------^^^^^^----
+   |         |                  |
+   |         |                  expected `()`, found `i32`
+   |         expected this to be `()`
    |
 help: consider using a semicolon here
    |
 LL |   f(|x| match x { tmp => { g(tmp); } });
    |                                  +
-help: try adding a return type
+help: consider using a semicolon here
    |
-LL |   f(|x| -> i32 match x { tmp => { g(tmp) } });
-   |         ++++++
+LL |   f(|x| match x { tmp => { g(tmp) } };);
+   |                                      +
 
 error[E0308]: mismatched types
   --> $DIR/issue-81943.rs:10:38
    |
 LL |     ($e:expr) => { match $e { x => { g(x) } } }
-   |                                      ^^^^ expected `()`, found `i32`
+   |                    ------------------^^^^----
+   |                    |                 |
+   |                    |                 expected `()`, found `i32`
+   |                    expected this to be `()`
 LL |   }
 LL |   f(|x| d!(x));
    |         ----- in this macro invocation
@@ -37,10 +41,10 @@ help: consider using a semicolon here
    |
 LL |     ($e:expr) => { match $e { x => { g(x); } } }
    |                                          +
-help: try adding a return type
+help: consider using a semicolon here
    |
-LL |   f(|x| -> i32 d!(x));
-   |         ++++++
+LL |     ($e:expr) => { match $e { x => { g(x) } }; }
+   |                                              +
 
 error: aborting due to 3 previous errors