Improve error reporting for closure return type mismatches

Author: FabianWolff

Diff for: compiler/rustc_typeck/src/check/check.rs

@@ -78,6 +78,7 @@ pub(super) fn check_fn<'a, 'tcx>(
     fn_id: hir::HirId,
     body: &'tcx hir::Body<'tcx>,
     can_be_generator: Option<hir::Movability>,
+    return_type_pre_known: bool,
 ) -> (FnCtxt<'a, 'tcx>, Option<GeneratorTypes<'tcx>>) {
     let mut fn_sig = fn_sig;
 
@@ -87,6 +88,7 @@ pub(super) fn check_fn<'a, 'tcx>(
     // in the case of closures, based on the outer context.
     let mut fcx = FnCtxt::new(inherited, param_env, body.value.hir_id);
     fcx.ps.set(UnsafetyState::function(fn_sig.unsafety, fn_id));
+    fcx.return_type_pre_known = return_type_pre_known;
 
     let tcx = fcx.tcx;
     let sess = tcx.sess;

Diff for: compiler/rustc_typeck/src/check/closure.rs

@@ -73,8 +73,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         debug!("check_closure: ty_of_closure returns {:?}", liberated_sig);
 
-        let generator_types =
-            check_fn(self, self.param_env, liberated_sig, decl, expr.hir_id, body, gen).1;
+        let return_type_pre_known = !liberated_sig.output().is_ty_infer();
+
+        let generator_types = check_fn(
+            self,
+            self.param_env,
+            liberated_sig,
+            decl,
+            expr.hir_id,
+            body,
+            gen,
+            return_type_pre_known,
+        )
+        .1;
 
         let parent_substs = InternalSubsts::identity_for_item(
             self.tcx,

Diff for: compiler/rustc_typeck/src/check/demand.rs

@@ -40,7 +40,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.suggest_missing_parentheses(err, expr);
         self.note_need_for_fn_pointer(err, expected, expr_ty);
         self.note_internal_mutation_in_method(err, expr, expected, expr_ty);
-        self.report_closure_infered_return_type(err, expected)
+        self.report_closure_inferred_return_type(err, expected);
     }
 
     // Requires that the two types unify, and prints an error message if
@@ -1106,29 +1106,26 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 
     // Report the type inferred by the return statement.
-    fn report_closure_infered_return_type(
+    fn report_closure_inferred_return_type(
         &self,
         err: &mut DiagnosticBuilder<'_>,
         expected: Ty<'tcx>,
     ) {
         if let Some(sp) = self.ret_coercion_span.get() {
-            // If the closure has an explicit return type annotation,
-            // then a type error may occur at the first return expression we
-            // see in the closure (if it conflicts with the declared
-            // return type). Skip adding a note in this case, since it
-            // would be incorrect.
-            if !err.span.primary_spans().iter().any(|&span| span == sp) {
-                let hir = self.tcx.hir();
-                let body_owner = hir.body_owned_by(hir.enclosing_body_owner(self.body_id));
-                if self.tcx.is_closure(hir.body_owner_def_id(body_owner).to_def_id()) {
-                    err.span_note(
-                        sp,
-                        &format!(
-                            "return type inferred to be `{}` here",
-                            self.resolve_vars_if_possible(expected)
-                        ),
-                    );
-                }
+            // If the closure has an explicit return type annotation, or if
+            // the closure's return type has been inferred from outside
+            // requirements (such as an Fn* trait bound), then a type error
+            // may occur at the first return expression we see in the closure
+            // (if it conflicts with the declared return type). Skip adding a
+            // note in this case, since it would be incorrect.
+            if !self.return_type_pre_known {
+                err.span_note(
+                    sp,
+                    &format!(
+                        "return type inferred to be `{}` here",
+                        self.resolve_vars_if_possible(expected)
+                    ),
+                );
             }
         }
     }

Diff for: compiler/rustc_typeck/src/check/fn_ctxt/mod.rs

@@ -111,6 +111,12 @@ pub struct FnCtxt<'a, 'tcx> {
     pub(super) enclosing_breakables: RefCell<EnclosingBreakables<'tcx>>,
 
     pub(super) inh: &'a Inherited<'a, 'tcx>,
+
+    /// True if the function or closure's return type is known before
+    /// entering the function/closure, i.e. if the return type is
+    /// either given explicitly or inferred from, say, an `Fn*` trait
+    /// bound. Used for diagnostic purposes only.
+    pub(super) return_type_pre_known: bool,
 }
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -137,6 +143,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 by_id: Default::default(),
             }),
             inh,
+            return_type_pre_known: true,
         }
     }
 

Diff for: compiler/rustc_typeck/src/check/mod.rs

@@ -391,7 +391,7 @@ fn typeck_with_fallback<'tcx>(
                 fn_sig,
             );
 
-            let fcx = check_fn(&inh, param_env, fn_sig, decl, id, body, None).0;
+            let fcx = check_fn(&inh, param_env, fn_sig, decl, id, body, None, true).0;
             fcx
         } else {
             let fcx = FnCtxt::new(&inh, param_env, body.value.hir_id);

Diff for: src/test/ui/closures/issue-87461.rs

@@ -0,0 +1,29 @@
+// Regression test for #87461.
+
+// edition:2021
+
+async fn func() -> Result<u16, u64> {
+    let _ = async {
+        Err(42u64)
+    }.await?;
+
+    Ok(())
+    //~^ ERROR: mismatched types [E0308]
+}
+
+async fn func2() -> Result<u16, u64> {
+    Err(42u64)?;
+
+    Ok(())
+    //~^ ERROR: mismatched types [E0308]
+}
+
+fn main() {
+    || -> Result<u16, u64> {
+        if true {
+            return Err(42u64);
+        }
+        Ok(())
+        //~^ ERROR: mismatched types [E0308]
+    };
+}

Diff for: src/test/ui/closures/issue-87461.stderr

@@ -0,0 +1,21 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-87461.rs:10:8
+   |
+LL |     Ok(())
+   |        ^^ expected `u16`, found `()`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-87461.rs:17:8
+   |
+LL |     Ok(())
+   |        ^^ expected `u16`, found `()`
+
+error[E0308]: mismatched types
+  --> $DIR/issue-87461.rs:26:12
+   |
+LL |         Ok(())
+   |            ^^ expected `u16`, found `()`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.