When encountering unexpected closure return type, point at return type/expression

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -653,6 +653,7 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
                 }))),
                 terr,
                 false,
+                None,
             );
             return Err(diag.emit());
         }
@@ -1070,6 +1071,7 @@ fn report_trait_method_mismatch<'tcx>(
         }))),
         terr,
         false,
+        None,
     );
 
     diag.emit()
@@ -1862,6 +1864,7 @@ fn compare_const_predicate_entailment<'tcx>(
             }))),
             terr,
             false,
+            None,
         );
         return Err(diag.emit());
     };

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -649,6 +649,7 @@ pub fn check_function_signature<'tcx>(
                 }))),
                 err,
                 false,
+                None,
             );
             return Err(diag.emit());
         }

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1147,6 +1147,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                 Some(self.param_env.and(trace.values)),
                                 e,
                                 true,
+                                None,
                             );
                         }
                     }

compiler/rustc_passes/src/check_attr.rs

@@ -2471,6 +2471,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                 }))),
                 terr,
                 false,
+                None,
             );
             diag.emit();
             self.abort.set(true);

compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs

@@ -1392,9 +1392,13 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         mut values: Option<ty::ParamEnvAnd<'tcx, ValuePairs<'tcx>>>,
         terr: TypeError<'tcx>,
         prefer_label: bool,
+        override_span: Option<Span>,
     ) {
-        let span = cause.span;
-
+        // We use `override_span` when we want the error to point at a `Span` other than
+        // `cause.span`. This is used in E0271, when a closure is passed in where the return type
+        // isn't what was expected. We want to point at the closure's return type (or expression),
+        // instead of the expression where the closure is passed as call argument.
+        let span = override_span.unwrap_or(cause.span);
         // For some types of errors, expected-found does not make
         // sense, so just ignore the values we were given.
         if let TypeError::CyclicTy(_) = terr {
@@ -2059,6 +2063,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             Some(param_env.and(trace.values)),
             terr,
             false,
+            None,
         );
         diag
     }

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -708,6 +708,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                     None,
                     TypeError::Sorts(ty::error::ExpectedFound::new(expected_ty, ct_ty)),
                     false,
+                    None,
                 );
                 diag
             }
@@ -931,14 +932,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             }
         }
         let hir_id = self.tcx.local_def_id_to_hir_id(obligation.cause.body_id);
-        let body_id = match self.tcx.hir_node(hir_id) {
-            hir::Node::Item(hir::Item {
-                kind: hir::ItemKind::Fn { body: body_id, .. }, ..
-            }) => body_id,
-            _ => return false,
-        };
-        let ControlFlow::Break(expr) = (FindMethodSubexprOfTry { search_span: span })
-            .visit_body(self.tcx.hir().body(*body_id))
+        let Some(body_id) = self.tcx.hir_node(hir_id).body_id() else { return false };
+        let ControlFlow::Break(expr) =
+            (FindMethodSubexprOfTry { search_span: span }).visit_body(self.tcx.hir().body(body_id))
         else {
             return false;
         };
@@ -1385,22 +1381,54 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 _ => (None, error.err),
             };
 
-            let msg = values
+            let (msg, span, closure_span) = values
                 .and_then(|(predicate, normalized_term, expected_term)| {
-                    self.maybe_detailed_projection_msg(predicate, normalized_term, expected_term)
+                    self.maybe_detailed_projection_msg(
+                        obligation.cause.span,
+                        predicate,
+                        normalized_term,
+                        expected_term,
+                    )
                 })
                 .unwrap_or_else(|| {
                     let mut cx = FmtPrinter::new_with_limit(
                         self.tcx,
                         Namespace::TypeNS,
                         rustc_session::Limit(10),
                     );
-                    with_forced_trimmed_paths!(format!("type mismatch resolving `{}`", {
-                        self.resolve_vars_if_possible(predicate).print(&mut cx).unwrap();
-                        cx.into_buffer()
-                    }))
+                    (
+                        with_forced_trimmed_paths!(format!("type mismatch resolving `{}`", {
+                            self.resolve_vars_if_possible(predicate).print(&mut cx).unwrap();
+                            cx.into_buffer()
+                        })),
+                        obligation.cause.span,
+                        None,
+                    )
                 });
-            let mut diag = struct_span_code_err!(self.dcx(), obligation.cause.span, E0271, "{msg}");
+            let mut diag = struct_span_code_err!(self.dcx(), span, E0271, "{msg}");
+            if let Some(span) = closure_span {
+                // Mark the closure decl so that it is seen even if we are pointing at the return
+                // type or expression.
+                //
+                // error[E0271]: expected `{[email protected]:41:16}` to be a closure that returns
+                //               `Unit3`, but it returns `Unit4`
+                //   --> $DIR/foo.rs:43:17
+                //    |
+                // LL |     let v = Unit2.m(
+                //    |                   - required by a bound introduced by this call
+                // ...
+                // LL |             f: |x| {
+                //    |                --- /* this span */
+                // LL |                 drop(x);
+                // LL |                 Unit4
+                //    |                 ^^^^^ expected `Unit3`, found `Unit4`
+                //    |
+                diag.span_label(span, "this closure");
+                if !span.overlaps(obligation.cause.span) {
+                    // Point at the binding corresponding to the closure where it is used.
+                    diag.span_label(obligation.cause.span, "closure used here");
+                }
+            }
 
             let secondary_span = (|| {
                 let ty::PredicateKind::Clause(ty::ClauseKind::Projection(proj)) =
@@ -1471,6 +1499,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 }),
                 err,
                 false,
+                Some(span),
             );
             self.note_obligation_cause(&mut diag, obligation);
             diag.emit()
@@ -1479,34 +1508,66 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
 
     fn maybe_detailed_projection_msg(
         &self,
+        mut span: Span,
         projection_term: ty::AliasTerm<'tcx>,
         normalized_ty: ty::Term<'tcx>,
         expected_ty: ty::Term<'tcx>,
-    ) -> Option<String> {
+    ) -> Option<(String, Span, Option<Span>)> {
         let trait_def_id = projection_term.trait_def_id(self.tcx);
         let self_ty = projection_term.self_ty();
 
         with_forced_trimmed_paths! {
             if self.tcx.is_lang_item(projection_term.def_id, LangItem::FnOnceOutput) {
                 let fn_kind = self_ty.prefix_string(self.tcx);
+                let (span, closure_span) = if let ty::Closure(def_id, _) = self_ty.kind() {
+                    let def_span = self.tcx.def_span(def_id);
+                    if let Some(local_def_id) = def_id.as_local()
+                        && let node = self.tcx.hir_node_by_def_id(local_def_id)
+                        && let Some(fn_decl) = node.fn_decl()
+                        && let Some(id) = node.body_id()
+                    {
+                        span = match fn_decl.output {
+                            hir::FnRetTy::Return(ty) => ty.span,
+                            hir::FnRetTy::DefaultReturn(_) => {
+                                let body = self.tcx.hir().body(id);
+                                match body.value.kind {
+                                    hir::ExprKind::Block(
+                                        hir::Block { expr: Some(expr), .. },
+                                        _,
+                                    ) => expr.span,
+                                    hir::ExprKind::Block(
+                                        hir::Block {
+                                            expr: None, stmts: [.., last], ..
+                                        },
+                                        _,
+                                    ) => last.span,
+                                    _ => body.value.span,
+                                }
+                            }
+                        };
+                    }
+                    (span, Some(def_span))
+                } else {
+                    (span, None)
+                };
                 let item = match self_ty.kind() {
                     ty::FnDef(def, _) => self.tcx.item_name(*def).to_string(),
                     _ => self_ty.to_string(),
                 };
-                Some(format!(
+                Some((format!(
                     "expected `{item}` to be a {fn_kind} that returns `{expected_ty}`, but it \
                      returns `{normalized_ty}`",
-                ))
+                ), span, closure_span))
             } else if self.tcx.is_lang_item(trait_def_id, LangItem::Future) {
-                Some(format!(
+                Some((format!(
                     "expected `{self_ty}` to be a future that resolves to `{expected_ty}`, but it \
                      resolves to `{normalized_ty}`"
-                ))
+                ), span, None))
             } else if Some(trait_def_id) == self.tcx.get_diagnostic_item(sym::Iterator) {
-                Some(format!(
+                Some((format!(
                     "expected `{self_ty}` to be an iterator that yields `{expected_ty}`, but it \
                      yields `{normalized_ty}`"
-                ))
+                ), span, None))
             } else {
                 None
             }

tests/ui/async-await/dont-ice-for-type-mismatch-in-closure-in-async.stderr

@@ -21,18 +21,14 @@ LL |         true
               found type `bool`
 
 error[E0271]: expected `{[email protected]:6:10}` to be a closure that returns `bool`, but it returns `Option<()>`
-  --> $DIR/dont-ice-for-type-mismatch-in-closure-in-async.rs:6:10
+  --> $DIR/dont-ice-for-type-mismatch-in-closure-in-async.rs:6:16
    |
-LL |       call(|| -> Option<()> {
-   |  _____----_^
-   | |     |
-   | |     required by a bound introduced by this call
-LL | |
-LL | |         if true {
-LL | |             false
-...  |
-LL | |     })
-   | |_____^ expected `bool`, found `Option<()>`
+LL |     call(|| -> Option<()> {
+   |     ---- ------^^^^^^^^^^
+   |     |    |     |
+   |     |    |     expected `bool`, found `Option<()>`
+   |     |    this closure
+   |     required by a bound introduced by this call
    |
    = note: expected type `bool`
               found enum `Option<()>`

tests/ui/closures/return-type-doesnt-match-bound.rs

@@ -0,0 +1,25 @@
+use std::error::Error;
+use std::process::exit;
+
+fn foo<F>(f: F) -> ()
+where
+    F: FnOnce() -> Result<(), Box<dyn Error>>,
+{
+    f().or_else(|e| -> ! { //~ ERROR to be a closure that returns
+        eprintln!("{:?}", e);
+        exit(1)
+    });
+}
+
+fn bar<F>(f: F) -> ()
+where
+    F: FnOnce() -> Result<(), Box<dyn Error>>,
+{
+    let c = |e| -> ! { //~ ERROR to be a closure that returns
+        eprintln!("{:?}", e);
+        exit(1)
+    };
+    f().or_else(c);
+}
+
+fn main() {}

tests/ui/closures/return-type-doesnt-match-bound.stderr

@@ -0,0 +1,37 @@
+error[E0271]: expected `{[email protected]:8:17}` to be a closure that returns `Result<(), _>`, but it returns `!`
+  --> $DIR/return-type-doesnt-match-bound.rs:8:24
+   |
+LL |     f().or_else(|e| -> ! {
+   |         ------- -------^
+   |         |       |      |
+   |         |       |      expected `Result<(), _>`, found `!`
+   |         |       this closure
+   |         required by a bound introduced by this call
+   |
+   = note: expected enum `Result<(), _>`
+              found type `!`
+note: required by a bound in `Result::<T, E>::or_else`
+  --> $SRC_DIR/core/src/result.rs:LL:COL
+
+error[E0271]: expected `{[email protected]:18:13}` to be a closure that returns `Result<(), _>`, but it returns `!`
+  --> $DIR/return-type-doesnt-match-bound.rs:18:20
+   |
+LL |     let c = |e| -> ! {
+   |             -------^
+   |             |      |
+   |             |      expected `Result<(), _>`, found `!`
+   |             this closure
+...
+LL |     f().or_else(c);
+   |         ------- - closure used here
+   |         |
+   |         required by a bound introduced by this call
+   |
+   = note: expected enum `Result<(), _>`
+              found type `!`
+note: required by a bound in `Result::<T, E>::or_else`
+  --> $SRC_DIR/core/src/result.rs:LL:COL
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0271`.

tests/ui/higher-ranked/trait-bounds/issue-62203-hrtb-ice.rs

@@ -36,12 +36,10 @@ trait Ty<'a> {
 
 fn main() {
     let v = Unit2.m(
-        L {
-            //~^ ERROR to be a closure that returns `Unit3`, but it returns `Unit4`
-            //~| ERROR type mismatch
+        L { //~ ERROR type mismatch
             f: |x| {
                 drop(x);
-                Unit4
+                Unit4 //~ ERROR to be a closure that returns `Unit3`, but it returns `Unit4`
             },
         },
     );