Rollup merge of #132156 - estebank:closure-return, r=Nadrieril,compiler-errors

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

```
error[E0271]: expected `{closure@fallback-closure-wrap.rs:18:40}` to be a closure that returns `()`, but it returns `!`
  --> $DIR/fallback-closure-wrap.rs:19:9
   |
LL |     let error = Closure::wrap(Box::new(move || {
   |                                        -------
LL |         panic!("Can't connect to server.");
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `!`
   |
   = note: expected unit type `()`
                   found type `!`
   = note: required for the cast from `Box<{closure@$DIR/fallback-closure-wrap.rs:18:40: 18:47}>` to `Box<dyn FnMut()>`
```

```
error[E0271]: expected `{closure@dont-ice-for-type-mismatch-in-closure-in-async.rs:6:10}` to be a closure that returns `bool`, but it returns `Option<()>`
  --> $DIR/dont-ice-for-type-mismatch-in-closure-in-async.rs:6:16
   |
LL |     call(|| -> Option<()> {
   |     ---- ------^^^^^^^^^^
   |     |          |
   |     |          expected `bool`, found `Option<()>`
   |     required by a bound introduced by this call
   |
   = note: expected type `bool`
              found enum `Option<()>`
note: required by a bound in `call`
  --> $DIR/dont-ice-for-type-mismatch-in-closure-in-async.rs:3:25
   |
LL | fn call(_: impl Fn() -> bool) {}
   |                         ^^^^ required by this bound in `call`
```

```
error[E0271]: expected `{closure@f670.rs:28:13}` to be a closure that returns `Result<(), _>`, but it returns `!`
    --> f670.rs:28:20
     |
28   |     let c = |e| -> ! {
     |             -------^
     |                    |
     |                    expected `Result<(), _>`, found `!`
...
32   |     f().or_else(c);
     |         ------- required by a bound introduced by this call
-Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs:1433:28
     |
     = note: expected enum `Result<(), _>`
                found type `!`
note: required by a bound in `Result::<T, E>::or_else`
    --> /home/gh-estebank/rust/library/core/src/result.rs:1406:39
     |
1406 |     pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
     |                                       ^^^^^^^^^^^^ required by this bound in `Result::<T, E>::or_else`
```

CC #111539.

2 files changed, 90 insertions, 24 deletions

diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
index 117b6a76425..db4a14356cc 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
+++ b/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 {
@@ -2057,6 +2061,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             Some(param_env.and(trace.values)),
             terr,
             false,
+            None,
         );
         diag
     }
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
index 6d39cbce3b7..6416c539f26 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs
+++ b/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;
         };
@@ -1385,9 +1381,14 @@ 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(
@@ -1395,12 +1396,39 @@ impl<'a, 'tcx> TypeErrCtxt<'a, '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 `{closure@foo.rs: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
             }