Suggest adding Result return type for associated method in E0277.

For following:

```rust
struct A;
impl A {
    fn test4(&self) {
        let mut _file = File::create("foo.txt")?;
        //~^ ERROR the `?` operator can only be used in a method
    }
```
Suggest:

```rust
impl A {
    fn test4(&self) -> Result<(), Box<dyn std::error::Error>> {
        let mut _file = File::create("foo.txt")?;
        //~^ ERROR the `?` operator can only be used in a method

    Ok(())
    }
}
```

For #125997

1 files changed, 36 insertions, 5 deletions

diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
index 885216e6216..9b949323a36 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs
@@ -4612,6 +4612,9 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         })
     }
 
+    // For E0277 when use `?` operator, suggest adding
+    // a suitable return type in `FnSig`, and a default
+    // return value at the end of the function's body.
     pub(super) fn suggest_add_result_as_return_type(
         &self,
         obligation: &PredicateObligation<'tcx>,
@@ -4622,19 +4625,47 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             return;
         }
 
+        // Only suggest for local function and associated method,
+        // because this suggest adding both return type in
+        // the `FnSig` and a default return value in the body, so it
+        // is not suitable for foreign function without a local body,
+        // and neighter for trait method which may be also implemented
+        // in other place, so shouldn't change it's FnSig.
+        fn choose_suggest_items<'tcx, 'hir>(
+            tcx: TyCtxt<'tcx>,
+            node: hir::Node<'hir>,
+        ) -> Option<(&'hir hir::FnDecl<'hir>, hir::BodyId)> {
+            match node {
+                hir::Node::Item(item) if let hir::ItemKind::Fn(sig, _, body_id) = item.kind => {
+                    Some((sig.decl, body_id))
+                }
+                hir::Node::ImplItem(item)
+                    if let hir::ImplItemKind::Fn(sig, body_id) = item.kind =>
+                {
+                    let parent = tcx.parent_hir_node(item.hir_id());
+                    if let hir::Node::Item(item) = parent
+                        && let hir::ItemKind::Impl(imp) = item.kind
+                        && imp.of_trait.is_none()
+                    {
+                        return Some((sig.decl, body_id));
+                    }
+                    None
+                }
+                _ => None,
+            }
+        }
+
         let node = self.tcx.hir_node_by_def_id(obligation.cause.body_id);
-        if let hir::Node::Item(item) = node
-            && let hir::ItemKind::Fn(sig, _, body_id) = item.kind
-            && let hir::FnRetTy::DefaultReturn(ret_span) = sig.decl.output
+        if let Some((fn_decl, body_id)) = choose_suggest_items(self.tcx, node)
+            && let hir::FnRetTy::DefaultReturn(ret_span) = fn_decl.output
             && self.tcx.is_diagnostic_item(sym::FromResidual, trait_pred.def_id())
             && trait_pred.skip_binder().trait_ref.args.type_at(0).is_unit()
             && let ty::Adt(def, _) = trait_pred.skip_binder().trait_ref.args.type_at(1).kind()
             && self.tcx.is_diagnostic_item(sym::Result, def.did())
         {
-            let body = self.tcx.hir().body(body_id);
             let mut sugg_spans =
                 vec![(ret_span, " -> Result<(), Box<dyn std::error::Error>>".to_string())];
-
+            let body = self.tcx.hir().body(body_id);
             if let hir::ExprKind::Block(b, _) = body.value.kind
                 && b.expr.is_none()
             {