Detect inherent method behind deref being shadowed by trait method

```
error[E0277]: the trait bound `Rc<RefCell<S>>: Borrow<S>` is not satisfied
  --> $DIR/shadowed-intrinsic-method-deref.rs:16:22
   |
LL |     let sb : &S = &s.borrow();
   |                      ^^^^^^ the trait `Borrow<S>` is not implemented for `Rc<RefCell<S>>`
   |
help: the trait `Borrow<S>` is not implemented for `Rc<RefCell<S>>`
      but trait `Borrow<RefCell<S>>` is implemented for it
  --> $SRC_DIR/alloc/src/rc.rs:LL:COL
   = help: for that trait implementation, expected `RefCell<S>`, found `S`
   = note: there's an inherent method on `RefCell<S>` of the same name, which can be auto-dereferenced from `&RefCell<T>`
help: to access the inherent method on `RefCell<S>`, use the fully-qualified path
   |
LL -     let sb : &S = &s.borrow();
LL +     let sb : &S = &RefCell::borrow(&s);
   |
```

In the example above, method `borrow` is available both on `<RefCell<S> as Borrow<S>>` *and* on `RefCell<S>`. Adding the import `use std::borrow::Borrow;` causes `s.borrow()` to find the former instead of the latter. We now point out that the other exists, and provide a suggestion on how to call it.

Author: estebank

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

@@ -3178,6 +3178,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
 
             self.suggest_tuple_wrapping(err, root_obligation, obligation);
         }
+        self.suggest_shadowed_inherent_method(err, obligation, trait_predicate);
     }
 
     fn add_help_message_for_fn_trait(

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

@@ -4916,6 +4916,79 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         }
     }
 
+    pub(super) fn suggest_shadowed_inherent_method(
+        &self,
+        err: &mut Diag<'_>,
+        obligation: &PredicateObligation<'tcx>,
+        trait_predicate: ty::PolyTraitPredicate<'tcx>,
+    ) {
+        let ObligationCauseCode::FunctionArg { call_hir_id, .. } = obligation.cause.code() else {
+            return;
+        };
+        let Node::Expr(call) = self.tcx.hir_node(*call_hir_id) else { return };
+        let hir::ExprKind::MethodCall(segment, rcvr, args, ..) = call.kind else { return };
+        let Some(typeck) = &self.typeck_results else { return };
+        let Some(rcvr_ty) = typeck.expr_ty_adjusted_opt(rcvr) else { return };
+        let rcvr_ty = self.resolve_vars_if_possible(rcvr_ty);
+        let autoderef = (self.autoderef_steps)(rcvr_ty);
+        for (ty, def_id) in autoderef.iter().filter_map(|(ty, obligations)| {
+            if let ty::Adt(def, _) = ty.kind()
+                && *ty != rcvr_ty.peel_refs()
+                && obligations.iter().all(|obligation| self.predicate_may_hold(obligation))
+            {
+                Some((ty, def.did()))
+            } else {
+                None
+            }
+        }) {
+            for impl_def_id in self.tcx.inherent_impls(def_id) {
+                if *impl_def_id == trait_predicate.def_id() {
+                    continue;
+                }
+                for m in self
+                    .tcx
+                    .provided_trait_methods(*impl_def_id)
+                    .filter(|m| m.name() == segment.ident.name)
+                {
+                    let fn_sig = self.tcx.fn_sig(m.def_id);
+                    if fn_sig.skip_binder().inputs().skip_binder().len() != args.len() + 1 {
+                        continue;
+                    }
+                    let rcvr_ty = fn_sig.skip_binder().input(0).skip_binder();
+                    let (mutability, _ty) = match rcvr_ty.kind() {
+                        ty::Ref(_, ty, hir::Mutability::Mut) => ("&mut ", ty),
+                        ty::Ref(_, ty, _) => ("&", ty),
+                        _ => ("", &rcvr_ty),
+                    };
+                    let path = self.tcx.def_path_str(def_id);
+                    err.note(format!(
+                        "there's an inherent method on `{ty}` of the same name, which can be \
+                         auto-dereferenced from `{rcvr_ty}`"
+                    ));
+                    err.multipart_suggestion(
+                        format!(
+                            "to access the inherent method on `{ty}`, use the fully-qualified path",
+                        ),
+                        vec![
+                            (
+                                call.span.until(rcvr.span),
+                                format!("{path}::{}({}", m.name(), mutability),
+                            ),
+                            match &args {
+                                [] => (
+                                    rcvr.span.shrink_to_hi().with_hi(call.span.hi()),
+                                    ")".to_string(),
+                                ),
+                                [first, ..] => (rcvr.span.between(first.span), ", ".to_string()),
+                            },
+                        ],
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+            }
+        }
+    }
+
     pub(super) fn explain_hrtb_projection(
         &self,
         diag: &mut Diag<'_>,

tests/ui-fulldeps/session-diagnostic/diagnostic-derive-doc-comment-field.stderr

@@ -13,6 +13,7 @@ help: the nightly-only, unstable trait `IntoDiagArg` is not implemented for `Not
 LL | struct NotIntoDiagArg;
    | ^^^^^^^^^^^^^^^^^^^^^
    = help: normalized in stderr
+   = note: there's an inherent method on `DiagInner` of the same name, which can be auto-dereferenced from `&mut DiagInner`
 note: required by a bound in `Diag::<'a, G>::arg`
   --> $COMPILER_DIR/rustc_errors/src/diagnostic.rs:LL:CC
   ::: $COMPILER_DIR/rustc_errors/src/diagnostic.rs:LL:CC

tests/ui-fulldeps/session-diagnostic/diagnostic-derive-inline.stderr

@@ -589,6 +589,7 @@ help: the nightly-only, unstable trait `IntoDiagArg` is not implemented for `Hel
 LL | struct Hello {}
    | ^^^^^^^^^^^^
    = help: normalized in stderr
+   = note: there's an inherent method on `DiagInner` of the same name, which can be auto-dereferenced from `&mut DiagInner`
 note: required by a bound in `Diag::<'a, G>::arg`
   --> $COMPILER_DIR/rustc_errors/src/diagnostic.rs:LL:CC
   ::: $COMPILER_DIR/rustc_errors/src/diagnostic.rs:LL:CC

tests/ui/methods/shadowed-intrinsic-method-deref.fixed

@@ -0,0 +1,21 @@
+//@ run-rustfix
+#![allow(unused_imports)]
+use std::rc::Rc;
+use std::cell::RefCell;
+use std::borrow::Borrow; // Without this import, the code would compile.
+
+pub struct S {
+    flag: bool,
+}
+
+type SCell = Rc<RefCell<S>>;
+
+fn main() {
+    // Type annotations just for clarity
+    let s : SCell = Rc::new(RefCell::new(S {flag: false}));
+    let sb : &S = &RefCell::borrow(&s);
+    //~^ ERROR: the trait bound `Rc<RefCell<S>>: Borrow<S>` is not satisfied [E0277]
+    //~| NOTE: the trait `Borrow<S>` is not implemented for `Rc<RefCell<S>>`
+    //~| NOTE: there's an inherent method on `RefCell<S>` of the same name
+    println!("{:?}", sb.flag);
+}

tests/ui/methods/shadowed-intrinsic-method-deref.rs

@@ -0,0 +1,21 @@
+//@ run-rustfix
+#![allow(unused_imports)]
+use std::rc::Rc;
+use std::cell::RefCell;
+use std::borrow::Borrow; // Without this import, the code would compile.
+
+pub struct S {
+    flag: bool,
+}
+
+type SCell = Rc<RefCell<S>>;
+
+fn main() {
+    // Type annotations just for clarity
+    let s : SCell = Rc::new(RefCell::new(S {flag: false}));
+    let sb : &S = &s.borrow();
+    //~^ ERROR: the trait bound `Rc<RefCell<S>>: Borrow<S>` is not satisfied [E0277]
+    //~| NOTE: the trait `Borrow<S>` is not implemented for `Rc<RefCell<S>>`
+    //~| NOTE: there's an inherent method on `RefCell<S>` of the same name
+    println!("{:?}", sb.flag);
+}

tests/ui/methods/shadowed-intrinsic-method-deref.stderr

@@ -0,0 +1,20 @@
+error[E0277]: the trait bound `Rc<RefCell<S>>: Borrow<S>` is not satisfied
+  --> $DIR/shadowed-intrinsic-method-deref.rs:16:22
+   |
+LL |     let sb : &S = &s.borrow();
+   |                      ^^^^^^ the trait `Borrow<S>` is not implemented for `Rc<RefCell<S>>`
+   |
+help: the trait `Borrow<S>` is not implemented for `Rc<RefCell<S>>`
+      but trait `Borrow<RefCell<S>>` is implemented for it
+  --> $SRC_DIR/alloc/src/rc.rs:LL:COL
+   = help: for that trait implementation, expected `RefCell<S>`, found `S`
+   = note: there's an inherent method on `RefCell<S>` of the same name, which can be auto-dereferenced from `&RefCell<T>`
+help: to access the inherent method on `RefCell<S>`, use the fully-qualified path
+   |
+LL -     let sb : &S = &s.borrow();
+LL +     let sb : &S = &RefCell::borrow(&s);
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.