Erase type params when suggesting fully qualified path

When suggesting the use of a fully qualified path for a method call that
is ambiguous because it has multiple candidates, erase type params in
the resulting code, as they would result in an error when applied. We
replace them with `_` in the output to rely on inference. There might be
cases where this still produces slighlty incomplete suggestions, but it
otherwise produces many more errors in relatively common cases.

Fix #96292

Author: estebank

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -736,8 +736,10 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     && let ExprKind::Path(hir::QPath::Resolved(_, path)) = expr.kind
                     && let [path_segment] = path.segments
                 {
+                    let mut eraser = TypeParamEraser(self.tcx);
                     let candidate_len = impl_candidates.len();
                     let suggestions = impl_candidates.iter().map(|candidate| {
+                        let candidate = candidate.super_fold_with(&mut eraser);
                         format!(
                             "{}::{}({})",
                             candidate, segment.ident, path_segment.ident
@@ -1037,3 +1039,18 @@ impl<'tcx> TypeFolder<'tcx> for ErrTypeParamEraser<'tcx> {
         }
     }
 }
+
+/// Replace type parameters with `ty::Infer(ty::Var)` to display `_`.
+struct TypeParamEraser<'tcx>(TyCtxt<'tcx>);
+
+impl<'tcx> TypeFolder<'tcx> for TypeParamEraser<'tcx> {
+    fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
+        self.0
+    }
+    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+        match t.kind() {
+            ty::Param(_) | ty::Error(_) => self.tcx().mk_ty_var(ty::TyVid::from_u32(0)),
+            _ => t.super_fold_with(self),
+        }
+    }
+}

src/test/ui/traits/do-not-mention-type-params-by-name-in-suggestion-issue-96292.rs

@@ -0,0 +1,20 @@
+struct Thing<X>(X);
+
+trait Method<T> {
+    fn method(self) -> T;
+}
+
+impl<X> Method<i32> for Thing<X> {
+    fn method(self) -> i32 { 0 }
+}
+
+impl<X> Method<u32> for Thing<X> {
+    fn method(self) -> u32 { 0 }
+}
+
+fn main() {
+    let thing = Thing(true);
+    thing.method();
+    //~^ ERROR type annotations needed
+    //~| ERROR type annotations needed
+}

src/test/ui/traits/do-not-mention-type-params-by-name-in-suggestion-issue-96292.stderr

@@ -0,0 +1,37 @@
+error[E0282]: type annotations needed
+  --> $DIR/do-not-mention-type-params-by-name-in-suggestion-issue-96292.rs:17:11
+   |
+LL |     thing.method();
+   |     ------^^^^^^--
+   |     |     |
+   |     |     cannot infer type for type parameter `T` declared on the trait `Method`
+   |     this method call resolves to `T`
+
+error[E0283]: type annotations needed
+  --> $DIR/do-not-mention-type-params-by-name-in-suggestion-issue-96292.rs:17:11
+   |
+LL |     thing.method();
+   |     ------^^^^^^--
+   |     |     |
+   |     |     cannot infer type for type parameter `T` declared on the trait `Method`
+   |     this method call resolves to `T`
+   |
+note: multiple `impl`s satisfying `Thing<bool>: Method<_>` found
+  --> $DIR/do-not-mention-type-params-by-name-in-suggestion-issue-96292.rs:7:1
+   |
+LL | impl<X> Method<i32> for Thing<X> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL | impl<X> Method<u32> for Thing<X> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: use the fully qualified path for the potential candidates
+   |
+LL |     <Thing<_> as Method<i32>>::method(thing);
+   |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+LL |     <Thing<_> as Method<u32>>::method(thing);
+   |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0282, E0283.
+For more information about an error, try `rustc --explain E0282`.