Detect when method call on argument could be removed to fulfill failed trait bound

When encountering

```rust
struct Foo;
struct Bar;
impl From<Bar> for Foo {
    fn from(_: Bar) -> Self { Foo }
}
fn qux(_: impl From<Bar>) {}
fn main() {
    qux(Bar.into());
}
```

Suggest removing `.into()`:

```
error[E0283]: type annotations needed
 --> f100.rs:8:13
  |
8 |     qux(Bar.into());
  |     ---     ^^^^
  |     |
  |     required by a bound introduced by this call
  |
  = note: cannot satisfy `_: From<Bar>`
note: required by a bound in `qux`
 --> f100.rs:6:16
  |
6 | fn qux(_: impl From<Bar>) {}
  |                ^^^^^^^^^ required by this bound in `qux`
help: try using a fully qualified path to specify the expected types
  |
8 |     qux(<Bar as Into<T>>::into(Bar));
  |         +++++++++++++++++++++++   ~
help: consider removing this method call, as the receiver has type `Bar` and `Bar: From<Bar>` can be fulfilled
  |
8 -     qux(Bar.into());
8 +     qux(Bar);
  |
```

Fix #71252

1 files changed, 19 insertions, 1 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index 335e6ff2822..00672af52ea 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -3961,6 +3961,24 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         if let Node::Expr(expr) = tcx.hir_node(arg_hir_id)
             && let Some(typeck_results) = &self.typeck_results
         {
+            if let hir::Expr { kind: hir::ExprKind::MethodCall(_, rcvr, _, _), .. } = expr
+                && let Some(ty) = typeck_results.node_type_opt(rcvr.hir_id)
+                && let Some(failed_pred) = failed_pred.to_opt_poly_trait_pred()
+                && let pred = failed_pred.map_bound(|pred| pred.with_self_ty(tcx, ty))
+                && self.predicate_must_hold_modulo_regions(&Obligation::misc(
+                    tcx, expr.span, body_id, param_env, pred,
+                ))
+            {
+                err.span_suggestion_verbose(
+                    expr.span.with_lo(rcvr.span.hi()),
+                    format!(
+                        "consider removing this method call, as the receiver has type `{ty}` and \
+                         `{pred}` trivially holds",
+                    ),
+                    "",
+                    Applicability::MaybeIncorrect,
+                );
+            }
             if let hir::Expr { kind: hir::ExprKind::Block(block, _), .. } = expr {
                 let inner_expr = expr.peel_blocks();
                 let ty = typeck_results
@@ -4096,7 +4114,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             }
         }
 
-        if let Node::Expr(expr) = tcx.hir_node(call_hir_id) {
+        if let Node::Expr(expr) = call_node {
             if let hir::ExprKind::Call(hir::Expr { span, .. }, _)
             | hir::ExprKind::MethodCall(
                 hir::PathSegment { ident: Ident { span, .. }, .. },