Rollup merge of #121100 - estebank:issue-71252, r=compiler-errors

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>` trivially holds
  |
8 -     qux(Bar.into());
8 +     qux(Bar);
  |
```

Fix #71252

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

@@ -3689,6 +3689,24 @@ impl<'tcx> 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
@@ -3824,7 +3842,7 @@ impl<'tcx> 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, .. }, .. },

tests/ui/async-await/issue-72442.stderr

@@ -8,6 +8,11 @@ LL |             let mut f = File::open(path.to_str())?;
    |
 note: required by a bound in `File::open`
   --> $SRC_DIR/std/src/fs.rs:LL:COL
+help: consider removing this method call, as the receiver has type `&Path` and `&Path: AsRef<Path>` trivially holds
+   |
+LL -             let mut f = File::open(path.to_str())?;
+LL +             let mut f = File::open(path)?;
+   |
 
 error: aborting due to 1 previous error
 

tests/ui/error-should-say-copy-not-pod.stderr

@@ -11,6 +11,11 @@ note: required by a bound in `check_bound`
    |
 LL | fn check_bound<T:Copy>(_: T) {}
    |                  ^^^^ required by this bound in `check_bound`
+help: consider removing this method call, as the receiver has type `&'static str` and `&'static str: Copy` trivially holds
+   |
+LL -     check_bound("nocopy".to_string());
+LL +     check_bound("nocopy");
+   |
 
 error: aborting due to 1 previous error
 

tests/ui/suggestions/issue-84973-blacklist.stderr

@@ -11,6 +11,11 @@ note: required by a bound in `f_copy`
    |
 LL | fn f_copy<T: Copy>(t: T) {}
    |              ^^^^ required by this bound in `f_copy`
+help: consider removing this method call, as the receiver has type `&'static str` and `&'static str: Copy` trivially holds
+   |
+LL -     f_copy("".to_string());
+LL +     f_copy("");
+   |
 
 error[E0277]: the trait bound `S: Clone` is not satisfied
   --> $DIR/issue-84973-blacklist.rs:16:13

tests/ui/trait-bounds/argument-with-unnecessary-method-call.rs

@@ -0,0 +1,11 @@
+struct Foo;
+struct Bar;
+impl From<Bar> for Foo {
+    fn from(_: Bar) -> Self { Foo }
+}
+fn qux(_: impl From<Bar>) {}
+fn main() {
+    qux(Bar.into()); //~ ERROR type annotations needed
+    //~| HELP try using a fully qualified path to specify the expected types
+    //~| HELP consider removing this method call, as the receiver has type `Bar` and `Bar: From<Bar>` trivially holds
+}

tests/ui/trait-bounds/argument-with-unnecessary-method-call.stderr

@@ -0,0 +1,27 @@
+error[E0283]: type annotations needed
+  --> $DIR/argument-with-unnecessary-method-call.rs:8:13
+   |
+LL |     qux(Bar.into());
+   |     ---     ^^^^
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = note: cannot satisfy `_: From<Bar>`
+note: required by a bound in `qux`
+  --> $DIR/argument-with-unnecessary-method-call.rs:6:16
+   |
+LL | fn qux(_: impl From<Bar>) {}
+   |                ^^^^^^^^^ required by this bound in `qux`
+help: try using a fully qualified path to specify the expected types
+   |
+LL |     qux(<Bar as Into<T>>::into(Bar));
+   |         +++++++++++++++++++++++   ~
+help: consider removing this method call, as the receiver has type `Bar` and `Bar: From<Bar>` trivially holds
+   |
+LL -     qux(Bar.into());
+LL +     qux(Bar);
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0283`.