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

2 files changed, 38 insertions, 0 deletions

diff --git a/tests/ui/trait-bounds/argument-with-unnecessary-method-call.rs b/tests/ui/trait-bounds/argument-with-unnecessary-method-call.rs
new file mode 100644
index 00000000000..d8fd1d44a98
--- /dev/null
+++ b/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
+}
diff --git a/tests/ui/trait-bounds/argument-with-unnecessary-method-call.stderr b/tests/ui/trait-bounds/argument-with-unnecessary-method-call.stderr
new file mode 100644
index 00000000000..49230c98a12
--- /dev/null
+++ b/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`.