E0562 should mention impl Trait in argument position

[kpreid]

impl Trait as a type is permitted in function parameter lists. However, the error message E0562 incorrectly states that it is only permitted in function or inherent method return positions.

/// OK
fn a(x: impl Into<String>) {}
/// OK
fn b() -> impl Into<String> { "bees" }
/// Error
struct C {
    x: impl Into<String>,
}

error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
 --> src/lib.rs:4:8
  |
4 |     x: impl Into<String>,
  |        ^^^^^^^^^^^^^^^^^

Being precise in this message will assist beginners (such as myself a few months ago) in understanding the boundaries of the language. Particularly, one might reasonably imagine equivalence between a function's parameter list and a single parameter whose type is a struct bundling several parameters, but impl Trait is available in the former and not the latter — whereas the error message implies that it is allowed in neither of those places.

Tangentially, it would be neat if the compiler gave suggestions to rewrite disallowed impl Trait as either an explicit parameter,struct C<S: impl Into<String>> { x: S }, or (especially in the case of impl Into) a constructor function, but that's much more work and more dependent on the context and perhaps even the specific trait.

Meta

This occurs on both stable 1.48.0 and nightly 1.50.0-nightly (2020-12-22 bb1fbbf84455fbad9afd), and the message is present in the same form in the current source code.

[Dylan-DPC]

Current output:

error[[E0562]](https://doc.rust-lang.org/nightly/error_codes/E0562.html): `impl Trait` only allowed in function and inherent method return types, not in field types
 --> src/lib.rs:7:8
  |
7 |     x: impl Into<String>,
  |        ^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0562`.
error: could not compile `playground` (lib) due to previous error