Improve errors about #[deprecated] attribute

[m-ou-se]

This change:

1. Turns #[deprecated] on a trait impl block into an error, which fixes Deprecation attribute on trait impl block is accepted but has no effect #78625;
2. Changes these and other errors about #[deprecated] to use the span of the attribute instead of the item; and
3. Turns this error into a lint, to make sure it can be capped with --cap-lints and doesn't break any existing dependencies.

Can be reviewed per commit.

---

Example:

struct X;

#[deprecated = "a"]
impl Default for X {
    #[deprecated = "b"]
    fn default() -> Self {
        X
    }
}

Before:

error: This deprecation annotation is useless
 --> src/main.rs:6:5
  |
6 | /     fn default() -> Self {
7 | |         X
8 | |     }
  | |_____^

After:

error: this `#[deprecated]' annotation has no effect
 --> src/main.rs:3:1
  |
3 | #[deprecated = "a"]
  | ^^^^^^^^^^^^^^^^^^^ help: try removing the deprecation attribute
  |
  = note: `#[deny(useless_deprecated)]` on by default

error: this `#[deprecated]' annotation has no effect
 --> src/main.rs:5:5
  |
5 |     #[deprecated = "b"]
  |     ^^^^^^^^^^^^^^^^^^^ help: try removing the deprecation attribute

[estebank]

Can this happen multiple times (if you have three deprecation attributes)? If so, reword the label to not make it technically incorrect.

Also, use prefer to use span_label when possible.

Suggested change

	struct_span_err!(diagnostic, attr.span, E0550, "multiple deprecated attributes")
	.span_note(*span, "first deprecation attribute here")
	struct_span_err!(diagnostic, attr.span, E0550, "multiple deprecated attributes")
	.span_label(attr.span, "repeated deprecated attribute")
	.span_label(*span, "prior deprecation attribute")

[m-ou-se]

It stops right after the first duplicate, so having three deprecation attributes wouldn't output any more diagnostics than two.

Is there documentation somewhere about all the different options for diagnostics, like span_label, span_suggestion_short, etc.?

[estebank]

https://github.com/rust-lang/rust/blob/master/compiler/rustc_errors/src/diagnostic.rs has all the methods that can be used and they have good docstrings for the most part.

[m-ou-se]

Thanks! Updated.

[estebank]

@bors r+

[bors]

📌 Commit 9c647d1 has been approved by estebank