Confusing warning when using `link_section` on `extern static`

[RalfJung]

Code

unsafe extern {
    #[unsafe(link_section = ".buffer")]
    pub static mut a: [u8; 1024];
}

Current output

warning: attribute should be applied to a function or static
  --> <source>:10:5
   |
10 |     #[unsafe(link_section = ".buffer")]
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
11 |     pub static mut a: [u8; 1024];
   |     ----------------------------- not a function or static
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: `#[warn(unused_attributes)]` on by default

warning: 1 warning emitted

Desired output

Not sure if the attribute should be allowed on extern statics... but if the answer is no, the warning should still be reworded to avoid saying that an extern static is "not a static".

Rationale and extra context

No response

Other cases

Rust Version

Rust 1.84

Anything else?

Example due by @moridinga

[Noratrieb]

This attribute does not make sense for external statics, it only makes sense for defined statics as it controls where they end up.

[tmiasko]

LLVM reference suggest that an explicit section is significant on declarations:

Either global variable definitions or declarations may have an explicit section to be placed in and may have an optional explicit alignment specified. If there is a mismatch between the explicit or inferred section information for the variable declaration and its definition the resulting behavior is undefined.

External declarations may have an explicit section specified. Section information is retained in LLVM IR for targets that make use of this information. Attaching section information to an external declaration is an assertion that its definition is located in the specified section. If the definition is located in a different section, the behavior is undefined.

[chenyukang]

this seems fixed by #145085

now the output is:

Compiling playground v0.0.1 (/playground)
warning: `extern` declarations without an explicit ABI are deprecated
 --> src/lib.rs:1:8
  |
1 | unsafe extern {
  |        ^^^^^^ help: explicitly specify the "C" ABI: `extern "C"`
  |
  = note: `#[warn(missing_abi)]` on by default

warning: the `link_section` attribute cannot be used on foreign statics
 --> src/lib.rs:2:14
  |
2 |     #[unsafe(link_section = ".buffer")]
  |              ^^^^^^^^^^^^
  |
  = help: the `link_section` attribute can be applied to functions and statics
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: `#[warn(unused_attributes)]` (part of `#[warn(unused)]`) on by default

cc @JonathanBrouwer

[JonathanBrouwer]

Awesome, indeed wasn't aware of this issue but my refactor of the relevant code happened to fix it. This doesn't seem to have a test yet, feel free someone to go do that :3