Misleading error on tuple struct field with docstring

[tbillington]

I tried this code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=48a7a316c9d4825be960974fd87a8d8d

struct Foo {
    a: Vec<(
        /// Docstring
        f32,
        f32,
    )>,
}

I expected to see this happen: Add docstring to field in tuple struct

Instead, this happened: Misleading second error "constant provided when a type was expected".

   Compiling playground v0.0.1 (/playground)
error: expected type, found doc comment `/// Docstring`
 --> src/lib.rs:3:9
  |
3 |         /// Docstring
  |         ^^^^^^^^^^^^^ expected type
  |
help: expressions must be enclosed in braces to be used as const generic arguments
  |
2 ~     a: Vec<{ (
3 |         /// Docstring
4 |         f32,
5 |         f32,
6 ~     ) }>,
  |

error[E0747]: constant provided when a type was expected
 --> src/lib.rs:2:12
  |
2 |       a: Vec<(
  |  ____________^
3 | |         /// Docstring
4 | |         f32,
5 | |         f32,
6 | |     )>,
  | |_____^

Meta

rustc --version --verbose:

rustc 1.76.0 (07dca489a 2024-02-04)
binary: rustc
commit-hash: 07dca489ac2d933c78d3c5158e3f43beefeb02ce
commit-date: 2024-02-04
host: aarch64-apple-darwin
release: 1.76.0
LLVM version: 17.0.6

[tbillington]

As an aside, would docstrings in "nested" tuples like this be a feature request? Docstrings are supported in tuple structs as of #87451.

[workingjubilee]

@tbillington This isn't a docstring on a tuple, this is a docstring on a generic. There is no "actual" tuple type there because it's effectively an argument to the Vec. If you want to document this type, you should probably separately declare and document its fields.

[workingjubilee]

This code works:

struct Foo {
    a: Vec<(
        // Comment
        f32,
        f32,
    )>,
}

So I suspect the error has something to do with the fact that a type in this position won't successfully have an attribute attached to it, because /// desugars into

#[doc = ""]

I believe the diagnostics should be updated to inform people of this.