Doc alias not allowed on associated constants and types in trait implementation block

[nvzqz]

I tried this code:

pub trait Trait {
    const VALUE: u32;
}

pub struct Struct;

impl Trait for Struct {
    #[doc(alias = "kValue")]
    const VALUE: u32 = 42;
}

Similarly, I tried:

trait Trait {
    type Value;
}

struct Struct;

impl Trait for Struct {
    #[doc(alias = "ValueType")]
    type Value = u32;
}

I expected to see this happen: the code to compile and searching kValue or ValueType in the docs to produce Struct::VALUE or Struct::Value respectively.

Instead, this happened: These failed to compile with the following errors:

error: #[doc(alias = "...")] isn't allowed on associated constant in trait implementation block

error: #[doc(alias = "...")] isn't allowed on type alias in implementation block

This is very surprising because:

• #[doc(alias)] works on fn trait members in impl blocks.
• Doc comments are allowed on associated constants and types in impl blocks.

Meta

rustc --version --verbose:

rustc 1.49.0 (e1884a8e3 2020-12-29)
binary: rustc
commit-hash: e1884a8e3c3e813aada8254edfa120e85bf5ffca
commit-date: 2020-12-29
host: x86_64-apple-darwin
release: 1.49.0

[nvzqz]

@rustbot modify labels: +A-doc-alias +A-traits +A-associated-items

[jyn514]

cc @GuillaumeGomez

[jyn514]

actually @ollie27 you said in #73721 (comment) that rustdoc should disallow this on associated constants, because they don't appear in the search index. But they do:
. So I think prohibiting them was incorrect.

[jyn514]

Oh I see - that's the constant for the trait itself, not for the implementation.

@nvzqz try moving it to type Value; in the trait definition, that should work.

[nvzqz]

@jyn514 my use case is that I'm wrapping a C library and would like to use the doc alias for the associated constant's original name in the specific type's impl. So it doesn't make sense to have it inside the trait.

Perhaps rustdoc should support including associated constants/types in the search index?