Rustdoc has different URLs for re-exported builtin derives than in the current crate

[jyn514]

I tried this code:

#![no_std]
#[doc(no_inline)]
pub use core::marker::Copy;

I expected to see this happen: Rustdoc links to https://doc.rust-lang.org/nightly/core/macro.Copy.html.

Instead, this happened: Rustdoc links to https://doc.rust-lang.org/nightly/core/marker/derive.Copy.html.

This is wrong in two ways:

1. It uses macro.Item.html and derive.Item.html inconsistently
2. It links to the marker submodule in the re-export crate but not when documenting core (pub macro defined in submodule is shown at the wrong path #74355).

This issue is only for tracking 1.

Thanks @danielhenrymantilla for the bug report!

Meta

rustdoc --version: rustdoc 1.50.0-nightly (0edce6f 2020-12-24)

[danielhenrymantilla]

This seems to stem from the way macro builtins and AST-cleaning work:

• Built-in macros are "#[lang]-declared" with a "dummy" macro definition, so they are seen, AST-wise / within rustdoc::clean, as the latter: as a TypeKind::Macro

• within a downstream crate that refers to it without inlining its definition, Res's DefKind has "correctly" resolved such macros to their actual nature (e.g., Derive).

FWIW, the "downstream crate that refers to a lang derive macro with a macro definition without inlining it" doesn't seem to happen in practice, hence it not being spotted. So we live in a world of macro.Copy.html and the like, which seems to be the bug, albeit a consistent one 😄