Intra-doc links are not resolved in inherited docs from upstream traits

[euclio]

I tried this code:

There are two crates, outer and inner. outer depends on inner.

outer/src/lib.rs:

use inner::SomeTrait;

pub struct SomeStruct;

impl SomeTrait for SomeStruct {
    fn foo() {}
}

outer/inner/src/lib.rs:

/// this is a trait
pub trait SomeTrait {
    /// this is a method for [SomeTrait]
    fn foo();
}

pub mod bar {
    use super::SomeTrait;

    pub struct BarStruct;

    impl SomeTrait for BarStruct {
        fn foo() {}
    }
}

I expected to see this happen: The trait implementation docs for SomeTrait::foo on outer::SomeStruct link to SomeTrait.

Instead, this happened: The link is not resolved, and there is no warning or indication that it failed. Here is a screenshot of the generated docs:

Notably, the docs on inner::bar::BarStruct resolve correctly despite being in a different module, indicating that this is a cross-crate issue.

Meta

rustc --version --verbose:

rustc 1.44.1 (c7087fe00 2020-06-17)
binary: rustc
commit-hash: c7087fe00d2ba919df1d813c040a5d47e43b0fe7
commit-date: 2020-06-17
host: x86_64-unknown-linux-gnu
release: 1.44.1
LLVM version: 9.0

[euclio]

cc @jyn514 who has been doing some work in this area recently.

[jyn514]

I expect this to be fixed by #73101. I'll test locally.

[jyn514]

Interesting, it was not fixed - rustdoc resolves links for associated items relative to the type, not relative to the trait. The lack of warning is definitely a bug, but I'm not sure the behavior is incorrect.

rust/src/librustdoc/passes/collect_intra_doc_links.rs

Line 475 in 6742382

ImplItem(Impl { ref for_, .. }) => {

[jyn514]

Oh hold on that shouldn't make a difference - the trait is in scope in both places. I have to look at this more, I'm not sure what's going on.

[euclio]

It's worth mentioning that these impl method docs are subtly broken in other ways (see my linked PR above) but as long as you use inline links you should be able to avoid those bugs.

[jyn514]

Reduced test case:

// inner.rs

/// [SomeType] links to [bar]
pub struct SomeType;
pub trait SomeTrait {}
/// [bar] links to [SomeTrait] and also [SomeType]
pub mod bar {}

// outer.rs
pub use inner::bar;

All the links on the documentation for outer::bar will be broken (even with the changes from #73101).

[jyn514]

This code looks wrong:

rust/src/librustdoc/passes/collect_intra_doc_links.rs

Lines 436 to 441 in 6742382

	let item_hir_id = if item.is_mod() {
	if let Some(def_id) = item.def_id.as_local() {
	Some(self.cx.tcx.hir().as_local_hir_id(def_id))
	} else {
	debug!("attempting to fold on a non-local item: {:?}", item);
	return self.fold_item_recur(item);

Not sure if it's related to the impl issue though.

[jyn514]

I have a fix for this in #73101. Thanks for the bug report!

[jyn514]

Update so all the discussion is in one place: My fix didn't do what I thought (it only worked for modules, not docs taken from the trait parent). However it will depend on the changes in #73101, so I'm leaving this be for now until that lands.

[jyn514]

... I think I found why this is broken. Oops.

rust/src/librustdoc/passes/collect_intra_doc_links.rs

Lines 830 to 837 in e25a67f

	// FIXME: if we can resolve intra-doc links from other crates, we can use the stock
	// `fold_crate`, but until then we should avoid scanning `krate.external_traits` since those
	// will never resolve properly
	fn fold_crate(&mut self, mut c: Crate) -> Crate {
	c.module = c.module.take().and_then(|module| self.fold_item(module));
	c
	}

[jyn514]

Removing that override gives a BorrowMut error. Investigating now.