fix: Exclude impls with errors from impl enumeration

[Wilfred]

Previously, rust-analyzer would include impl methods from impl Foo for NoSuchType, which would lead to rust-analyzer reporting nonexistent type errors.

This is noticeable when depending on tokio with feature="fs" and cfg(test) enabled. The following code would produce an invalid type error.

use std::io::Read;
fn f<T>(mut o: Option<T>) {
    o.take().unwrap();
}

rust-analyzer was incorrectly resolving the .take() to Read::take() instead of Option::take().

Inside tokio, it uses mockall:mock! to define a MockFile, and then does impl Read for &'_ MockFile. However, since mockall is a dev dependency of tokio and we don't include dev dependencies of dependencies, rust-analyzer just sees impl Read for &'_ NoSuchType.

Instead, exclude traits whose self type is error.

AI disclosure: Partially written by Codex and GPT-5.5

[Veykril]

Not opposed to this, it sounds like the right thing to do here, but why is cfg(test) set for you in tokio which I presume is a dependency of your workspace? That asks for weird issues

[Wilfred]

Yeah, this is the curse of a monorepo: we don't really know which libraries you're actively working on, so I defensively set cfg(test) essentially everywhere so you can always go-to-def on test helpers. I noticed this when using rust-project as a discover command.

It looks like I should modify rust-project to exclude cfg(test) on imported libraries like tokio, since our monorepo (using reindeer) doesn't actually have dev-dependencies for imported libraries so cfg(test) is genuinely wrong there.

The bug was a spooky action-at-a-distance issue: adding tokio to the transitive set of dependencies causes spurious red squiggles and it took me a while to get to the bottom of it. This fix does feel appropriately defensive.

I see CI is red, I'll fix that too.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.