Switch next solver to use a specific associated type for trait def id

[ChayimFriedman2]

The compiler just puts DefId in there, but rust-analyzer uses different types for each kind of item.

See the Zulip discussion. In short, it will be a tremendous help to r-a to use specific associated types, while for the solver and the compiler it's a small change. So I ported TraitId, as a proof of concept and it's also likely the most impactful.

r? types

[bors]

☔ The latest upstream changes (presumably #144991) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

This PR was rebased onto a different master commit! Check out the changes with our range-diff.

[bors]

☔ The latest upstream changes (presumably #145600) made this pull request unmergeable. Please resolve the merge conflicts.

[lcnr]

sorry for not replying to this PR up until now 😅

I am somewhat torn about this and avoided forming a final opinion up until now. I think this is generally useful and I am in favor of this change.

I do feel like ideally this should not limited to the solver and we actually have some struct AssocItemDefId { inner: DefId } outside of the solver as well. We already have struct BodyId for the HIR.

I feel like the main issue is to actually use these IDs everywhere 😅 I don't know what exactly I want and changing stuff here results in a lot of churn/merge conflicts

so yeah, want to take some time to properly sort out my thoughts here, am generally in favor, but don't have the time to do that rn

[ChayimFriedman2]

@lcnr That's exactly why I opened this PR: so you can see (and I can be sure) that it's not, in fact, a big change.

Doing it throughout the compiler might be interesting but it definitely will be a big change, and I don't want to do that. I also didn't create this PR to have better type safety within the solver (although this is nice), but to help rust-analyzer.

[lcnr]

Can you rename TraitSolverLangItem to SolverLangItem?

TraitSolverTrait is hard to read.

Also, rename uses of trait_def_id to just trait_id maybe?

then r=me. While I'd like to maybe extend that to rustc, I agree that it doesn't make the trait solver harder to read and it's useful for r-a

View changes since this review

[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.

[ChayimFriedman2]

@lcnr Did what you requested, except renaming all trait_def_id to trait_id because I fear this will cause a lot of conflicts, and trait_def_id is not incorrect either.

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

warning: `test_docs` (lib doc) generated 4 warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.57s
   Generated /checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/test_docs/index.html
npm ERR! code ECONNRESET
npm ERR! network aborted
npm ERR! network This is a problem related to network connectivity.
npm ERR! network In most cases you are behind a proxy or have bad network settings.
npm ERR! network 
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npm ERR! network 'proxy' config is set properly.  See: 'npm help config'

npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2025-08-25T14_59_48_442Z-debug-0.log
npm install did not exit successfully

thread 'main' panicked at src/tools/rustdoc-gui-test/src/main.rs:69:10:
unable to install browser-ui-test: Custom { kind: Other, error: "npm install returned exit code exit status: 1" }
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/788da80fcfcef3f34c90def5baa32813e39a1a41/library/std/src/panicking.rs:697:5
   1: core::panicking::panic_fmt
             at /rustc/788da80fcfcef3f34c90def5baa32813e39a1a41/library/core/src/panicking.rs:75:14

[ChayimFriedman2]

That must be a spurious failure.