Add dependent binders and explicit trait evidence representation

[enginespot]

Introduce the type-level representation needed to carry dependent binder entries and compiler-internal trait evidence through rustc. Evidence is kept separate from ordinary generic arguments so later solver and canonical-query code can preserve the proof identity associated with a projection.

The motivation is the HRTB/GAT behavior tracked in #107572, #86702, and #121437. The current E0582 check conservatively rejects some callable bounds even when the same GAT projection appears on both the input and output sides. This representation gives later stages enough information to distinguish that case from a projection whose normalized output is independent of the bound lifetime.

This change adds:

• dependent binder entries and explicit evidence states;
• evidence-indexed aliases;
• folding, visiting, relation, persistence, diagnostics, and printing support;
• low-level coverage for the new representation.

Testing:

• ./x check
• ./x test --stage 2 tests/ui-fulldeps
• ./x test src/tools/tidy

Related: #107572, #86702, and #121437.

This design is informed by @RalfJung's explicit trait-evidence discussion in #32330 and by @compiler-errors and @SkiFire13's analysis of projection normalization and lifetime constraints in #107572 and #86702.

[rustbot]

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @khyperia (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

Please see the contribution instructions and our LLM policy for more information.

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler, types
• compiler, types expanded to 76 candidates
• Random selection from 18 candidates

[jackh726]

This needs significant discussion with the types team; Zulip is the right place for that.

I'll leave this open for now, but am going to mark this as experimental. It is not going to be reviewed without discussion.

[rustbot]

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

changes to the core type system

cc @lcnr

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

changes to the core type system

cc @lcnr

[rust-log-analyzer]

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

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

failures:

---- [ui] tests/ui-fulldeps/dependent-evidence-codec.rs stdout ----

error: test did not exit with success! code=Some(101) so test would pass with `run-fail`
status: exit status: 101
command: cd "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui-fulldeps/dependent-evidence-codec" && RUSTC="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" RUST_TEST_THREADS="4" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui-fulldeps/dependent-evidence-codec/a" "--sysroot" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2" "/checkout/tests/ui-fulldeps/auxiliary/dependent-evidence-codec-input.rs"
stdout: none
--- stderr -------------------------------

thread 'rustc' (206617) panicked at /checkout/tests/ui-fulldeps/dependent-evidence-codec.rs:303:5:
assertion failed: decoder.in_progress.is_empty()

Important

For more information how to resolve CI failures of this job, visit this link.

[enginespot]

@jackh726 Thanks for the guidance. I’ve started a discussion in #t-types
(https://rust-lang.zulipchat.com/#narrow/channel/144729-t-types/topic/Early.20design.20feedback.20on.20HRTB.2FGAT.20evidence.20handling.20.28PR.20.23162/with/623947679).
I proposed the core idea and used AI tools extensively in this work. I hope to learn from the team and help
move this issue toward a solution.

[rust-bors]

☔ The latest upstream changes (presumably #162827) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.