solver.md

The UI suite under nightly's default trait solver

Since compiler-team MCP #1014 (rust-lang/rust#160895), nightly builds use the new trait solver everywhere by default (CFG_DEFAULT_NEXT_SOLVER_GLOBALLY), while compiletest passes -Znext-solver=coherence to every UI test, so the suite checks the old solver only. Code that nightly users compile is therefore not what the suite checks.

rustc/ui-mirth-lab solver-diff compiles every UI test both ways with stock nightly-2026-10-06, the way its //@ headers say (tests needing auxiliary crates or another target, and tests that set -Znext-solver themselves, are left out):

rustc/ui-mirth-lab solver-diff --rustc <rustc> --tests <rust>/tests/ui --out <dir>

17,716 tests compiled both ways; 300 differ: 271 fail either way with a different first error, 26 compile under the pinned solver and fail under nightly's default, and 3 crash under the default where the pinned solver reports the expected error.

Crashes

test	under nightly's default
codegen/normalization-overflow/recursion-issue-122823.rs	error: internal compiler error: … failed to resolve instance for <&mut Peekable<std::iter:
codegen/normalization-overflow/recursion-issue-131342.rs	error: internal compiler error: … failed to resolve instance for <&mut Peekable<&mut Peeka
codegen/normalization-overflow/recursion-issue-92004.rs	error: internal compiler error: … failed to resolve instance for <&mut Peekable<std::iter:

failed to resolve instance while instantiating a recursive Peekable chain, where the tests expect "reached the recursion limit". The new solver crashed on these in July and August too (with -Znext-solver=globally); what changed between nightly-2026-08-15 and nightly-2026-10-06 is the default, so every nightly user now gets the crash. Found first by the UI coverage run (coverage.md), as a test that crashes outside compiletest and passes inside it.

Compile under the pinned solver, fail under the default

test	first error under nightly's default
async-await/drop-tracking-unresolved-typeck-results.rs	error: lifetime bound not satisfied
async-await/return-type-notation/issue-110963-early.rs	error: lifetime bound not satisfied
async-await/witness-auto-trait/higher-ranked-auto-trait-1.rs	error: higher-ranked subtype error
async-await/witness-auto-trait/higher-ranked-auto-trait-12.rs	error: higher-ranked lifetime error
async-await/witness-auto-trait/higher-ranked-auto-trait-15.rs	error: lifetime bound not satisfied
async-await/witness-auto-trait/higher-ranked-auto-trait-17.rs	error: higher-ranked subtype error
async-await/witness-auto-trait/higher-ranked-auto-trait-18.rs	error: higher-ranked lifetime error
async-await/witness-auto-trait/higher-ranked-auto-trait-4.rs	error: higher-ranked subtype error
async-await/witness-auto-trait/higher-ranked-auto-trait-5.rs	error: higher-ranked lifetime error
async-await/witness-auto-trait/higher-ranked-auto-trait-6.rs	error: lifetime bound not satisfied
async-await/witness-auto-trait/higher-ranked-auto-trait-8.rs	error: higher-ranked lifetime error
async-await/witness-auto-trait/higher-ranked-auto-trait-9.rs	error: higher-ranked lifetime error
borrowck/alias-liveness/escaping-bounds.rs	error[E0283]: type annotations needed
delegation/impl-trait.rs	error[E0282]: type annotations needed
impl-trait/recursive-impl-trait-type-direct.rs	error[E0282]: type annotations needed
impl-trait/recursive-type-alias-impl-trait-declaration-too-subtle-2.rs	error: item does not constrain …
implied-bounds/gluon_salsa.rs	error[E0309]: the associated type … may not live long enough
transmutability/alignment/align-pass.rs	error[E0277]: … cannot be safely transmuted into …
transmutability/references/accept_assume_lifetime_extension.rs	error[E0277]: … cannot be safely transmuted into …
transmutability/references/recursive-wrapper-types-bit-compatible.rs	error[E0277]: … cannot be safely transmuted into …
transmutability/references/recursive-wrapper-types.rs	error[E0277]: … cannot be safely transmuted into …
transmutability/references/u8-to-unit.rs	error[E0277]: … cannot be safely transmuted into …
transmutability/references/unit-to-itself.rs	error[E0277]: … cannot be safely transmuted into …
transmutability/safety/assume/should_accept_if_ref_src_has_safety_invariant.rs	error[E0277]: … cannot be safely transmuted into …
transmutability/transmute-higher-ranked.rs	error[E0277]: … cannot be safely transmuted into …
type-alias-impl-trait/struct-assignment-validity.rs	error[E0391]: cycle detected when computing type of …

These look like the fallout #160895 tracks (higher-ranked auto traits of async blocks, transmutability, type annotations); each is for someone working on the solver to sort into known and new. The list is facts, not reports.