Skip redundant next-solver fulfillment scans

[anonrig]

This change was implemented by a Cursor cloud agent. It is intended for this fork, not as an upstream rust-lang/rust PR.

Skip walking the next-solver pending fulfillment queue when no pending goal can have been unstalled.

Refs rust-lang#159933. Related idea: rust-lang#161348, but this keeps a simpler conservative check:

• Track the smallest type vid equated, sub-unified, or instantiated since the last fulfillment pass (snapshot-aware via the existing infer undo log).
• Track the largest stalled type vid / sub-root among pending goals.
• Skip the retain_mut scan when every pending goal is type-var-only and trackable, opaque storage is unchanged, and every infer change is to a strictly newer vid.
• Int/float/const stalls, mixed opaque counts, and -Zdisable-fast-paths take the existing path.

Missing a min_changed bump is conservative (extra scans). Missing a bump on equate / instantiate / sub_unify would be incorrect; those three paths all record the change.

Measurements

Same-commit stage1 rustc, --crate-type=lib --edition=2024 --emit=metadata. Median of 4 runs after warmup.

Workload	Before	After	Change
rust-lang#159933 N=2000 Vec::push(Default::default())	3.484s	1.078s	69.1% faster
rust-lang#159933 N=4000	13.459s	4.061s	69.8% faster
400 independent Default/unwrap_or_default bindings	0.055s	0.054s	~1.7% (noise)

The 10%+ win is on next-solver typeck of large function bodies with many stalled obligations, which is the default solver on this tree.

---

Note

Medium Risk
Touches next-solver fulfillment and type-inference undo logging. A missed min_changed bump on equate/instantiate/sub_unify could skip needed re-evaluation; the skip is otherwise conservative.

Overview
Next-solver fulfillment can now skip walking the pending obligation queue when no pending goal could have been unstalled (rustc#159933).

It tracks the smallest type vid equated, sub-unified, or instantiated since the last fulfillment pass (snapshot-aware via the infer undo log) and the largest stalled type vid among pending goals. The scan is skipped only when every pending goal is type-var-only and trackable, opaque storage is unchanged, and every infer change is to a strictly newer vid. Int/float/const stalls, mixed opaque counts, overflow, and -Zdisable-fast-paths keep the existing full walk.

A UI test covers a long chain of stalled Default obligations that must still resolve when a later u8 constraint appears.

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Shared min-changed reset breaks skip

High Severity

reset_min_changed_ty_vid clears InferCtxt-global change tracking after any fulfillment scan. Temporary ObligationCtxt / FulfillmentCtxt instances used during typeck (e.g. coercion) share that state with the main fulfillment engine, so a successful nested scan can wipe a relevant min_changed and cause the outer context to take the skip path even though pending goals were unstalled. Those goals are later reported as ambiguity instead of being re-solved or hard-errored.

Additional Locations (2)

• compiler/rustc_trait_selection/src/solve/fulfill.rs#L204-L218
• compiler/rustc_infer/src/infer/type_variable.rs#L198-L205

 

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