Type fallback refactorings by WaffleLapkin · Pull Request #159026 · rust-lang/rust

WaffleLapkin · 2026-07-09T16:32:06Z

r? types

Don't merge different kinds of infer variables into Ty before matching them back
Do fallback only on root vars (this is semantically equivalent to what we currently do; unifying any of the infer vars from the same equivalence class necesserily affects all of them) (this makes Implement "low priority impls" #159003 a bit less hacky though)
Do some simplification that has been allowed by the previous changes

WaffleLapkin · 2026-07-09T19:12:45Z

Type fallback refactorings

rust-bors · 2026-07-09T21:19:26Z

☀️ Try build successful (CI)
Build commit: 6a25a06 (6a25a0621d4f43872d80fc0aba25c7aedfc92c0f)
Base parent: 3664b37 (3664b37017c529cad0f0ed259769f15743c21a6a)

rust-timer · 2026-07-10T00:52:33Z

Finished benchmarking commit (6a25a06): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.2%, 0.3%]	4
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.3%, -2.0%]	12
All ❌✅ (primary)	0.2%	[0.2%, 0.3%]	4

Max RSS (memory usage)

Results (primary 3.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.4%	[2.6%, 4.1%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.4%	[2.6%, 4.1%]	2

Cycles

Results (secondary -5.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.3%	[-14.7%, -2.8%]	7
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%, secondary 0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.1%]	59
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	22
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.1%]	59

Bootstrap: 487.967s -> 488.861s (0.18%)
Artifact size: 388.37 MiB -> 388.42 MiB (0.01%)

WaffleLapkin · 2026-07-14T12:30:22Z

I think the perf regression is noise.
@rustbot label: +perf-regression-triaged

lcnr · 2026-07-21T08:25:55Z

+    /// - Unconstrained floats are replaced with `f64`, except when there is a trait predicate
+    ///   `f32: From<{float}>`, in which case `f32` is used as the fallback instead.


can you change this comment to name the FCW we emit here so that we update this comment when removing the fallback

View changes since the review

lcnr · 2026-07-21T08:29:39Z

+                if self.root_var(vid) != vid {
+                    return None;
+                }
                match self.probe(vid) {
                    TypeVariableValue::Unknown { .. } => Some(vid),
                    TypeVariableValue::Known { .. } => None,
                }


you can merge this with ena version 0.14.4, see #158447

There's an operation which does the "is known" and "get root var" with one access

View changes since the review

lcnr · 2026-07-21T08:31:16Z

            .map(|&ty_id| self.shallow_resolve(Ty::new_var(self.tcx, ty_id)))
            .filter_map(|ty| ty.ty_vid())
            .map(|vid| self.root_var(vid))


that should also be a probe_with_root_vid i think? seems like a very cumbersome way to get the root vid of a ty var

View changes since the review

lcnr · 2026-07-21T08:33:38Z

-                    "calculate_diverging_fallback: root_vid={:?} reaches {:?}",
+            let unsafe_infer_vars = OnceCell::new();
+            for &root_vid in &diverging_root_vids {
+                self.lint_never_type_fallback_flowing_into_unsafe_code(


why are we using a OnceCell here instead of a if !diverging_root_vids.is_empty() around the whole block here?

View changes since the review

lcnr · 2026-07-21T08:34:38Z

unsure about the last commit after applying my other review comments, but r=me after nits on the other changes

WaffleLapkin added 2 commits July 9, 2026 18:05

split fallback of different infer var kinds

2b4dc91

only do fallback on root vars

9dcdb29

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 9, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jul 9, 2026

Auto merge of #159026 - WaffleLapkin:fallback-refactorings, r=<try>

6a25a06

Type fallback refactorings

rustbot added the T-types Relevant to the types team, which will review and decide on the PR/issue. label Jul 9, 2026

rustbot assigned oli-obk Jul 9, 2026

Urgau added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 9, 2026

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jul 10, 2026

WaffleLapkin added 3 commits July 14, 2026 13:57

simplify never type fallback

c1686cc

tiny refactor to f32 type fallback

619c9f1

use a helper for getting unresolved root vars

8992f0e

WaffleLapkin force-pushed the fallback-refactorings branch from 68f06e3 to 8992f0e Compare July 14, 2026 12:01

rustbot added the perf-regression-triaged The performance regression has been triaged. label Jul 14, 2026

lcnr reviewed Jul 21, 2026

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oli-obk assigned lcnr and unassigned oli-obk Jul 21, 2026

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Type fallback refactorings#159026

Type fallback refactorings#159026
WaffleLapkin wants to merge 5 commits into
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WaffleLapkin:fallback-refactorings

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		/// - Unconstrained floats are replaced with `f64`, except when there is a trait predicate
		/// `f32: From<{float}>`, in which case `f32` is used as the fallback instead.

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WaffleLapkin commented Jul 9, 2026 • edited by Urgau Loading Uh oh! There was an error while loading. Please reload this page.

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rust-timer commented Jul 10, 2026

Overall result: ❌✅ regressions and improvements - please read:

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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WaffleLapkin commented Jul 9, 2026 •

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