Next steps for FnDef binder changes (instantiate most FnDef binders) by addiesh · Pull Request #159403 · rust-lang/rust

addiesh · 2026-07-16T16:48:59Z

This change is a part of the ongoing effort to clean up binder/turbofishing behavior in the compiler (#156581).

This PR instantiates binders that were previously created via ty::Binder::dummy() with meaningfully bound vars.

r? oli-obk

Call me Turbofishmael.
Some years ago- never mind how long precisely-
having little or no money in my purse,
and nothing particular to interest me in C++,
I thought I would sail about a little and see the rusty part of the world.

rustbot · 2026-07-16T16:49:05Z

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @oli-obk (or someone else) some time within the next two weeks.

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addiesh · 2026-07-16T19:53:37Z

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)
Click to see the possible cause of the failure (guessed by this bot)

what happened during the rebase??? lemme rollback and try see what broke

oli-obk · 2026-07-20T16:04:34Z

+            TraitItemKind::Fn(_, _) => Ty::new_fn_def(
+                tcx,
+                def_id.to_def_id(),
+                ty::Binder::bind_with_vars(
+                    ty::GenericArgs::identity_for_item(tcx, def_id),
+                    tcx.late_bound_vars(item.hir_id()),
+                ),
+            ),


preexisting, but considering how duplicated this is across the various invocations, add a a closure above for creating such a fn def and just invoke it in each site.

View changes since the review

oli-obk · 2026-07-20T16:05:12Z

-                let args = ty::GenericArgs::identity_for_item(tcx, def_id);
-                // FIXME(156581): actually instantiate the binder correctly (turbofishing/fndef changes)
-                Ty::new_fn_def(tcx, ctor.to_def_id(), ty::Binder::dummy(args))
+                let fn_sig = tcx.fn_sig(ctor.to_def_id()).instantiate_identity().skip_norm_wip();


this is still going through fn_sig, take the late_bound_vars route here (and everywhere below), too

View changes since the review

you'll probably want to add a late_bound_vars_for_def_id that internally just invokes local_def_id_to_hir_id and then uses that HirId for late_bound_vars.

oli-obk · 2026-07-20T16:08:52Z

        Ok(a_ty)
    }

+    fn relate_ty_args_bound(


how much is relate_ty_args used? Could we just have a single function, or if that's too verbose, implement relate_ty_args in the trait with a default impl that calls relate_ty_args_bound and remove all impl overrides of relate_ty_args?

View changes since the review

oli-obk · 2026-07-20T16:14:22Z

 }
+
+pub fn combine_ty_args_bound<Infcx, I, R>(
+    infcx: &Infcx,


same thing here, avoid this duplication, maybe just by implementing combine_ty_args in terms of combine_ty_args_bound, but ideally just by having bound vars on combine_ty_args and adjusting all the call sites (if it's not too many). If the latter is reasonably doable, then do it in a separate commit before all the other commits in this PR

View changes since the review

rustbot · 2026-07-20T16:18:57Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

addiesh · 2026-07-20T20:19:21Z

@rustbot ready :3

oli-obk · 2026-07-21T14:54:53Z

@bors try @rust-timer queue

Next steps for FnDef binder changes (instantiate most FnDef binders)

rust-bors · 2026-07-21T17:00:29Z

☀️ Try build successful (CI)
Build commit: b2c0bf0 (b2c0bf0eafe2c8ec02d1dc04054ef086c27e56a4)
Base parent: f65b272 (f65b272fc92b1e7527dea4a224430a249ab25c2d)

rust-timer · 2026-07-21T18:49:12Z

Finished benchmarking commit (b2c0bf0): comparison URL.

Overall result: ❌ regressions - 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 rustc-perf
@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)	0.3%	[0.2%, 0.4%]	16
Regressions ❌ (secondary)	0.3%	[0.1%, 0.7%]	12
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	0.3%	[-0.2%, 0.4%]	17

Max RSS (memory usage)

Results (primary 0.9%, secondary -4.4%)

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

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.6%, 1.1%]	5
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.4%	[-4.4%, -4.4%]	1
All ❌✅ (primary)	0.9%	[0.6%, 1.1%]	5

Cycles

Results (primary 2.3%, secondary 3.7%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	3.7%	[3.1%, 4.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.3%	[2.3%, 2.3%]	1

Binary size

Results (primary 0.2%, secondary 0.3%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 0.5%]	77
Regressions ❌ (secondary)	0.3%	[0.0%, 1.0%]	64
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.0%, 0.5%]	77

Bootstrap: 487.415s -> 488.409s (0.20%)
Artifact size: 387.80 MiB -> 388.33 MiB (0.14%)

rustbot · 2026-07-23T08:49:38Z

This PR was rebased onto a different main 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.

rust-bors · 2026-07-26T10:31:28Z

☔ The latest upstream changes (presumably #159942) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

rustbot assigned oli-obk Jul 16, 2026

addiesh force-pushed the call-me-turbofishmael branch 3 times, most recently from 014b214 to d780036 Compare July 16, 2026 19:40