Avoid Vec allocation in TyCtxt::mk_place_elem by kevinheavey · Pull Request #155778 · rust-lang/rust

kevinheavey · 2026-04-25T15:00:52Z

mk_place_elem appends a single PlaceElem to an existing (interned) projection. The current implementation copies the projection into a fresh Vec, pushes the new element, and re-interns the slice, which allocates on every call.

Feed the elements through mk_place_elems_from_iter so that CollectAndApply's hand-unrolled stack fast path (up to 9 elements, in rustc_type_ir::interner) kicks in for the common case of short projections and the Vec allocation is skipped entirely. The behavior is identical for longer projections (the fast path falls back to a Vec internally).

`mk_place_elem` appends a single `PlaceElem` to an existing (interned) projection. The current implementation copies the projection into a fresh `Vec`, pushes the new element, and re-interns the slice, which allocates on the heap on every call. Feed the elements through `mk_place_elems_from_iter` so that `CollectAndApply`'s hand-unrolled stack fast path (up to 9 elements, in `rustc_type_ir::interner`) kicks in for the common case of short projections and the `Vec` allocation is skipped entirely. The behavior is identical for longer projections (the fast path falls back to a `Vec` internally).

rustbot · 2026-04-25T15:00:57Z

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cjgillot · 2026-04-26T16:00:40Z

Seems reasonable, let's see what perf says.

@bors try @rust-timer queue

…c, r=<try> Avoid Vec allocation in TyCtxt::mk_place_elem

rust-bors · 2026-04-26T18:11:16Z

☀️ Try build successful (CI)
Build commit: 7a0916d (7a0916d081b78b4e94da1fd8af8f3a954ad92e9c, parent: c7fe5e9d1eb0ef8cc033961956d60f8e2e91e741)

rust-timer · 2026-04-26T18:51:33Z

Finished benchmarking commit (7a0916d): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (primary 0.4%, secondary -1.8%)

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

	mean	range	count
Regressions ❌ (primary)	1.5%	[0.7%, 2.3%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.9%	[-1.9%, -1.9%]	1
Improvements ✅ (secondary)	-1.8%	[-1.8%, -1.8%]	1
All ❌✅ (primary)	0.4%	[-1.9%, 2.3%]	3

Cycles

Results (secondary -4.3%)

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)	-4.3%	[-4.3%, -4.3%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.0%, 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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	11
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	4
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	11

Bootstrap: 488.641s -> 488.25s (-0.08%)
Artifact size: 393.90 MiB -> 393.84 MiB (-0.02%)

JohnTitor

LGTM! Waiting for @cjgillot checks in.

View changes since this review

cjgillot · 2026-04-26T22:22:26Z

@bors r+

rust-bors · 2026-04-26T22:22:28Z

📌 Commit 5cd9d92 has been approved by cjgillot

It is now in the queue for this repository.

JohnTitor · 2026-04-26T22:42:44Z

@bors r-

rust-bors · 2026-04-26T22:42:48Z

This pull request was unapproved.

View changes since this unapproval

JohnTitor · 2026-04-26T22:43:03Z

@bors r=cjgillot,JohnTitor

rust-bors · 2026-04-26T22:43:06Z

📌 Commit 5cd9d92 has been approved by cjgillot,JohnTitor

It is now in the queue for this repository.

Rollup of 12 pull requests Successful merges: - #149624 (Fix requires_lto targets needing lto set in cargo) - #155317 (`std::io::Take`: Clarify & optimize `BorrowedBuf::set_init` usage.) - #155579 (Make Rcs and Arcs use pointer comparison for unsized types) - #155588 (Implement more traits for FRTs) - #155708 (Fix heap overflow in slice::join caused by misbehaving Borrow) - #155778 (Avoid Vec allocation in TyCtxt::mk_place_elem) - #151014 (std: sys: process: uefi: Add program searching) - #155682 (Add boxing suggestions for `impl Trait` return type mismatches) - #155770 (Avoid misleading closure return type note) - #155818 (Convert attribute `FinalizeFn` to fn pointer) - #155829 (rustc_attr_parsing: use a `try {}` in `or_malformed`) - #155835 (couple of `crate_name` cleanups)

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 25, 2026

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rust-bors Bot pushed a commit that referenced this pull request Apr 26, 2026

Auto merge of #155778 - kevinheavey:perf-mk-place-elem-avoid-vec-allo…

7a0916d

…c, r=<try> Avoid Vec allocation in TyCtxt::mk_place_elem

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 26, 2026

JohnTitor approved these changes Apr 26, 2026

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jhpratt mentioned this pull request Apr 27, 2026

Rollup of 12 pull requests #155851

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rust-bors Bot merged commit c20a92e into rust-lang:main Apr 27, 2026
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rustbot added this to the 1.97.0 milestone Apr 27, 2026

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rust-timer commented Apr 26, 2026

Overall result: no relevant changes - no action needed

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