Prepare to make `rustc_trivial_field_reads` usable in user code by tehforsch · Pull Request #164217 · rust-lang/rust

tehforsch · 2026-10-12T16:05:13Z

Zulip discussion:
#t-lang > diagnostics attribute to ignore code for unused_code lints
#t-compiler/help > #160621 (Allow ignoring fns for dead code analysis)

Open questions I have:

Some of the Debug impls are not pointed out as usage sites. For example this test:

#[derive(Debug)]
enum TupleVariant {
    Variant1(i32), //~ ERROR: variant `Variant1` is never constructed
    Variant2,
}

fn main() {
	let _ = TupleVariant::Variant2;
}

now loses its the note in the error message:

note: `TupleVariant` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis

but in my mind this also makes sense - the Debug impl never constructs that Variant1 and even if rustc_trivial_field_reads is removed from the debug impl, the "variant never constructed" warning would still occur. Note that this doesn't apply to Clone impls, which do technically construct that variant although that brings me to point 2

If I am reading the code right, the whole point of the ignored_variant_stack is to make sure we don't mark a variant of an enum live from within a match arm that matches on that exact enum variant. But isn't that exactly what the Clone impl does? The inner match arm of it would look like Variant1(x) => Variant1(x). I looked into necessary_variants and found that it directly ignores the kind of variant we encounter, namely PatKind::TupleStruct(hir::QPath::TypeRelative(...)) but I don't really understand why and whether that's a bug or whether I am missing something

panstromek · 2026-10-12T16:57:28Z

@bors try @rust-timer queue

Prepare to make `rustc_trivial_field_reads` usable in user code

rust-bors · 2026-10-12T17:56:23Z

☀️ Try build successful (CI)
Build commit: e790d10 (e790d107e7868a817cc9ec9592a9b2d5fc634365)
Base parent: 0093837 (0093837afdf14ec7069c81da19cd51ed7d3476d9)

rust-timer · 2026-10-12T18:42:04Z

Finished benchmarking commit (e790d10): 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%	[0.1%, 0.7%]	47
Regressions ❌ (secondary)	0.4%	[0.2%, 0.8%]	19
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.1%, 0.7%]	47

Max RSS (memory usage)

Results (primary 5.1%)

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

	mean	range	count
Regressions ❌ (primary)	5.1%	[5.1%, 5.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	5.1%	[5.1%, 5.1%]	1

Cycles

Results (primary 2.6%, secondary 2.1%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.2%, 3.0%]	3
Regressions ❌ (secondary)	2.1%	[2.1%, 2.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.6%	[2.2%, 3.0%]	3

Binary size

Results (secondary 0.0%)

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)	0.0%	[0.0%, 0.0%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 488.816s -> 489.518s (0.14%)
Artifact size: 406.86 MiB -> 406.85 MiB (-0.00%)

mejrs · 2026-10-12T21:39:29Z

@bors try @rust-timer queue

Prepare to make `rustc_trivial_field_reads` usable in user code

rust-bors · 2026-10-12T22:39:24Z

☀️ Try build successful (CI)
Build commit: a64a0d1 (a64a0d13d3c0d5e6be02384b77719f7a3d8a651f)
Base parent: d64de46 (d64de46d0aab619b69efe68a99d43d11434b6924)

rust-timer · 2026-10-13T00:10:04Z

Finished benchmarking commit (a64a0d1): 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 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%	[0.1%, 0.6%]	48
Regressions ❌ (secondary)	0.4%	[0.2%, 0.9%]	16
Improvements ✅ (primary)	-0.4%	[-0.6%, -0.2%]	3
Improvements ✅ (secondary)	-0.5%	[-0.7%, -0.2%]	35
All ❌✅ (primary)	0.2%	[-0.6%, 0.6%]	51

Max RSS (memory usage)

Results (secondary 2.8%)

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)	2.8%	[2.8%, 2.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -1.3%)

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)	-1.3%	[-1.3%, -1.3%]	1
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%, 0.2%]	74
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	53
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	74

Bootstrap: 490.869s -> 490.424s (-0.09%)
Artifact size: 406.63 MiB -> 406.91 MiB (0.07%)

mejrs · 2026-10-13T10:59:11Z

Let's run it again, i think the improvements are spurious.

@bors try @rust-timer queue

Allow `rustc_trivial_field_reads` on inherent methods Add test for multiple usage sites

tehforsch · 2026-11-10T09:16:53Z

Thanks for the reviews :) Squashed into two commits
r? @mu001999

rustbot · 2026-11-10T09:16:56Z

Requested reviewer is already assigned to this pull request.

Please choose another assignee.

mu001999 · 2026-11-10T09:24:56Z

Thanks! @bors r=mejrs,mu001999 rollup=never

rust-bors · 2026-11-10T09:25:00Z

📌 Commit 70426a6 has been tentatively approved by mejrs,mu001999

It will be put into the queue for this repository once PR CI succeeds.

JonathanBrouwer · 2026-11-10T11:13:01Z

@bors rollup=iffy
Perf effect is reasonable enough this can enter a rollup

rust-bors · 2026-11-10T11:58:08Z

⌛ Testing commit 70426a6 with merge 2e70d67...

Workflow: https://github.com/rust-lang/rust/actions/runs/42811773796

Prepare to make `rustc_trivial_field_reads` usable in user code Zulip discussion: [#t-lang > diagnostics attribute to ignore code for unused_code lints](https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/diagnostics.20attribute.20to.20ignore.20code.20for.20unused_code.20lints/with/614949580) [#t-compiler/help > #160621 (Allow ignoring fns for dead code analysis)](https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/.23160621.20.28Allow.20ignoring.20fns.20for.20dead.20code.20analysis.29/with/629007836) Open questions I have: 1. Some of the `Debug` impls are not pointed out as usage sites. For example this test: ```rust #[derive(Debug)] enum TupleVariant { Variant1(i32), //~ ERROR: variant `Variant1` is never constructed Variant2, } fn main() { let _ = TupleVariant::Variant2; } ``` now loses its the note in the error message: ``` note: `TupleVariant` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis ``` but in my mind this also makes sense - the `Debug` impl never constructs that `Variant1` and even if `rustc_trivial_field_reads` is removed from the debug impl, the "variant never constructed" warning would still occur. Note that this doesn't apply to `Clone` impls, which do technically construct that variant although that brings me to point 2 2. If I am reading the code right, the whole point of the `ignored_variant_stack` is to make sure we don't mark a variant of an enum live from within a match arm that matches on that exact enum variant. But isn't that exactly what the `Clone` impl does? The inner match arm of it would look like `Variant1(x) => Variant1(x)`. I looked into `necessary_variants` and found that it directly ignores the kind of variant we encounter, namely `PatKind::TupleStruct(hir::QPath::TypeRelative(...))` but I don't really understand why and whether that's a bug or whether I am missing something

JonathanBrouwer · 2026-11-10T12:14:42Z

@bors yield
Yielding to rollup

rust-bors · 2026-11-10T12:14:46Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/42811773796

The next pull request likely to be tested is #165752.

…1999 Prepare to make `rustc_trivial_field_reads` usable in user code Zulip discussion: [#t-lang > diagnostics attribute to ignore code for unused_code lints](https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/diagnostics.20attribute.20to.20ignore.20code.20for.20unused_code.20lints/with/614949580) [#t-compiler/help > rust-lang#160621 (Allow ignoring fns for dead code analysis)](https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/.23160621.20.28Allow.20ignoring.20fns.20for.20dead.20code.20analysis.29/with/629007836) Open questions I have: 1. Some of the `Debug` impls are not pointed out as usage sites. For example this test: ```rust #[derive(Debug)] enum TupleVariant { Variant1(i32), //~ ERROR: variant `Variant1` is never constructed Variant2, } fn main() { let _ = TupleVariant::Variant2; } ``` now loses its the note in the error message: ``` note: `TupleVariant` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis ``` but in my mind this also makes sense - the `Debug` impl never constructs that `Variant1` and even if `rustc_trivial_field_reads` is removed from the debug impl, the "variant never constructed" warning would still occur. Note that this doesn't apply to `Clone` impls, which do technically construct that variant although that brings me to point 2 2. If I am reading the code right, the whole point of the `ignored_variant_stack` is to make sure we don't mark a variant of an enum live from within a match arm that matches on that exact enum variant. But isn't that exactly what the `Clone` impl does? The inner match arm of it would look like `Variant1(x) => Variant1(x)`. I looked into `necessary_variants` and found that it directly ignores the kind of variant we encounter, namely `PatKind::TupleStruct(hir::QPath::TypeRelative(...))` but I don't really understand why and whether that's a bug or whether I am missing something

…uwer Rollup of 24 pull requests Successful merges: - #165709 (remove 'x miri' smoke test from PR CI) - #164217 (Prepare to make `rustc_trivial_field_reads` usable in user code) - #165569 (`Move` trait feature gate) - #165575 (Upgrade bundled musl to 1.2.6) - #163241 (Add enclosing item context when pointing at `FnMut` bound) - #163992 (add `core::arch::{x86, x86_64}::x87_f80`) - #165047 (Change the span building of paths from first-to-last to chained) - #165458 (fix incorrect byte count suffix on MSVC) - #165490 (Mark the compile unit for `-Zdebuginfo-for-profiling`) - #165586 (Remove `backend_optimization_level`.) - #165646 (fix NumScalableVectors::from_field_count) - #165655 (Unignore passing GCC backend tests) - #165740 (miri subtree update) - #165753 (avoid Terminator:None in MIR inliner) - #153261 (Partially stabilize `ptr_alignment_type` as `alignment_type`) - #164559 (Windows: fix Dir::remove and Dir::rename following symlinks) - #165562 (stop using gce in a test) - #165681 (Add safety requirements for Pattern and Searcher) - #165682 (interpret: ensure newly created unions have uninit in their padding) - #165693 (Move `check_attr` to attribute parsing) - #165715 (windows: check for null before wide encoding) - #165717 (mpsc_sync::stream_send_recv_stress: wait for all threads to finish) - #165733 (docs(num): remove needless whitespaces) - #165739 (remove NOP File::flush from fs tests) Failed merges: - #165727 (Normalize "beta release channel" in `const-supertrait-unstable` test) - #164218 (Tweak private re-export errors to point at correct span and have more context)

Rollup merge of #164217 - tehforsch:fix-160621, r=mejrs,mu001999 Prepare to make `rustc_trivial_field_reads` usable in user code Zulip discussion: [#t-lang > diagnostics attribute to ignore code for unused_code lints](https://rust-lang.zulipchat.com/#narrow/channel/213817-t-lang/topic/diagnostics.20attribute.20to.20ignore.20code.20for.20unused_code.20lints/with/614949580) [#t-compiler/help > #160621 (Allow ignoring fns for dead code analysis)](https://rust-lang.zulipchat.com/#narrow/channel/182449-t-compiler.2Fhelp/topic/.23160621.20.28Allow.20ignoring.20fns.20for.20dead.20code.20analysis.29/with/629007836) Open questions I have: 1. Some of the `Debug` impls are not pointed out as usage sites. For example this test: ```rust #[derive(Debug)] enum TupleVariant { Variant1(i32), //~ ERROR: variant `Variant1` is never constructed Variant2, } fn main() { let _ = TupleVariant::Variant2; } ``` now loses its the note in the error message: ``` note: `TupleVariant` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis ``` but in my mind this also makes sense - the `Debug` impl never constructs that `Variant1` and even if `rustc_trivial_field_reads` is removed from the debug impl, the "variant never constructed" warning would still occur. Note that this doesn't apply to `Clone` impls, which do technically construct that variant although that brings me to point 2 2. If I am reading the code right, the whole point of the `ignored_variant_stack` is to make sure we don't mark a variant of an enum live from within a match arm that matches on that exact enum variant. But isn't that exactly what the `Clone` impl does? The inner match arm of it would look like `Variant1(x) => Variant1(x)`. I looked into `necessary_variants` and found that it directly ignores the kind of variant we encounter, namely `PatKind::TupleStruct(hir::QPath::TypeRelative(...))` but I don't really understand why and whether that's a bug or whether I am missing something

…uwer Rollup of 24 pull requests Successful merges: - rust-lang/rust#165709 (remove 'x miri' smoke test from PR CI) - rust-lang/rust#164217 (Prepare to make `rustc_trivial_field_reads` usable in user code) - rust-lang/rust#165569 (`Move` trait feature gate) - rust-lang/rust#165575 (Upgrade bundled musl to 1.2.6) - rust-lang/rust#163241 (Add enclosing item context when pointing at `FnMut` bound) - rust-lang/rust#163992 (add `core::arch::{x86, x86_64}::x87_f80`) - rust-lang/rust#165047 (Change the span building of paths from first-to-last to chained) - rust-lang/rust#165458 (fix incorrect byte count suffix on MSVC) - rust-lang/rust#165490 (Mark the compile unit for `-Zdebuginfo-for-profiling`) - rust-lang/rust#165586 (Remove `backend_optimization_level`.) - rust-lang/rust#165646 (fix NumScalableVectors::from_field_count) - rust-lang/rust#165655 (Unignore passing GCC backend tests) - rust-lang/rust#165740 (miri subtree update) - rust-lang/rust#165753 (avoid Terminator:None in MIR inliner) - rust-lang/rust#153261 (Partially stabilize `ptr_alignment_type` as `alignment_type`) - rust-lang/rust#164559 (Windows: fix Dir::remove and Dir::rename following symlinks) - rust-lang/rust#165562 (stop using gce in a test) - rust-lang/rust#165681 (Add safety requirements for Pattern and Searcher) - rust-lang/rust#165682 (interpret: ensure newly created unions have uninit in their padding) - rust-lang/rust#165693 (Move `check_attr` to attribute parsing) - rust-lang/rust#165715 (windows: check for null before wide encoding) - rust-lang/rust#165717 (mpsc_sync::stream_send_recv_stress: wait for all threads to finish) - rust-lang/rust#165733 (docs(num): remove needless whitespaces) - rust-lang/rust#165739 (remove NOP File::flush from fs tests) Failed merges: - rust-lang/rust#165727 (Normalize "beta release channel" in `const-supertrait-unstable` test) - rust-lang/rust#164218 (Tweak private re-export errors to point at correct span and have more context)

rust-timer · 2026-11-11T13:49:41Z

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (b076b10): comparison URL.

Overall result: ❌ regressions - please read:

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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%	[0.1%, 0.3%]	16
Regressions ❌ (secondary)	0.3%	[0.1%, 0.9%]	21
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	2
All ❌✅ (primary)	0.2%	[0.1%, 0.3%]	16

Max RSS (memory usage)

Results (primary -2.7%)

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)	-2.7%	[-2.7%, -2.7%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.7%	[-2.7%, -2.7%]	1

Cycles

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

Binary size

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

Artifact size: 406.28 MiB -> 406.40 MiB (0.03%)

…uwer Rollup of 24 pull requests Successful merges: - rust-lang/rust#165709 (remove 'x miri' smoke test from PR CI) - rust-lang/rust#164217 (Prepare to make `rustc_trivial_field_reads` usable in user code) - rust-lang/rust#165569 (`Move` trait feature gate) - rust-lang/rust#165575 (Upgrade bundled musl to 1.2.6) - rust-lang/rust#163241 (Add enclosing item context when pointing at `FnMut` bound) - rust-lang/rust#163992 (add `core::arch::{x86, x86_64}::x87_f80`) - rust-lang/rust#165047 (Change the span building of paths from first-to-last to chained) - rust-lang/rust#165458 (fix incorrect byte count suffix on MSVC) - rust-lang/rust#165490 (Mark the compile unit for `-Zdebuginfo-for-profiling`) - rust-lang/rust#165586 (Remove `backend_optimization_level`.) - rust-lang/rust#165646 (fix NumScalableVectors::from_field_count) - rust-lang/rust#165655 (Unignore passing GCC backend tests) - rust-lang/rust#165740 (miri subtree update) - rust-lang/rust#165753 (avoid Terminator:None in MIR inliner) - rust-lang/rust#153261 (Partially stabilize `ptr_alignment_type` as `alignment_type`) - rust-lang/rust#164559 (Windows: fix Dir::remove and Dir::rename following symlinks) - rust-lang/rust#165562 (stop using gce in a test) - rust-lang/rust#165681 (Add safety requirements for Pattern and Searcher) - rust-lang/rust#165682 (interpret: ensure newly created unions have uninit in their padding) - rust-lang/rust#165693 (Move `check_attr` to attribute parsing) - rust-lang/rust#165715 (windows: check for null before wide encoding) - rust-lang/rust#165717 (mpsc_sync::stream_send_recv_stress: wait for all threads to finish) - rust-lang/rust#165733 (docs(num): remove needless whitespaces) - rust-lang/rust#165739 (remove NOP File::flush from fs tests) Failed merges: - rust-lang/rust#165727 (Normalize "beta release channel" in `const-supertrait-unstable` test) - rust-lang/rust#164218 (Tweak private re-export errors to point at correct span and have more context)

tehforsch force-pushed the fix-160621 branch from c06e80c to cf722b6 Compare October 12, 2026 16:12