Shallowly bail from coerce_unsized more

[compiler-errors]

We do a lot of coercion in HIR typeck. Most of the time we're just coercing a type to itself, but we always try coerce_unsized even if it's not necessary.

Let's avoid doing that by adding a fast path to coerce_unsized; see the comment in that function.

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit e0b68fe with merge 6a35eeb...

[compiler-errors]

TODO: test with a fast path for &str: CoerceUnsized<&str>.

[bors]

☀️ Try build successful - checks-actions
Build commit: 6a35eeb (6a35eeb37ae3dfbce8f5159463405fb81cddd28a)

[rust-timer]

Finished benchmarking commit (6a35eeb): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@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
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-0.7%	[-4.5%, -0.1%]	60
Improvements ✅ (secondary)	-6.7%	[-21.6%, -0.1%]	58
All ❌✅ (primary)	-0.7%	[-4.5%, -0.1%]	60

Max RSS (memory usage)

Results (primary -1.2%, secondary -5.0%)

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)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-1.2%	[-1.2%, -1.2%]	2
Improvements ✅ (secondary)	-5.8%	[-10.6%, -1.6%]	9
All ❌✅ (primary)	-1.2%	[-1.2%, -1.2%]	2

Cycles

Results (primary -4.5%, secondary -15.4%)

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)	-4.5%	[-5.5%, -3.2%]	6
Improvements ✅ (secondary)	-15.4%	[-25.4%, -6.1%]	27
All ❌✅ (primary)	-4.5%	[-5.5%, -3.2%]	6

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
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-	-	0

Bootstrap: 688.989s -> 689.016s (0.00%)
Artifact size: 371.96 MiB -> 372.08 MiB (0.03%)

[compiler-errors]

holy fuck lol

[compiler-errors]

@bors2 try @rust-timer queue

[rust-bors]

⌛ Trying commit 3d115ac with merge 1eb1a24…

To cancel the try build, run the command @bors2 try cancel.

[compiler-errors]

Anyways this is ready to review.

r? @lcnr

[rustbot]

r? @petrochenkov

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They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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[rust-log-analyzer]

The job aarch64-gnu-llvm-19-1 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

test [crashes] tests/crashes/122823.rs ... ok
test [crashes] tests/crashes/125841.rs ... ok
test [crashes] tests/crashes/125801.rs ... ok
test [crashes] tests/crashes/126268.rs ... ok
2025-06-24T17:37:08.303374Z ERROR compiletest::runtest: fatal error, panic: "crashtest no longer crashes/triggers ICE, hooray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`."
test [crashes] tests/crashes/126269.rs ... FAILED
test [crashes] tests/crashes/126443.rs ... ok
test [crashes] tests/crashes/126667.rs ... ok
test [crashes] tests/crashes/126680.rs ... ok
test [crashes] tests/crashes/126696.rs ... ok
---
test [crashes] tests/crashes/130970.rs ... ok
test [crashes] tests/crashes/131046.rs ... ok
test [crashes] tests/crashes/131292.rs ... ignored, only executed when the architecture is x86_64
test [crashes] tests/crashes/131295.rs ... ok
2025-06-24T17:37:09.173889Z ERROR compiletest::runtest: fatal error, panic: "crashtest no longer crashes/triggers ICE, hooray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`."
test [crashes] tests/crashes/131048.rs ... FAILED
test [crashes] tests/crashes/131052.rs ... ok
test [crashes] tests/crashes/131347.rs ... ok
test [crashes] tests/crashes/131373.rs ... ok
test [crashes] tests/crashes/131406.rs ... ok
---
test [crashes] tests/crashes/133808.rs ... ok
test [crashes] tests/crashes/133966.rs ... ok
test [crashes] tests/crashes/134061.rs ... ok
test [crashes] tests/crashes/133965.rs ... ok
2025-06-24T17:37:10.226182Z ERROR compiletest::runtest: fatal error, panic: "crashtest no longer crashes/triggers ICE, hooray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`."
test [crashes] tests/crashes/134217.rs ... FAILED
test [crashes] tests/crashes/131342.rs ... ok
test [crashes] tests/crashes/134174.rs ... ok
test [crashes] tests/crashes/134587.rs ... ok
test [crashes] tests/crashes/134615.rs ... ok
---
failures:

---- [crashes] tests/crashes/126269.rs stdout ----

error: crashtest no longer crashes/triggers ICE, hooray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`.

thread '[crashes] tests/crashes/126269.rs' panicked at src/tools/compiletest/src/runtest/crashes.rs:17:18:
fatal error
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- [crashes] tests/crashes/131048.rs stdout ----

error: crashtest no longer crashes/triggers ICE, hooray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`.

thread '[crashes] tests/crashes/131048.rs' panicked at src/tools/compiletest/src/runtest/crashes.rs:17:18:
fatal error

---- [crashes] tests/crashes/134217.rs stdout ----

error: crashtest no longer crashes/triggers ICE, hooray! Please give it a meaningful name, add a doc-comment to the start of the test explaining why it exists and move it to tests/ui or wherever you see fit. Adding 'Fixes #<issueNr>' to your PR description ensures that the corresponding ticket is auto-closed upon merge. If you want to see verbose output, set `COMPILETEST_VERBOSE_CRASHES=1`.

thread '[crashes] tests/crashes/134217.rs' panicked at src/tools/compiletest/src/runtest/crashes.rs:17:18:
fatal error

[rust-bors]

☀️ Try build successful (CI)
Build commit: 1eb1a24 (1eb1a24f58bb3d49e9be66d408afa9adf2b30a78, parent: 36b21637e93b038453924d3c66821089e71d8baa)

[compiler-errors]

Crashes should be fixed by #142976; they're only coincidentally fixed here by the fast path bc we don't end up trying coerce_unsized.

[rust-timer]

Finished benchmarking commit (1eb1a24): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@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%, 0.2%]	1
Regressions ❌ (secondary)	0.4%	[0.3%, 0.4%]	2
Improvements ✅ (primary)	-0.8%	[-4.5%, -0.1%]	80
Improvements ✅ (secondary)	-5.5%	[-21.6%, -0.0%]	73
All ❌✅ (primary)	-0.8%	[-4.5%, 0.2%]	81

Max RSS (memory usage)

Results (primary -0.9%, secondary -3.8%)

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

	mean	range	count
Regressions ❌ (primary)	1.3%	[1.3%, 1.3%]	1
Regressions ❌ (secondary)	3.9%	[3.9%, 4.0%]	2
Improvements ✅ (primary)	-3.1%	[-3.1%, -3.1%]	1
Improvements ✅ (secondary)	-5.2%	[-10.6%, -1.5%]	11
All ❌✅ (primary)	-0.9%	[-3.1%, 1.3%]	2

Cycles

Results (primary -2.0%, secondary -12.2%)

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)	-2.0%	[-4.1%, -0.8%]	26
Improvements ✅ (secondary)	-12.2%	[-24.0%, -0.9%]	31
All ❌✅ (primary)	-2.0%	[-4.1%, -0.8%]	26

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 689.383s -> 687.827s (-0.23%)
Artifact size: 371.94 MiB -> 372.03 MiB (0.02%)