[PERF] partition shims into CGU of their characteristic types, if possible

[compiler-errors]

We throw all non-InstanceKind::Item mono items into the same fallback CGU. This seems to have been a thing since #32779. This PR investigates: what happens to incremental codegen if we didn't do that, but tried a bit harder to partition shims into the same CGU as their characteristic types?

r? @ghost

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 8ce6e6e with merge 01c9c97...

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 3695a33 with merge 155e6b9...

[rust-log-analyzer]

The job x86_64-gnu-llvm-19 failed! Check out the build log: (web) (plain)

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#18 exporting to docker image format
#18 sending tarball 21.0s done
#18 DONE 27.7s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-19]
[CI_JOB_NAME=x86_64-gnu-llvm-19]
debug: `DISABLE_CI_RUSTC_IF_INCOMPATIBLE` configured.
---
sccache: Listening on address 127.0.0.1:4226
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-19', '--enable-llvm-link-shared', '--set', 'rust.randomize-layout=true', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'build.print-step-timings', '--enable-verbose-tests', '--set', 'build.metrics', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'rust.lld=false', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--set', 'gcc.download-ci-gcc=true', '--enable-new-symbol-mangling']
configure: build.build          := x86_64-unknown-linux-gnu
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-19/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.randomize-layout := True
configure: rust.thin-lto-import-instr-limit := 10
---
---- [codegen-units] tests/codegen-units/partitioning/local-drop-glue.rs stdout ----

The following items were assigned to wrong codegen units:

fn <Struct as std::ops::Drop>::drop
  expected: local_drop_glue-fallback.cgu[External] 
  actual:   local_drop_glue[External] 

fn std::ptr::drop_in_place::<(u32, Struct)> - shim(Some((u32, Struct)))
  expected: local_drop_glue-fallback.cgu[External] 
  actual:   local_drop_glue.volatile[External] 

fn std::ptr::drop_in_place::<Outer> - shim(Some(Outer))
  expected: local_drop_glue-fallback.cgu[External] 
  actual:   local_drop_glue.volatile[External] 

fn std::ptr::drop_in_place::<Struct> - shim(Some(Struct))
  expected: local_drop_glue-fallback.cgu[External] 
  actual:   local_drop_glue.volatile[External] 

fn std::ptr::drop_in_place::<mod1::Struct2> - shim(Some(mod1::Struct2))
  expected: local_drop_glue-fallback.cgu[External] 
  actual:   local_drop_glue-mod1.volatile[External] 


thread '[codegen-units] tests/codegen-units/partitioning/local-drop-glue.rs' panicked at src/tools/compiletest/src/runtest/codegen_units.rs:103:13:
explicit panic
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- [codegen-units] tests/codegen-units/partitioning/vtable-through-const.rs stdout ----

The following items were assigned to wrong codegen units:

fn <mod1::NeedsDrop as std::ops::Drop>::drop
  expected: vtable_through_const-fallback.cgu[External] 
  actual:   vtable_through_const-mod1[External] 

fn std::ptr::drop_in_place::<mod1::NeedsDrop> - shim(Some(mod1::NeedsDrop))
  expected: vtable_through_const-fallback.cgu[External] 
  actual:   vtable_through_const-mod1.volatile[External] 


thread '[codegen-units] tests/codegen-units/partitioning/vtable-through-const.rs' panicked at src/tools/compiletest/src/runtest/codegen_units.rs:103:13:
explicit panic

---- [codegen-units] tests/codegen-units/partitioning/extern-drop-glue.rs stdout ----

The following items were assigned to wrong codegen units:

fn std::ptr::drop_in_place::<LocalStruct> - shim(Some(LocalStruct))
  expected: extern_drop_glue-fallback.cgu[External] 
  actual:   extern_drop_glue.volatile[External] 

fn std::ptr::drop_in_place::<cgu_extern_drop_glue::Struct> - shim(Some(cgu_extern_drop_glue::Struct))
  expected: extern_drop_glue-fallback.cgu[External] 
  actual:   cgu_extern_drop_glue-in-extern_drop_glue.volatile[External] 

fn std::ptr::drop_in_place::<mod1::LocalStruct> - shim(Some(mod1::LocalStruct))
  expected: extern_drop_glue-fallback.cgu[External] 
  actual:   extern_drop_glue-mod1.volatile[External] 


thread '[codegen-units] tests/codegen-units/partitioning/extern-drop-glue.rs' panicked at src/tools/compiletest/src/runtest/codegen_units.rs:103:13:
explicit panic

[bors]

☀️ Try build successful - checks-actions
Build commit: 155e6b9 (155e6b96fcb60833ff6194a1e370d66371850f47)

[rust-timer]

Finished benchmarking commit (155e6b9): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	4.9%	[0.2%, 17.9%]	30
Regressions ❌ (secondary)	4.3%	[0.2%, 23.6%]	8
Improvements ✅ (primary)	-2.0%	[-4.2%, -0.3%]	5
Improvements ✅ (secondary)	-1.2%	[-2.7%, -0.2%]	8
All ❌✅ (primary)	3.9%	[-4.2%, 17.9%]	35

Max RSS (memory usage)

Results (primary 0.4%, secondary -0.7%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.4%	[1.2%, 9.9%]	8
Regressions ❌ (secondary)	2.5%	[2.3%, 2.8%]	3
Improvements ✅ (primary)	-3.7%	[-5.0%, -1.2%]	6
Improvements ✅ (secondary)	-3.9%	[-5.2%, -2.1%]	3
All ❌✅ (primary)	0.4%	[-5.0%, 9.9%]	14

Cycles

Results (primary 6.6%, secondary 5.3%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	8.2%	[0.8%, 19.7%]	18
Regressions ❌ (secondary)	9.1%	[0.8%, 26.4%]	4
Improvements ✅ (primary)	-2.7%	[-4.2%, -1.7%]	3
Improvements ✅ (secondary)	-2.3%	[-2.9%, -1.7%]	2
All ❌✅ (primary)	6.6%	[-4.2%, 19.7%]	21

Binary size

Results (primary 0.4%, secondary 0.5%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.0%, 1.8%]	59
Regressions ❌ (secondary)	4.6%	[0.0%, 14.4%]	10
Improvements ✅ (primary)	-0.3%	[-0.7%, -0.0%]	25
Improvements ✅ (secondary)	-2.5%	[-5.4%, -0.0%]	14
All ❌✅ (primary)	0.4%	[-0.7%, 1.8%]	84

Bootstrap: 775.708s -> 776.167s (0.06%)
Artifact size: 366.25 MiB -> 366.31 MiB (0.02%)