Externally implementable items #146348

jdonszelmann · 2025-09-08T21:04:17Z

Supersedes #140010
Tracking issue: #125418

Getting started:

#![feature(eii)]

#[eii(eii1)]
pub fn decl1(x: u64) 
// body optional (it's the default)
{
    println!("default {x}");
}

// in another crate, maybe
#[eii1]
pub fn decl2(x: u64) {
    println!("explicit {x}");
}

fn main() {
    decl1(4);
}

tiny perf regression, underlying issue makes multiple things in the compiler slow, not just EII, planning to solve those separately.
No codegen_gcc support, they don't have bindings for weak symbols yet but could
No windows support yet for weak definitions

This PR merges the implementation of EII for just llvm + not windows, doesn't yet contain like a new panic handler implementation or alloc handler. With this implementation, it would support implementing the panic handler in terms of EII already since it requires no default implementation so no weak symbols

The PR has been open in various forms for about a year now, but I feel that having some implementation merged to build upon

jdonszelmann · 2025-09-08T21:12:23Z

compiler/rustc_passes/src/reachable.rs

        // `SymbolExportLevel::Rust` export level but may end up being exported in dylibs.
        || codegen_attrs.flags.contains(CodegenFnAttrFlags::USED_COMPILER)
        || codegen_attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER)
+        // Right now, the only way to get "foreign item symbol aliases" is by being an EII-implementation.


TODO: do we need this:

option 1: we do and implement our own reachability analysis based on it separate from RUSTC_STD_INTERNAL_SYMBOL.

~~option 2: we don't cause we also use RUSTC_STD_INTERNAL_SYMBOL on all EIIs. In that case we should rename STD_INTERNAL_SYMBOL~~

option 3: same as option 2 but maybe we shouldn't use RUSTC_STD_INTERNAL_SYMBOL in the first place; we leave it as-is and create a new flag thta's like it but specifically for EIIs and named something else like RUSTC_USED_BY_NONDIRECT_DEP_CRATE or whatever nicer name we can think of that (as long as it doesn't use STD since it's not really specific to that anymore)

despite STD_INTERNAL_SYMBOL working, and we can test with it for a bit, this is not the way we should do it. i.e. not option 2.

jdonszelmann · 2025-09-08T21:14:02Z

tests/ui/eii/default/local_crate_explicit.rs

+
+#[eii(eii1)]
+pub fn decl1(x: u64) {
+    //~^ WARN function `decl1` is never used


It took me a little bit to decide what is the expected behavior here. We might want to remove this warning. Defaults are in some way expected to be unused. However, here we can prove it because the explicit impl is in the same crate hence the warning.

removing the warning actually makes the code a tiny bit trickier

jdonszelmann · 2025-09-09T20:10:50Z

@bors2 try @rust-timer queue

[DONT MERGE] externally implementable items

rust-bors · 2025-09-09T22:16:01Z

☀️ Try build successful (CI)
Build commit: d5a6633 (d5a66337e06d3b77a1ab8f6793055535195c3ae9, parent: 364da5d88d772fa40fb20353443595385443ac25)

rust-timer · 2025-09-09T23:40:37Z

Finished benchmarking commit (d5a6633): comparison URL.

Overall result: ❌ regressions - please read the text below

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	0.3%	[0.1%, 1.5%]	19
Regressions ❌ (secondary)	0.4%	[0.1%, 1.0%]	44
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	4
All ❌✅ (primary)	0.3%	[0.1%, 1.5%]	19

Max RSS (memory usage)

Results (primary 1.7%, secondary 3.9%)

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

	mean	range	count
Regressions ❌ (primary)	1.7%	[0.6%, 2.3%]	6
Regressions ❌ (secondary)	4.7%	[1.1%, 11.2%]	16
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.9%, -1.2%]	2
All ❌✅ (primary)	1.7%	[0.6%, 2.3%]	6

Cycles

Results (primary 3.1%, 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)	3.1%	[3.1%, 3.1%]	1
Regressions ❌ (secondary)	3.2%	[2.5%, 3.6%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.8%	[-4.1%, -2.0%]	9
All ❌✅ (primary)	3.1%	[3.1%, 3.1%]	1

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)	0.0%	[0.0%, 0.1%]	93
Regressions ❌ (secondary)	0.1%	[0.0%, 0.3%]	61
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.1%]	93

Bootstrap: 468.052s -> 471.14s (0.66%)
Artifact size: 387.52 MiB -> 390.25 MiB (0.71%)

jdonszelmann · 2025-09-10T02:44:26Z

Well, shit. I think I know some fixes but I hoped this wouldn't happen

compiler/rustc_hir_analysis/src/check/wfcheck.rs

fmease · 2025-12-01T15:57:23Z

As discussed privately, renaming pending @bors r-

EII collection queries

rustbot · 2025-12-01T16:17:33Z

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.

jdonszelmann · 2025-12-01T16:20:43Z

@bors r=lcnr,oli-obk

bors · 2025-12-01T16:20:45Z

📌 Commit cbd6653 has been approved by lcnr,oli-obk

It is now in the queue for this repository.

Externally implementable items Supersedes #140010 Tracking issue: #125418 Getting started: ```rust #![feature(eii)] #[eii(eii1)] pub fn decl1(x: u64) // body optional (it's the default) { println!("default {x}"); } // in another crate, maybe #[eii1] pub fn decl2(x: u64) { println!("explicit {x}"); } fn main() { decl1(4); } ``` - tiny perf regression, underlying issue makes multiple things in the compiler slow, not just EII, planning to solve those separately. - No codegen_gcc support, they don't have bindings for weak symbols yet but could - No windows support yet for weak definitions This PR merges the implementation of EII for just llvm + not windows, doesn't yet contain like a new panic handler implementation or alloc handler. With this implementation, it would support implementing the panic handler in terms of EII already since it requires no default implementation so no weak symbols The PR has been open in various forms for about a year now, but I feel that having some implementation merged to build upon

bors · 2025-12-02T02:54:58Z

⌛ Testing commit cbd6653 with merge 02d7db1...

rust-log-analyzer · 2025-12-02T04:21:59Z

The job aarch64-apple failed! Check out the build log: (web) (plain enhanced) (plain)

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

---- [ui] tests/ui/eii/default/call_default.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/stage2/bin/rustc" "/Users/runner/work/rust/rust/tests/ui/eii/default/call_default.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/Users/runner/.cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/Users/runner/work/rust/rust/vendor" "--sysroot" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/stage2" "--target=aarch64-apple-darwin" "--check-cfg" "cfg(test,FALSE)" "-O" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-o" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/eii/default/call_default/a" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/Users/runner/work/rust/rust/build/aarch64-apple-darwin/native/rust-test-helpers" "-L" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/eii/default/call_default/auxiliary"
stdout: none
--- stderr -------------------------------
error: linking with `cc` failed: exit status: 1
   |
   = note:  "cc" "/var/folders/8s/1jkm89h96qjdtjr7q3bll2vh0000gn/T/rustc5zoobT/symbols.o" "<2 object files omitted>" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/eii/default/call_default/auxiliary/libdecl_with_default.rlib" "<sysroot>/lib/rustlib/aarch64-apple-darwin/lib/{libstd-*,libpanic_unwind-*,libobject-*,libmemchr-*,libaddr2line-*,libgimli-*,libcfg_if-*,librustc_demangle-*,libstd_detect-*,libhashbrown-*,librustc_std_workspace_alloc-*,libminiz_oxide-*,libadler2-*,libunwind-*,liblibc-*,librustc_std_workspace_core-*,liballoc-*,libcore-*,libcompiler_builtins-*}.rlib" "-lSystem" "-lc" "-lm" "-arch" "arm64" "-mmacosx-version-min=11.0.0" "-L" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/native/rust-test-helpers" "-L" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/eii/default/call_default/auxiliary" "-o" "/Users/runner/work/rust/rust/build/aarch64-apple-darwin/test/ui/eii/default/call_default/a" "-Wl,-dead_strip" "-nodefaultlibs" "-Wl,-rpath,@loader_path/../../../../../stage2/lib/rustlib/aarch64-apple-darwin/lib"
   = note: some arguments are omitted. use `--verbose` to show all linker arguments
   = note: Undefined symbols for architecture arm64:
             "_decl1", referenced from:
                 __RNvCsjMfW7Z0Cixe_12call_default4main in a.call_default.e65edcad8a79f346-cgu.0.rcgu.o
           ld: symbol(s) not found for architecture arm64
           clang: error: linker command failed with exit code 1 (use -v to see invocation)
           

error: aborting due to 1 previous error
------------------------------------------

bors · 2025-12-02T04:22:32Z

💔 Test failed - checks-actions