Use the .drectve section for exporting symbols from dlls on Windows #142568

bjorn3 · 2025-06-16T09:24:31Z

While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used.

Necessary for and extracted out of #142366.

Thanks @dpaoliello for figuring out this weird quirk of link.exe!

rustbot · 2025-06-16T09:24:35Z

r? @wesleywiser

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rustbot · 2025-06-16T09:24:36Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

compiler/rustc_codegen_ssa/src/back/linker.rs

bjorn3 · 2025-06-16T13:29:19Z

@bors2 try jobs=x86_64-msvc-1,x86_64-msvc-2

rust-bors · 2025-06-16T13:29:23Z

⌛ Trying commit 5e2efb6 with merge 7bae343…

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

Use the .drectve section for exporting symbols from dlls on Windows While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used. Necessary for and extracted out of #142366. Thanks `@dpaoliello` for figuring out this weird quirk of link.exe! try-job: x86_64-msvc-1 try-job: x86_64-msvc-2

rust-bors · 2025-06-16T14:51:23Z

💔 Test failed

CI ❌

bjorn3 · 2025-06-16T15:29:16Z

@bors2 try jobs=x86_64-msvc-1,x86_64-msvc-2

rust-bors · 2025-06-16T15:29:20Z

⌛ Trying commit c353c7d with merge 3dadabc…

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

Use the .drectve section for exporting symbols from dlls on Windows While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used. Necessary for and extracted out of #142366. Thanks `@dpaoliello` for figuring out this weird quirk of link.exe! try-job: x86_64-msvc-1 try-job: x86_64-msvc-2

dpaoliello

LGTM.

One overall design question: is it supported for folks to run the linker themselves AND specify their own def file? A couple of my changes broke the Chromium and Fuschia folks, so I'm a little paranoid now about changing anything to do with linking that may affect them.

compiler/rustc_codegen_ssa/src/back/linker.rs

bjorn3 · 2025-06-16T16:58:23Z

One overall design question: is it supported for folks to run the linker themselves AND specify their own def file?

This PR on it's own has no effect at all on people running the linker themself. Both the .DEF file and the symbols.o file are only generated when rustc itself runs the linker.

As for a future PR that will use weak definitions, those symbols are not meant to be exported from a staticlib anyway. The only case that could break is if someone (Chromium) tries to link rlibs together into a dll and wants to export all symbols, which we don't officially support anyway. The Chromium people have had to adaot their build system for rustc changes anyway and I expect it to not all that hard to adapt to said future weak symbol usage too.

rust-bors · 2025-06-16T17:09:05Z

💔 Test failed

CI ❌

dpaoliello · 2025-06-16T17:32:23Z

💔 Test failed

CI ❌

The test executable failed with 0xc0000135 which is STATUS_DLL_NOT_FOUND

Looks like this is a test for shaking out unused functions: is the compiler supposed to do that, or is it relying on the linker?

bjorn3 · 2025-06-17T09:00:00Z

@bors2 try jobs=x86_64-msvc-1,x86_64-msvc-2

rust-bors · 2025-06-17T09:00:05Z

⌛ Trying commit 8c88ff4 with merge e97fbdf…

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

Use the .drectve section for exporting symbols from dlls on Windows While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used. Necessary for and extracted out of #142366. Thanks `@dpaoliello` for figuring out this weird quirk of link.exe! try-job: x86_64-msvc-1 try-job: x86_64-msvc-2

rust-bors · 2025-06-17T11:30:12Z

☀️ Try build successful (CI)
Build commit: e97fbdf (e97fbdf63e618534ac0929eef0229de9fb477dad, parent: 55d436467c351b56253deeba209ae2553d1c243f)

bjorn3 · 2025-06-17T11:42:13Z

Squashed everything and reverted some now unnecessary changes.

@rustbot ready

wesleywiser · 2025-06-20T18:30:35Z

@bors r+

wesleywiser · 2025-06-24T16:16:13Z

@bors r+

bors · 2025-06-24T16:16:16Z

📌 Commit d40bee8 has been approved by wesleywiser

It is now in the queue for this repository.

…=wesleywiser Use the .drectve section for exporting symbols from dlls on Windows While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used. Necessary for and extracted out of rust-lang#142366. Thanks `@dpaoliello` for figuring out this weird quirk of link.exe!

bors · 2025-06-24T19:44:19Z

☔ The latest upstream changes (presumably #142962) made this pull request unmergeable. Please resolve the merge conflicts.

While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used.

bjorn3 · 2025-06-26T11:45:55Z

@rustbot ready

wesleywiser · 2025-06-30T22:07:07Z

@bors r+

bors · 2025-06-30T22:07:10Z

📌 Commit 7539730 has been approved by wesleywiser

It is now in the queue for this repository.

…=wesleywiser Use the .drectve section for exporting symbols from dlls on Windows While it would be reasonable to expect the Windows linker to handle linker args in the .drectve section identical to cli arguments, as it turns out exporting weak symbols only works when the /EXPORT is in the .drectve section, not when it is a linker argument or when a .DEF file is used. Necessary for and extracted out of rust-lang#142366. Thanks `@dpaoliello` for figuring out this weird quirk of link.exe!

Rollup of 12 pull requests Successful merges: - #141829 (Specialize sleep_until implementation for unix (except mac)) - #141847 (Explain `TOCTOU` on the top of `std::fs`, and reference it in functions) - #142138 (Add `Vec::into_chunks`) - #142321 (Expose elf abi on ppc64 targets) - #142568 (Use the .drectve section for exporting symbols from dlls on Windows) - #142886 (ci: aarch64-gnu: Stop skipping `panic_abort_doc_tests`) - #143038 (avoid suggesting traits from private dependencies) - #143194 (fix bitcast of single-element SIMD vectors) - #143206 (Align attr fixes) - #143258 (Don't recompute `DisambiguatorState` for every RPITIT in trait definition) - #143260 (Use the correct export kind for __rust_alloc_error_handler_should_panic) - #143274 (ci: support optional jobs) r? `@ghost` `@rustbot` modify labels: rollup

jhpratt · 2025-07-02T01:54:23Z

Not certain, but this seems likely to be the cause of #143305 (comment)

@bors r-

rustbot assigned wesleywiser Jun 16, 2025

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 Jun 16, 2025

bjorn3 force-pushed the windows_symbols_o_export branch from e63c76b to 94ec88c Compare June 16, 2025 09:25

bjorn3 commented Jun 16, 2025

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bjorn3 force-pushed the windows_symbols_o_export branch from 8c88ff4 to fc88eae Compare June 17, 2025 09:01

bjorn3 force-pushed the windows_symbols_o_export branch from fc88eae to e6aa201 Compare June 17, 2025 11:41

This was referenced Jun 17, 2025

[WIP] Use weak aliases for the allocator shim #142366

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[win][aarch64] Fix linking statics on Arm64EC, take 2 #142742

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bjorn3 force-pushed the windows_symbols_o_export branch from 809b270 to d40bee8 Compare June 21, 2025 16:59

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workingjubilee mentioned this pull request Jun 24, 2025

Rollup of 15 pull requests #142971

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bjorn3 force-pushed the windows_symbols_o_export branch from d40bee8 to aa98f4d Compare June 24, 2025 19:47

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bjorn3 force-pushed the windows_symbols_o_export branch from aa98f4d to 7539730 Compare June 24, 2025 20:03

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Rollup of 12 pull requests #143305

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Use the .drectve section for exporting symbols from dlls on Windows #142568

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Use the .drectve section for exporting symbols from dlls on Windows #142568

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