initial implementation of the darwin_objc unstable feature #145660

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jbatez wants to merge 4 commits into rust-lang:master from jbatez:darwin_objc

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jbatez commented Aug 20, 2025 •

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Tracking issue: #145496

rustbot assigned lcnr

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rustbot commented Aug 20, 2025

rustbot has assigned @lcnr.
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rustbot added A-attributes A-LLVM A-tidy O-apple S-waiting-on-review T-bootstrap T-compiler T-libs labels

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rustbot commented Aug 20, 2025

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann

rustbot assigned tmandry and unassigned lcnr

jbatez mentioned this pull request

Tracking Issue for darwin_objc #145496

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jdonszelmann commented Aug 20, 2025

I looked through the attribute changes and they look good to me. I can't judge the rest. @tmandry feel free to approve in both our names once you agree with the rest :)

madsmtm reviewed

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madsmtm left a comment

Looked through it, it seems to match what I tried to do with the objc2 macros those years back (which is not to say that it is correct, I had no idea what I was doing back then).

Also, it would be helpful with more comments, and a few codegen tests - I get why you haven't done that yet, it's bothersome while we're still somewhat discussing what the best approach is.

Note: I'm still a bit unsure that this is actually the correct approach going forward, I suspect there might be more value in the future from something like the define_in_every_cgu_used you first proposed?
At the very least we'll also need some way to get protocol references, we might also need metaclass references, and possibly more if we're to support static NSString and support fully statically declared classes.

But I think it's fine to move forwards with this in the current state, then we can always figure that sort of stuff out well before stabilisation.

compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs Outdated Show resolved Hide resolved

library/core/src/os/mod.rs Outdated Show resolved Hide resolved

compiler/rustc_hir/src/attrs/encode_cross_crate.rs Outdated Show resolved Hide resolved

compiler/rustc_feature/src/builtin_attrs.rs Outdated Show resolved Hide resolved

compiler/rustc_codegen_llvm/src/consts.rs Show resolved Hide resolved

compiler/rustc_codegen_llvm/src/base.rs Show resolved Hide resolved

compiler/rustc_codegen_llvm/src/consts.rs Show resolved Hide resolved

compiler/rustc_codegen_llvm/src/consts.rs

Comment on lines +592 to +601

    
                      let mut classrefs = self.objc_classrefs.borrow_mut();

                      if let Some(classref) = classrefs.get(&classname).copied() {

                          return classref;

                      }

Contributor

madsmtm Aug 20, 2025

Just so we're on the same page: It is basically this part that is the main feature here, right? I.e. that the symbol references are directly present in all translation units, and that they are deduplicated.

Author

jbatez Aug 21, 2025

Yep. This self.objc_classrefs is a per-CGU cache that allows us to deduplicate references within a codegen unit. But since this function ends up getting called in each CGU that references a class, we end up with definitions in all the referencing translations units and let the linker deduplicate across them. I'll add a comment to summarize that.

compiler/rustc_codegen_llvm/src/consts.rs

Comment on lines +614 to +623

    
                              let extern_sym = format!("OBJC_CLASS_$_{}", classname.as_str());

                              let extern_llty = self.get_objc_class_t();

                              self.declare_global(&extern_sym, extern_llty)

Contributor

madsmtm Aug 20, 2025

Q: Does this add the unnamed_addr attribute as well?

Author

jbatez Aug 21, 2025

No, should it? Clang doesn't generate unnamed_addr here, and my understanding is unnamed_addr is only useful for definitions with initializers, not external declarations like this.

compiler/rustc_codegen_llvm/src/consts.rs

    
                          });

                          set_global_alignment(self, g, self.tcx.data_layout.pointer_align().abi);

                          llvm::set_initializer(g, llval);

                          llvm::set_linkage(g, llvm::Linkage::PrivateLinkage);

Contributor

madsmtm Aug 20, 2025

Q: In some places, you use PrivateLinkage, in others InternalLinkage. I assume this is intentional?

Author

jbatez Aug 21, 2025

I just try to match what Clang does with these. My guess is they'd all work if they were all the same, but by mimicking Clang I figure it's more likely to continue to work with future linker versions.

Member

bjorn3 Aug 21, 2025

PrivateLinkage is basically InternalLinkage except that no symbol is generated in the object file.

Author

jbatez commented Aug 21, 2025

... At the very least we'll also need some way to get protocol references, we might also need metaclass references, and possibly more if we're to support static NSString and support fully statically declared classes.

But I think it's fine to move forwards with this in the current state, then we can always figure that sort of stuff out well before stabilisation.

NSString constants were next on my list and should be pretty straightforward. I haven't looked into protocols or metaclasses yet, but I did take a quick peek at static class definitions and, wow, that's a lot more complicated; I'm not personally planning to tackle that any time soon.

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jbatez force-pushed the darwin_objc branch from c08321b to 34b7d0f Compare

August 21, 2025 16:21

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rustbot commented Aug 21, 2025

This PR was rebased onto a different master 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.


          initial implementation of the darwin_objc unstable feature

3955a18

jbatez force-pushed the darwin_objc branch from 34b7d0f to 3955a18 Compare

August 21, 2025 16:22

Author

jbatez commented Aug 21, 2025

I added tests/codegen-llvm/darwin-objc.rs. In the process, I discovered, at some point, OBJC_CLASSLIST_REFERENCES_$_ and OBJC_SELECTOR_REFERENCES_ were getting optimized away and their initial values were getting used directly. I'm pretty sure this wasn't the case when I originally wrote this months ago. Adding them to llvm.compiler.used (which Clang does as well, fwiw) solved the problem. In the process, I had to convert compiler_used_statics to a RefCell, but I think that's fine.

jbatez added 2 commits

August 21, 2025 09:39


          verify objc classes and selectors are loaded from OBJC_CLASSLIST_REFE…

461dc01

…RENCES_$_ and OBJC_SELECTOR_REFERENCES_


          make tests/codegen-llvm/darwin-objc.rs no_core so we can test more ta…

5e7442e

…rgets

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          workaround doc generation error on non-darwin platforms

13fe4bc

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rust-log-analyzer commented Aug 22, 2025

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

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

test [codegen] tests/codegen-llvm/zip.rs ... ok

failures:

---- [codegen] tests/codegen-llvm/darwin-objc.rs#i686_apple_darwin stdout ----

error in revision `i686_apple_darwin`: compilation failed!
status: exit status: 101
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/codegen-llvm/darwin-objc.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=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--cfg" "i686_apple_darwin" "--check-cfg" "cfg(test,FALSE,i686_apple_darwin,x86_64_apple_darwin,aarch64_apple_darwin,i386_apple_ios,x86_64_apple_ios,armv7s_apple_ios,aarch64_apple_ios,aarch64_apple_ios_sim)" "-O" "-Cdebug-assertions=no" "-Zcodegen-source-order" "--emit" "llvm-ir" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/codegen-llvm/darwin-objc.i686_apple_darwin/darwin-objc.ll" "-A" "internal_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "--target" "i686-apple-darwin" "-Cpanic=abort" "-Cforce-unwind-tables=yes" "--extern" "minicore=/checkout/obj/build/aarch64-unknown-linux-gnu/test/codegen-llvm/darwin-objc.i686_apple_darwin/libminicore.rlib"
stdout: none
--- stderr -------------------------------
warning: unused import: `minicore::*`
##[warning]  --> /checkout/tests/codegen-llvm/darwin-objc.rs:33:5
   |
33 | use minicore::*;
   |     ^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default

Global variable initializer type does not match global variable type!
ptr @OBJC_MODULES
rustc-LLVM ERROR: Broken module found, compilation aborted!
------------------------------------------



failures:

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bors commented Aug 22, 2025

☔ The latest upstream changes (presumably #145728) made this pull request unmergeable. Please resolve the merge conflicts.

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