Fix the wasm32-unknown-unknown target feature/cfg bug - #161718
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nnethercote wants to merge 4 commits into
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Fix the wasm32-unknown-unknown target feature/cfg bug#161718nnethercote wants to merge 4 commits into
nnethercote wants to merge 4 commits into
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Due to some bad ordering of session/config initialization code -- more about that in subsequent commits -- `cfg(target_has_threads)` fails to be set for the `wasm32-unknown-unknown` platform when `-Ctarget-feature=+atomics` is specified. This commit modifies a test to demonstrate the bug; as written the test passes.
- `parse_check_cfg` has a single call site and is followed by a call to `fill_well_known`. - `parse_cfg` has two call sites and in both cases is followed by a call to `build_configuration`. This commit moves the follow-up calls into the functions, simplifying `run_compiler`.
Currently it modifies the `Session` and the `Cfg` (and the `Cfg` afterwards is put into the `Session`). It also takes a `CodegenBackend`. Those are some heavyweight arguments. This commit moves the `Session` modifications to the caller so the `&mut Session` isn't necessary, and passes in the `TargetConfig` instead of the whole `CodegenBackend`, plus some other small arguments. `add_configuration` ends up more clearly about modifying the `Cfg`. This is a step towards untangling session/backend initialization.
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LLM disclosure: an LLM identified this bug and made some suggestions on how to fix it and test the fix. I wrote all the code and text myself. |
RalfJung
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Aug 25, 2026
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Ah nice, with my recent refactors here we can now funnel everything through |
Currently, `parse_cfg` calls `build_configuration`, which calls `default_configuration`, which calls `sess.target.singlethread(&sess.internal_target_features)`. But `sess.internal_target_features` hasn't been set at this point and is empty! This commit moves the setting of `sess.internal_target_features` before the `parse_cfg` call to fix this ordering bug. This results in the `cfg(target_has_threads)` being correctly set on `wasm32-unknown-unknown` when `-Ctarget-feature=+atomics` is specified. Note: I have plans to make this kind of ordering bug difficult/impossible in a follow-up (e.g. rust-lang#161432).
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Urgau
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| /// Converts strings provided as `--check-cfg [specs]` into a `CheckCfg`. | ||
| pub(crate) fn parse_check_cfg(dcx: DiagCtxtHandle<'_>, specs: Vec<String>) -> CheckCfg { | ||
| pub(crate) fn parse_check_cfg(sess: &Session, specs: Vec<String>) -> CheckCfg { |
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A bit surprising that we take a Session to do some parsing, maybe the functions should be renamed to parse_and_build_...?
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Due to some bad ordering of session/config initialization code,
cfg(target_has_threads)fails to be set for thewasm32-unknown-unknownplatform when-Ctarget-feature=+atomicsis specified. This PR fixes the problem. Details in individual commits.r? @Mark-Simulacrum