[WIP] Use the dummy codegen backend #4648
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bjorn3
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Oct 23, 2025
| // `exported_non_generic_symbols` and `reachable_non_generics` provided by rustc always returns | ||
| // an empty result if `tcx.sess.opts.output_types.should_codegen()` is false. | ||
| // In addition we need to add #[used] symbols to exported_symbols for `lookup_link_section`. | ||
| local_providers.exported_non_generic_symbols = |tcx, LocalCrate| { |
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On second thought we may still need this for #[used].
bjorn3
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Oct 23, 2025
bjorn3
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Oct 23, 2025
| /// Also disable the MIR pass that inserts an alignment check on every pointer dereference. Miri | ||
| /// does that too, and with a better error message. | ||
| pub const MIRI_DEFAULT_ARGS: &[&str] = &[ | ||
| "-Zcodegen-backend=dummy", |
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The dummy codegen backend is a builtin codegen backend that is only capable of emitting rlibs containing the crate metadata. For every other crate type it emits an error after codegen. This is fine for miri as for --crate-type bin, miri exits before codegen.
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Oct 28, 2025
…apkin Allow codegen backends to indicate which crate types they support This way cargo will drop the unsupported crate types for crates that specify multiple crate types. This is a prerequisite for rust-lang/miri#4648.
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…, r=WaffleLapkin Allow codegen backends to indicate which crate types they support This way cargo will drop the unsupported crate types for crates that specify multiple crate types. This is a prerequisite for rust-lang/miri#4648.
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Oct 28, 2025
Rollup merge of #148177 - bjorn3:codegen_backend_crate_types, r=WaffleLapkin Allow codegen backends to indicate which crate types they support This way cargo will drop the unsupported crate types for crates that specify multiple crate types. This is a prerequisite for rust-lang/miri#4648.
This allows getting rid of all the argument mangling hacks as well as the exported_non_generic_symbols override hack by doing cargo build instead of cargo check. This is also a prerequisite for using native jit mode support in miri that I hope to add to rustc in the future to be used with cg_clif too. Currently this still has two issues: The dummy backend can't build dylibs, but cargo tries to build libstd as dylib. And required target features are not set by the dummy backend. Both of these require rustc side changes.
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☔ The latest upstream changes made this pull request unmergeable. Please resolve the merge conflicts. |
bjorn3
commented
Oct 29, 2025
| } else { | ||
| // For all other kinds of tests, we can just add our flag. | ||
| cmd.arg("--emit=metadata"); | ||
| cmd.arg("--emit=metadata,link"); |
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Is this still needed in the first place?
The library workspace sets this already nowadays.
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Oct 29, 2025
…es, r=JonathanBrouwer Handle default features and -Ctarget-features in the dummy backend This prevents a warning about ABI relevant target features not being set on x86 and arm. In addition it is required for miri to report correct features in is_*_feature_detected!() if miri switches to the dummy backend. Required for rust-lang/miri#4648
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This allows getting rid of all the argument mangling hacks as well as the exported_non_generic_symbols override hack by doing cargo build instead of cargo check. This is also a prerequisite for using native jit mode support in miri that I hope to add to rustc in the future to be used with cg_clif too.
Currently this still has two issues:
The dummy backend can't build dylibs, but cargo tries to build libstd as dylib.(rust-lang/rust#148177) And required target features are not set by the dummy backend. Both of these require rustc side changes.I'm opening this draft PR for early feedback on this approach before I spent time fixing these issues on the rustc side.