[WIP] Use the dummy codegen backend #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. And required target features are not set by the dummy backend. Both of these require rustc side changes.
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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
commented
Oct 23, 2025
| // Patch `--extern` filenames, since Cargo sometimes passes stub `.rlib` files: | ||
| // https://github.com/rust-lang/miri/issues/1705 | ||
| if arg == "--extern" { | ||
| forward_patched_extern_arg(&mut args, &mut cmd); |
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The forward_patched_extern_arg function can likely be removed as the fake lib file hack has been removed.
bjorn3
commented
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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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.
I'm opening this draft PR for early feedback on this approach before I spent time fixing these issues on the rustc side. You can test this PR right now by removing the
crate-types = ["rlib", "dylib"]from~/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/Cargo.toml.