Adding Target Specification and Patching Crates

for an existing target using `--print target-spec-json`: ``` rustc -Z unstable-options --target=wasm32-unknown-unknown --print target-spec-json ``` Save that JSON to a file and modify it as appropriate for your target. ### Adding a target specification Once you have filled out a JSON specification and been able to compile somewhat successfully, you can copy the specification into the compiler itself. You will need to add a line to the big table inside of the `supported_targets` macro in the `rustc_target::spec` module. You will then add a corresponding file for your new target containing a `target` function. Look for existing targets to use as examples. After adding your target to the `rustc_target` crate you may want to add `core`, `std`, ... with support for your new target. In that case you will probably need access to some `target_*` cfg. Unfortunately when building with stage0 (a precompiled compiler), you'll get an error that the target cfg is unexpected because stage0 doesn't know about the new target specification and we pass `--check-cfg` in order to tell it to check. To fix the errors you will need to manually add the unexpected value to the different `Cargo.toml` in `library/{std,alloc,core}/Cargo.toml`. Here is an example for adding `NEW_TARGET_ARCH` as `target_arch`: *`library/std/Cargo.toml`*: ```diff [lints.rust.unexpected_cfgs] level = "warn" check-cfg = [ 'cfg(bootstrap)', - 'cfg(target_arch, values("xtensa"))', + # #[cfg(bootstrap)] NEW_TARGET_ARCH + 'cfg(target_arch, values("xtensa", "NEW_TARGET_ARCH"))', ``` To use this target in bootstrap, we need to explicitly add the target triple to the `STAGE0_MISSING_TARGETS` list in `src/bootstrap/src/core/sanity.rs`. This is necessary because the default compiler bootstrap uses does not recognize the new target we just added. Therefore, it should be added to `STAGE0_MISSING_TARGETS` so that the bootstrap is aware that this target is not yet supported by the stage0 compiler. ```diff const STAGE0_MISSING_TARGETS: &[&str] = &[ + "NEW_TARGET_TRIPLE" ]; ``` ## Patching crates You may need to make changes to crates that the compiler depends on,

This section details how to incorporate a target specification into the Rust compiler by adding it to the `supported_targets` macro and creating a corresponding target file with a `target` function. It also explains how to handle `target_*` cfg options when building with stage0, which involves manually adding the new target to the `Cargo.toml` files of the `std`, `alloc`, and `core` libraries. Additionally, the process of patching crates that the compiler depends on is mentioned.