`rustdoc` cannot use dependencies with target modifiers

[ojeda]

If one builds a dependency with a target modifier set, then rustdoc will fail to document a crate using that dependency.

Either rustdoc should skip checking target modifiers, or if it checks, then it should actually save the ones set. #133138 creates and fills a target_modifiers variable, but it is not used afterwards -- #144523 fixes that.

As a workaround, for older compilers, -Cunsafe-allow-abi-mismatch= can be used in rustdoc invocations.

This affects the Linux kernel when building the documentation with e.g. SCS enabled -- reported by Konrad Dybcio: https://lore.kernel.org/rust-for-linux/36cdc798-524f-4910-8b77-d7b9fac08d77@oss.qualcomm.com/

Cc: @azhogin

I tried this code:

RUSTC_BOOTSTRAP=1 rustc --edition=2024 --target=aarch64-unknown-none-softfloat --sysroot=/dev/null --emit=metadata -Zfixed-x18 --crate-type rlib --crate-name core $(rustc --print sysroot)/lib/rustlib/src/rust/library/core/src/lib.rs

echo '#![allow(internal_features)]
#![feature(compiler_builtins)]
#![compiler_builtins]
#![no_builtins]
#![no_std]
' | RUSTC_BOOTSTRAP=1 rustdoc --edition=2021 --target=aarch64-unknown-none-softfloat --sysroot=/dev/null -Zfixed-x18 --extern core=libcore.rmeta -

I expected to see this happen: no error.

Instead, this happened: error:

error: mixing `-Zfixed-x18` will cause an ABI mismatch in crate `rust_out`
  |
  = help: the `-Zfixed-x18` flag modifies the ABI so Rust crates compiled with different values of this flag cannot be used together safely
  = note: unset `-Zfixed-x18` in this crate is incompatible with `-Zfixed-x18=` in dependency `core`
  = help: set `-Zfixed-x18=` in this crate or unset `-Zfixed-x18` in `core`
  = help: if you are sure this will not cause problems, you may use `-Cunsafe-allow-abi-mismatch=fixed-x18` to silence this error

Meta

rustc --version --verbose:

rustc 1.88.0 (6b00bc388 2025-06-23)