rustdoc: save target modifiers

`rustdoc` was filling a `target_modifiers` variable, but it was not
using the result.

In turn, that means that trying to use a dependency that set a target
modifier fails.

For instance, running:

```sh
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)]
' | RUSTC_BOOTSTRAP=1 rustdoc --edition=2021 --target=aarch64-unknown-none-softfloat --sysroot=/dev/null -Zfixed-x18 --extern core=libcore.rmeta -
```

will fail with:

```text
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
```

Thus save the targets modifiers in `Options` to then pass it to the
session options, so that eventually the diff can be performed as expected
in `report_incompatible_target_modifiers()`.

Fixes: https://github.com/rust-lang/rust/issues/144521
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

2 files changed, 6 insertions, 0 deletions

diff --git a/src/librustdoc/config.rs b/src/librustdoc/config.rs
index 986390dbaa0..57456e906de 100644
--- a/src/librustdoc/config.rs
+++ b/src/librustdoc/config.rs
@@ -173,6 +173,9 @@ pub(crate) struct Options {
 
     /// Arguments to be used when compiling doctests.
     pub(crate) doctest_build_args: Vec<String>,
+
+    /// Target modifiers.
+    pub(crate) target_modifiers: BTreeMap<OptionsTargetModifiers, String>,
 }
 
 impl fmt::Debug for Options {
@@ -846,6 +849,7 @@ impl Options {
             unstable_features,
             expanded_args: args,
             doctest_build_args,
+            target_modifiers,
         };
         let render_options = RenderOptions {
             output,
diff --git a/src/librustdoc/core.rs b/src/librustdoc/core.rs
index bd57bb21e63..e89733b2f6d 100644
--- a/src/librustdoc/core.rs
+++ b/src/librustdoc/core.rs
@@ -214,6 +214,7 @@ pub(crate) fn create_config(
         scrape_examples_options,
         expanded_args,
         remap_path_prefix,
+        target_modifiers,
         ..
     }: RustdocOptions,
     render_options: &RenderOptions,
@@ -277,6 +278,7 @@ pub(crate) fn create_config(
         } else {
             OutputTypes::new(&[])
         },
+        target_modifiers,
         ..Options::default()
     };