rust: kbuild: remove --remap-path-prefix workarounds

Commit 8cf5b3f836 ("Revert "kbuild, rust: use -fremap-path-prefix
to make paths relative"") removed `--remap-path-prefix` from the build
system, so the workarounds are not needed anymore.

Thus remove them.

Note that the flag has landed again in parallel in this cycle in
commit dda135077e ("rust: build: remap path to avoid absolute path"),
together with `--remap-path-scope=macro` [1]. However, they are gated on
`rustc-option-yn, --remap-path-scope=macro`, which means they are both
only passed starting with Rust 1.95.0 [2]:

  `--remap-path-scope` is only stable in Rust 1.95, so use `rustc-option`
  to detect its presence. This feature has been available as
  `-Zremap-path-scope` for all versions that we support; however due to
  bugs in the Rust compiler, it does not work reliably until 1.94. I opted
  to not enable it for 1.94 as it's just a single version that we missed.

In turn, that means the workarounds removed here should not be needed
again (even with the flag added again above), since:

  - `rustdoc` now recognizes the `--remap-path-prefix` flag since Rust
    1.81.0 [3] (even if it is still an unstable feature [4]).

  - The Internal Compiler Error [5] that the comment mentions was fixed in
    Rust 1.87.0 [6]. We tested that was the case in a previous version
    of this series by making the workaround conditional [7][8].

...which are both older versions than Rust 1.95.0.

We will still need to skip `--remap-path-scope` for `rustdoc` though,
since `rustdoc` does not support that one yet [4].

Link: https://github.com/rust-lang/rust/issues/111540 [1]
Link: https://github.com/rust-lang/rust/pull/147611 [2]
Link: https://github.com/rust-lang/rust/pull/107099 [3]
Link: https://doc.rust-lang.org/nightly/rustdoc/unstable-features.html#--remap-path-prefix-remap-source-code-paths-in-output [4]
Link: https://github.com/rust-lang/rust/issues/138520 [5]
Link: https://github.com/rust-lang/rust/pull/138556 [6]
Link: https://lore.kernel.org/rust-for-linux/20260401114540.30108-9-ojeda@kernel.org/ [7]
Link: https://lore.kernel.org/rust-for-linux/20260401114540.30108-10-ojeda@kernel.org/ [8]
Link: https://patch.msgid.link/20260405235309.418950-2-ojeda@kernel.org
Acked-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/Makefile

@@ -145,14 +145,10 @@ rustdoc_modifiers_workaround := $(if $(call rustc-min-version,108800),-Cunsafe-a
 # Similarly, for doctests (https://github.com/rust-lang/rust/issues/146465).
 doctests_modifiers_workaround := $(rustdoc_modifiers_workaround)$(if $(call rustc-min-version,109100),$(comma)sanitizer)
 
-# `rustc` recognizes `--remap-path-prefix` since 1.26.0, but `rustdoc` only
-# since Rust 1.81.0. Moreover, `rustdoc` ICEs on out-of-tree builds since Rust
-# 1.82.0 (https://github.com/rust-lang/rust/issues/138520). Thus workaround both
-# issues skipping the flag. The former also applies to `RUSTDOC TK`.
 quiet_cmd_rustdoc = RUSTDOC $(if $(rustdoc_host),H, ) $<
       cmd_rustdoc = \
 	OBJTREE=$(abspath $(objtree)) \
-	$(RUSTDOC) $(filter-out $(skip_flags) --remap-path-prefix=%,$(if $(rustdoc_host),$(rust_common_flags),$(rust_flags))) \
+	$(RUSTDOC) $(filter-out $(skip_flags),$(if $(rustdoc_host),$(rust_common_flags),$(rust_flags))) \
 		$(rustc_target_flags) -L$(objtree)/$(obj) \
 		-Zunstable-options --generate-link-to-definition \
 		--output $(rustdoc_output) \
@@ -338,7 +334,7 @@ quiet_cmd_rustdoc_test_kernel = RUSTDOC TK $<
 	rm -rf $(objtree)/$(obj)/test/doctests/kernel; \
 	mkdir -p $(objtree)/$(obj)/test/doctests/kernel; \
 	OBJTREE=$(abspath $(objtree)) \
-	$(RUSTDOC) --test $(filter-out --remap-path-prefix=%,$(rust_flags)) \
+	$(RUSTDOC) --test $(rust_flags) \
 		-L$(objtree)/$(obj) --extern ffi --extern pin_init \
 		--extern kernel --extern build_error --extern macros \
 		--extern bindings --extern uapi \