rust: compile libcore with edition 2024 for 1.87+

Rust 1.87 (released on 2025-05-15) compiles core library with edition
2024 instead of 2021 [1]. Ensure that the edition matches libcore's
expectation to avoid potential breakage.

[ The `rustc_library`'s `skip_flags` change is needed for Rust < 1.87,
  since otherwise we would skip the `--edition=2021` we just added,
  ending up with no edition specified, and thus an old one. - Miguel ]

Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Link: rust-lang/rust#138162 [1]
Closes: Rust-for-Linux#1163
Signed-off-by: Gary Guo <gary@garyguo.net>
Reported-by: est31 <est31@protonmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20250517085600.2857460-1-gary@garyguo.net
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: nbdd0121

rust/Makefile

@@ -60,6 +60,12 @@ endif
 core-cfgs = \
     --cfg no_fp_fmt_parse
 
+ifeq ($(call rustc-min-version,108700),y)
+core-cfgs += --edition=2024
+else
+core-cfgs += --edition=2021
+endif
+
 # `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
@@ -106,7 +112,7 @@ rustdoc-macros: $(src)/macros/lib.rs FORCE
 
 # Starting with Rust 1.82.0, skipping `-Wrustdoc::unescaped_backticks` should
 # not be needed -- see https://github.com/rust-lang/rust/pull/128307.
-rustdoc-core: private skip_flags = -Wrustdoc::unescaped_backticks
+rustdoc-core: private skip_flags = -Wrustdoc::unescaped_backticks --edition=2021
 rustdoc-core: private rustc_target_flags = $(core-cfgs)
 rustdoc-core: $(RUST_LIB_SRC)/core/src/lib.rs FORCE
 	+$(call if_changed,rustdoc)
@@ -416,7 +422,7 @@ quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L
       cmd_rustc_library = \
 	OBJTREE=$(abspath $(objtree)) \
 	$(if $(skip_clippy),$(RUSTC),$(RUSTC_OR_CLIPPY)) \
-		$(filter-out $(skip_flags),$(rust_flags) $(rustc_target_flags)) \
+		$(filter-out $(skip_flags),$(rust_flags)) $(rustc_target_flags) \
 		--emit=dep-info=$(depfile) --emit=obj=$@ \
 		--emit=metadata=$(dir $@)$(patsubst %.o,lib%.rmeta,$(notdir $@)) \
 		--crate-type rlib -L$(objtree)/$(obj) \
@@ -483,7 +489,7 @@ $(obj)/helpers/helpers.o: $(src)/helpers/helpers.c $(recordmcount_source) FORCE
 $(obj)/exports.o: private skip_gendwarfksyms = 1
 
 $(obj)/core.o: private skip_clippy = 1
-$(obj)/core.o: private skip_flags = -Wunreachable_pub
+$(obj)/core.o: private skip_flags = -Wunreachable_pub --edition=2021
 $(obj)/core.o: private rustc_objcopy = $(foreach sym,$(redirect-intrinsics),--redefine-sym $(sym)=__rust$(sym))
 $(obj)/core.o: private rustc_target_flags = $(core-cfgs)
 $(obj)/core.o: $(RUST_LIB_SRC)/core/src/lib.rs \