[Buildroot] [PATCH 1/4] package/rust: fix bootstrap with Rust≥1.83

[El Mehdi YOUNES]

Rust includes an option `download-ci-llvm` in config.toml that enables
downloading prebuilt LLVM binaries from Rust's CI infrastructure instead
of building LLVM from source. This option helps speed up the bootstrap
process and is enabled by default starting from Rust 1.83.

However, starting from commit [1] the bootstrap process performs
a strict check via the function
check_incompatible_options_for_ci_llvm().

This validation, implemented in the function
check_incompatible_options_for_ci_llvm(), checks for any incompatible
custom options such as `llvm.ldflags`, `llvm.cflags`, `targets`, etc.

If any of these are set locally and differ from the values used to build
the CI-provided LLVM, the build fails immediately with the error:

  ERROR: Setting `llvm.ldflags` is incompatible with
         `llvm.download-ci-llvm`.

Buildroot explicitly sets `llvm.ldflags` in rust.mk to ensure proper
host linking during the build of host-rust. Removing this setting may
introduce portability or reliability issues across toolchains.

To address the issue without compromising the build environment, this
patch disables the use of CI-provided LLVM by setting:

  [llvm]
  download-ci-llvm = false

This follows the recommendation from the Rust bootstrap script itself.

Note: this is a temporary workaround to restore compatibility with Rust
≥1.83. Other solutions will be investigated to avoid disabling the use
of prebuilt LLVM in the future, while preserving Buildroot’s reproducible
build setup.

[1] https://github.com/rust-lang/rust/commit/9df7680ecf698bf7087616b595774ee1023d3c7b

Signed-off-by: El Mehdi YOUNES <elmehdi.younes at smile.fr>
---
 package/rust/rust.mk | 1 +
 1 file changed, 1 insertion(+)

diff --git a/package/rust/rust.mk b/package/rust/rust.mk
index 29c03594d2..6481e40561 100644
--- a/package/rust/rust.mk
+++ b/package/rust/rust.mk
@@ -48,6 +48,7 @@ define HOST_RUST_CONFIGURE_CMDS
 		echo '[target.$(RUSTC_TARGET_NAME)]'; \
 		echo 'cc = "$(TARGET_CROSS)gcc"'; \
 		echo '[llvm]'; \
+		echo 'download-ci-llvm = false'; \
 		echo 'ninja = false'; \
 		echo 'ldflags = "$(HOST_LDFLAGS)"'; \
 	) > $(@D)/config.toml
-- 
2.43.0