[PATCH v4 2/2] arm64: rust: Enable Rust support for AArch64

[Matthew Maurer]

On Fri, Oct 20, 2023 at 8:53 AM Jamie Cunliffe <Jamie.Cunliffe at arm.com> wrote:
> +KBUILD_RUSTFLAGS += --target=aarch64-unknown-none -Ctarget-feature="-neon"
If I have aarch64-unknown-none target support already installed in my
local Rust compiler (either build a custom compiler and enable the
target, as is my real case, or as the common case, run `rustup target
add aarch64-unknown-none`), this produces a build error I didn't get
from the previous target.json approach:

error[E0464]: multiple candidates for `rmeta` dependency `core` found
  |
  = note: candidate #1:
/usr/local/google/home/mmaurer/android/linux/out/cache/65c6e8ea/common/rust/libcore.rmeta
  = note: candidate #2:
/usr/local/google/home/mmaurer/android/linux/prebuilts/rust/linux-x86/1.72.1/lib/rustlib/aarch64-unknown-none/lib/libcore-690113bb4de6f7fb.rlib

Are we sure that we want to try to use `--target=aarch64-unknown-none`
rather than a custom target.json? If our libcore and their libcore may
be compiled differently (and they can be, because we are controlling
additional compilation options), it seems to me that we ought to be
defining our own target.

If we really do want to go ahead with using targets directly form
rustc, this patch should add `--sysroot=/dev/null` or a similar
override to `rustc_library_cmd` and `rust_common_cmd` to prevent rustc
from picking up any already installed sysroot libraries.