`cc` doesn't seem to respect `CARGO_TARGET_{TARGET_TRIPLE}_LINKER` on Android

[JohnTitor]

I've found cc >= 1.0.53 doesn't seem to respect CARGO_TARGET_{TARGET_TRIPLE}_LINKER.
In the libc crate, e pass aarch64-linux-android28-clang as CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER here: https://github.com/rust-lang/libc/blob/4d0643b769a4c1051908419e3cf25e6b46762dea/ci/docker/aarch64-linux-android/Dockerfile#L30-L33
But the test emits this failure (full log: https://github.com/rust-lang/libc/runs/1266042807?check_suite_focus=true):

  running: "aarch64-linux-android21-clang" "-O0" "-DANDROID" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-Wall" "-Wextra" "-Wall" "-Wextra" "-Werror" "-Wno-unused-parameter" "-Wno-type-limits" "-Wno-address-of-packed-member" "-Wno-unknown-warning-option" "-Wno-deprecated-declarations" "-D_GNU_SOURCE" "-o" "/checkout/target/aarch64-linux-android/debug/build/libc-test-1fbfc2c2b96c32dc/out/main.o" "-c" "/checkout/target/aarch64-linux-android/debug/build/libc-test-1fbfc2c2b96c32dc/out/main.c"
  cargo:warning=/checkout/target/aarch64-linux-android/debug/build/libc-test-1fbfc2c2b96c32dc/out/main.c:4442:24: error: use of undeclared identifier 'fmemopen'; did you mean 'freopen'?
  cargo:warning=                return fmemopen;
  cargo:warning=                       ^~~~~~~~
  cargo:warning=                       freopen
  cargo:warning=/android/ndk-aarch64/bin/../sysroot/usr/include/stdio.h:286:7: note: 'freopen' declared here
  cargo:warning=FILE* freopen(const char* __path, const char* __mode, FILE* __fp);
  cargo:warning=      ^

This means the build uses aarch64-linux-android21-clang, i.e., API 21 NDK.

I think #495 is the cause since it's released as 1.0.53, cc @Dushistov.

[alexcrichton]

AFAIK this crate has never read that env var, it's always expected CC to be set

[JohnTitor]

AFAIK this crate has never read that env var, it's always expected CC to be set

Ahh, yes, cc now reads CC_aarch64_linux_android (i.e. CC_TARGET) only, I guess?
As mentioned in #459, CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER affected the build with cc previously. And the behavior for that certainly changed between 1.0.52 and 1.0.53.
Thanks for clarifying, I'm going to close this as there's no actionable item here :)