Use target.rustflags instead of build.rustflags

[Kobzol]

Fixes: #56

[jonasmalacofilho]

One of the new tests failed on my system (see accompanying comment), but otherwise the PR works as designed. Thanks a lot!

[jonasmalacofilho]

A very minor note, but this assumes the tests run on a system that hasn't set -Ctarget-cpu=native on a matching target.rustflags... which, of course, mine has, causing the test to fail.

Suggestion:

diff --git a/tests/integration/pgo.rs b/tests/integration/pgo.rs
index a60e19828f27..3a33ee5f8ca0 100644
--- a/tests/integration/pgo.rs
+++ b/tests/integration/pgo.rs
@@ -292,13 +292,13 @@ fn test_override_build_rustflags_from_config() -> anyhow::Result<()> {
         ".cargo/config.toml",
         r#"
 [build]
-rustflags = ["-Ctarget-cpu=native"]
+rustflags = ["-Ctarget-cpu=imaginary"]
 "#,
     );
 
     let output = project.cmd(&["build", "--", "-v"]).run()?;
     println!("{}", output.stderr());
-    assert!(!output.stderr().contains("-Ctarget-cpu=native"));
+    assert!(!output.stderr().contains("-Ctarget-cpu=imaginary"));
     assert!(output.stderr().contains("-Cprofile-generate"));
     output.assert_ok();

[Kobzol]

This won't work, the test would fail because cargo does not know the imaginary target. It would be nice to disallow Cargo to load external config files, but I haven't found any way to do it.

[jonasmalacofilho]

Actually, i tested it (both with and without my local ~/.cargo/config.toml), and it works. The imaginary target is removed before it's sent to rustc (and AFAIK, and the test seems to confirm, cargo doesn't check it, only rustc).

[Kobzol]

Hmm, it failed with -Ctarget-cpu=foo on my machine, by Rust saying that it doesn't know the foo target (as expected).

Aah, I see now, I replaced it everywhere, it should have only been done for this single test. Sorry.

[Kobzol]

That's a good idea to add a CLI flag for allowing swapping between build/target rustflags! It can be implemented later though, I'd like to merge this as is, to let people try it.