run-pass/lib-defaults.rs warns of redundant linker flag

[pnkfelix]

This test:

rust/src/test/run-pass/lib-defaults.rs

Lines 13 to 18 in fccde00

	// compile-flags: -lrust_test_helpers
	#[link(name = "rust_test_helpers", kind = "static")]
	extern "C" {
	pub fn rust_dbg_extern_identity_u32(x: u32) -> u32;
	}

is causing the following compile-time output to stderr (on Linux; not sure about other hosts):

warning: redundant linker flag specified for library `rust_test_helpers`

This is the command line that compiletest is constructing:

command: "/home/pnkfelix/Dev/Mozilla/issue53764/rust-53764/objdir-dbgopt/build/x86_64-unknown-linux-gnu/stage1/bin/rustc" "/home/pnkfelix/Dev/Mozilla/issue53764/rust-53764/src/test/run-pass/lib-defaults.rs" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-C" "prefer-dynamic" "-o" "/home/pnkfelix/Dev/Mozilla/issue53764/rust-53764/objdir-dbgopt/build/x86_64-unknown-linux-gnu/test/run-pass/lib-defaults/a" "-Crpath" "-Zunstable-options" "-Lnative=/home/pnkfelix/Dev/Mozilla/issue53764/rust-53764/objdir-dbgopt/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-lrust_test_helpers" "-L" "/home/pnkfelix/Dev/Mozilla/issue53764/rust-53764/objdir-dbgopt/build/x86_64-unknown-linux-gnu/test/run-pass/lib-defaults/auxiliary"

I haven't delved deeply into what our requirements are for linkage directives. I'm guessing that rustc is able to construct the desired -lrust_test_helpers from the presence of the #[link(name = "rust_test_helpers", kind = "static")], but it might be good to know if that is the case on all of our platforms?

(I.e. maybe we should strive to remove the // compile-flags: -lrust_test_helpers directive from this test, or if that fails, maybe it conditional on which platform we are testing on...)

In any case I am mainly filling this ticket so I have something to link to in the test when I explain why I'm ignoring the stderr output from the compiler in this case.

[pnkfelix]

(the same output is issued for Mac OS X, which leads me to wonder if it is also issued on Windows? If that is the case, then I'd be inclined to add a .stderr file for this test rather than use // dont-check-compiler-stderr)

[pnkfelix]

I've confirmed that I've seen the warning issued on both Linux and OS X.