Dead code warnings if the environment contains RUSTFLAGS

[dtolnay]

We currently write out a .cargo/config containing [build] rustflags = ["-A", "dead_code"]:

trybuild/src/run.rs

Lines 178 to 187 in 32f36c4

	let mut rustflags = Vec::new();
	for &lint in IGNORED_LINTS {
	rustflags.push("-A".to_owned());
	rustflags.push(lint.to_owned());
	}
	Config {
	build: Build { rustflags },
	}

This fails to take effect if the shell has RUSTFLAGS defined, as that overrides whatever is in .cargo/config.

Maybe instead of using .cargo/config we need to pass Cargo a RUSTFLAGS consisting of $RUSTFLAGS -A dead_code in the case that a RUSTFLAGS exists already.

FYI @jonhoo

[dtolnay]

Fixed in trybuild 1.0.5. Tested by running RUSTFLAGS="-C target-cpu=native" cargo test against jonhoo/proc-macro-workshop@382f32a and observing that the dead code warnings no longer appear.