Remove most of the logic in fake rustc once `-Ztarget-applies-to-host=true` lands

[jyn514]

rust-lang/cargo#10395 made it possible to apply RUSTFLAGS to build scripts and proc-macros. Bootstrap currently special-cases this in fake rustc:

rust/src/bootstrap/bin/rustc.rs

Lines 110 to 135 in 06460fe

	// FIXME(rust-lang/cargo#5754) we shouldn't be using special env vars
	// here, but rather Cargo should know what flags to pass rustc itself.
	// Override linker if necessary.
	if let Ok(host_linker) = env::var("RUSTC_HOST_LINKER") {
	cmd.arg(format!("-Clinker={}", host_linker));
	}
	if env::var_os("RUSTC_HOST_FUSE_LD_LLD").is_some() {
	cmd.arg("-Clink-args=-fuse-ld=lld");
	}
	if let Ok(s) = env::var("RUSTC_HOST_CRT_STATIC") {
	if s == "true" {
	cmd.arg("-C").arg("target-feature=+crt-static");
	}
	if s == "false" {
	cmd.arg("-C").arg("target-feature=-crt-static");
	}
	}
	if stage == "0" {
	// Cargo doesn't pass RUSTFLAGS to proc_macros:
	// https://github.com/rust-lang/cargo/issues/4423
	// Set `--cfg=bootstrap` explicitly instead.
	cmd.arg("--cfg=bootstrap");
	}

We should switch to using -Ztarget-applies-to-host=true on the next cargo update.
cc @Mark-Simulacrum @ehuss @jonhoo (thank you again @jonhoo for fixing this! ❤️)

@rustbot label: +A-rustbuild +C-enhancement

[jonhoo]

Ah, so, unfortunately, -Ztarget-applies-to-host=true doesn't quite do what it says on the box. I discuss that issue in rust-lang/cargo#10395 (comment). Basically, you'll want to move towards =false, not =true, since =false is the "more correct and consistent" behavior. And then you'd want something like -Zhost-config on top of that. Alternatively my proposal in rust-lang/cargo#10395 (comment) was to adopt something along the lines of https://github.com/alexcrichton/cc-rs/pull/9 for RUSTFLAGS, which I think could work pretty well and would also address this use-case.

Also cc @alexcrichton since this may be a useful real-world example of where the current behavior falls short.

[jyn514]

@jonhoo is the difference between the three modes documented somewhere? I don't have any clear idea what they do.

[jonhoo]

The unstable docs should explain exactly what the feature does (and doesn't) do. The challenge is that =true matches Cargo's current behavior exactly, and =false makes rustflags not apply more often, with the expectation that users will be using [host] instead.

See rust-lang/cargo#10462 for the new envvar proposal though