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

@Mark-Simulacrum

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

@alexcrichton

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.

@jonhoo

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

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

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Remove most of the logic in fake rustc once `-Ztarget-applies-to-host=true` lands #94556

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	// 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");
	}

Remove most of the logic in fake rustc once -Ztarget-applies-to-host=true lands #94556

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jonhoo commented on Mar 3, 2022

jyn514 commented on Mar 6, 2022

jonhoo commented on Mar 7, 2022

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Remove most of the logic in fake rustc once `-Ztarget-applies-to-host=true` lands #94556