No way to pass environment information to compiler flags defined in Cargo configuration

[CJKay]

The .cargo/config file, through its build.rustflags key, allows you to add arbitrary compiler flags, which are appended to those passed by Cargo at build-time. These flags, however, are not parsed or evaluated in any way, which means that no environment information can be used in these flags.

Steps

1. Create a new Cargo binary project:

cargo new --bin test-case

2. Insert into .cargo/config:

[build]
rustflags = ["-C", "link-arg=-Wl,--defsym=FOO=${BAR}"]

3. Export BAR:

export BAR=0

4. Build the binary:

cargo build

5. Note the output of the error, which shows that the ${BAR} variable was not expanded:

  = note: /usr/bin/ld:--defsym:1: ignoring invalid character `{' in expression
          /usr/bin/ld:--defsym:1: syntax error
          collect2: error: ld returned 1 exit status

Possible Solution(s)

There are two obvious solutions to this:

1. Expand rustflags before passing them to the compiler.
2. Allow build.rs to override arbitrary compiler flags.

Notes

Output of cargo version: cargo 1.39.0-nightly (fe0e5a4 2019-09-04)

The original issue that prompted this is that there is no way to pass the Cargo output directory as a part of the -Map linker flag, which means that the map file is always generated in the source directory (highly undesirable).

[Luminoth]

It would be amazing if either this or #5376 happened. For our project we have to pass a dynamic sysroot through RUSTFLAGS ('-C link-arg=--sysroot=$SYSROOT') which means we have to pass the entire RUSTFLAGS through our build system rather than just the one argument (or having it set an env var to be expanded by Cargo).

[epage]

Side note: #12193 requests other ways of modifying RUSTFLAGS