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

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 (fe0e5a48b 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).

Start with Cargo's handling of the .cargo/config build.rustflags key and reproduce the reported cargo build case using BAR and ${BAR}. Compare the proposed expansion and build.rs approaches, then define and test behavior for passing environment-derived values to compiler flags without breaking existing configuration.