In Debian Rust package builds we would like to apply e.g. -C linker and --remap-path-prefix in all situations. We currently do this by setting CARGO_HOME to point to a directory containing a config file with contents similar to the following:

[source.crates-io]
replace-with = "dh-cargo-registry"

[source.dh-cargo-registry]
directory = "$PWD/vendor" # or /usr/share/cargo/registry

[build]
rustflags = [... "-C", "linker=arm-linux-gnueabi-gcc", "-C", "link-arg=-Wl,-z,relro", "--remap-path-prefix", "$PWD=/usr/share/cargo/registry/cargo-0.31.0"]

However if RUSTFLAGS is set (e.g. to empty) in the environment, the above build.rustflags will be totally ignored.

The only method we have currently, to absolutely ensure that certain flags are set, is to append to RUSTFLAGS itself, but this is ugly because it does not support escaping of spaces.

The proposal here is to change env_args to instead use all the sources listed there, in order of priority - that is, set the effective rust flags to be config/build.rustflags + Cargo.toml/target.rustflags + RUSTFLAGS, with later flags taking precedence.