Make `CARGO_BUILD_TARGET` accept multiple target triples

[madsmtm]

Problem

It is really nice that Cargo supports compiling for multiple platform target triples (not to be confused with the artifacts that Cargo produces). However, it is inconvenient that this capability does not extend to CARGO_BUILD_TARGET.

Proposed Solution

Make CARGO_BUILD_TARGET take multiple values, potentially comma-separated, such that CARGO_BUILD_TARGET=aarch64-apple-darwin,aarch64-apple-ios cargo build would work the same as cargo build --target aarch64-apple-darwin --target aarch64-apple-ios.

Perhaps we could even extend --target to allow comma-separated target triples too?

As a bit of prior art, x.py in rust-lang/rust supports comma-separated target triples.

Notes

This wouldn't work with custom target triple paths that contain commas. How common are these? And would it be okay to break, since custom target json specs are unstable anyhow?

This was also discussed in #10473, but nothing further was done about this yet (from my understanding?)

[epage]

To clarify, this is talking specifically about CARGO_BUILD_TARGET and not build.target which accepts multiple values as of #10473.

(sometimes people refer to env or config to refer to both)

[weihanglo]

To myself I am still hoping -Zadvanced-env instead of a special case.

[ehuss]

We talked about this a little in our team meeting, but didn't come to a conclusion. We noted that there we already support space-separated lists in environment variables, but because build.target is defined using a custom one/many deserializer, it doesn't support space-separation like it would if it used the StringList or UnmergedStringList type. One option is to just switch it to use our standard type and remove the custom impl.

The other option as weihang noted above is to support a more general-purpose array encoding for environment variables. For example, one problem with space-separation (or comma separation) is that custom spec .json files include paths which could include spaces, which would break here if we didn't use some encoding like TOML.

[madsmtm]

We know a few crucial details about target triples that may be able to help, though: They contain neither spaces, slashes, nor dots.

So how about requiring custom target-spec.json files to contain a path separator? So the following would all be valid:

cargo check --target aarch64-apple-darwin,x86_64-unknown-linux   # Like --features a,b
cargo check --target "aarch64-apple-darwin x86_64-unknown-linux" # Like --features "a b"
cargo check --target ./aarch64-custom.json
cargo check --target .\aarch64-custom.json # Windows
cargo check --target aarch64-apple-darwin,x86_64-unknown-linux --target ./aarch64-custom.json

Alternatively (or additionally), require that they end with .json, same idea. Might also need to restrict it so that only one custom target is supported per --target / in CARGO_BUILD_TARGET.

I'd be up for implementing either of these options, I find I use Cargo's multi-build feature very often now that I know about it ;)