Lazy installation of targets listed in rust-toolchain.toml

Problem you are trying to solve

My team builds our software for 5 different Rust targets:

• aarch64-apple-darwin
• x86_64-apple-darwin
• x86_64-unknown-linux-gnu
• wasm32-unknown-unknown
• x86_64-pc-windows-msvc

When building for macOS, we typically only build aarch64-apple-darwin, but sometimes also build for Intel Macs when we're putting together a release. Similarly, on Linux, we sometimes are compiling for the host architecture, but sometimes (especially in CI) compiling for wasm.

If we don't include our extra targets in rust-toolchain.toml, we'll run into errors when running cargo with a cross-compilation target (unless we explicitly install the toolchain). If we do include the extra targets, they'll get installed on every machine and CI runner, whether it makes sense or not. (We really don't need the macOS targets when we're on a Linux CI runner trying to make sure our wasm binary compiles.)

While it's not that hard to explicitly install the toolchain where we need it, it's extra friction vs. sticking it in the targets list and having it automatically installed.

Solution you'd like

It would be nice if there were a way to keep all of our supported targets listed in rust-toolchain.toml, but defer installation of the target until cargo is invoked for that particular target.

It's the best of both worlds - we get automatic installation of the cross-compilation targets we need, but we don't end up installing them eagerly on systems where it doesn't make sense.

As a strawman, one could imagine something like the following:

[toolchain]
channel = "1.80.1"
components = ["rustfmt", "clippy"]
# List all targets that we support.
targets = [
    "aarch64-apple-darwin",
    "x86_64-apple-darwin",
    "x86_64-unknown-linux-gnu",
    "wasm32-unknown-unknown",
    "x86_64-pc-windows-msvc",
]
# Only install the target on the host machine at the time it is actually needed.
lazy-target-installation = true
profile = "minimal"

Notes

No response

Start by tracing how rustup handles the targets listed in rust-toolchain.toml when cargo is invoked for a cross-compilation target. Define the behavior for lazy-target-installation, including when a target is installed and when it remains absent; done means the documented configuration installs only the target actually requested.