Define target compatibilities in Cargo.toml

[mankinskin]

Problem:
It is difficult to recognize what targets a crate is compatible with. With WASM coming about, it has become more important to know if a crate is WASM-compatible or not, i.e. builds and runs on a wasm32 architecture. One has to try building the crate for the target manually, and if there are issues one needs to find the dependency which may be causing incompatibility by recursing down the dependency tree, following the compiler errors.

Solution:

[build]
targets = [
    "wasm32-unknown-unknown",
    "wasm32-wasi",
    "x86_64-unknown-linux-gnu",
    "i586-pc-windows-msvc",
]

• Allow crate authors to define compatible targets in the crate configuration
• check those targets when publishing the crate
• let cargo make a suggestion to add a target to the list, after target has been checked successfully
• check the compatible targets of dependencies and point out compatibility conflicts

This would make the compatibility visible in the crate, and make it easier to find compatibility issues.
Another idea is to show the compatible targets in the documentation on docs.rs, to make it even easier to see what targets a crate can be run on.

[ehuss]

This looks like a duplicate of #6179. Would you mind adding this as a comment over there? The discussion there is a little focused on workspaces, but I think getting all the different use cases together would be helpful.