Filter toolchain index by arch/target

[foresterre]

In my opinion, cargo-msrv should try to find the minimum supported version binary for a specific arch. Different MSRV on different architecture is more acceptable. We can't ask a developer who working on RISC-V to follow the MSRV generated from x86_64.

For example, if there is a new architecture implementation coming in Rust 1.999999 or whatever, developer will not able to compile some crate with rustc v1.999998, so the MSRV for that architecture should be 1.999999.

I agree.

Cargo itself explicitly states that the rust-version key in the Cargo manifest affects all targets in the package. That is, only one MSRV can be set for a package (if you want 'official cargo support').

For a tool like cargo-msrv however, it may very well be desirable to search for the MSRV independently for each target, and maybe optionally set the MSRV per target specifically. It would then be up to the crate developer to decide which MSRV to pick from the set of MSRV's per target (assuming no target will be removed, this might always be the highest MSRV in the set).

Originally posted by @foresterre in #585 (comment)