Only use metadata for current target

[jonhoo]

Unlike most other cargo tools, cargo metadata counter-intuitively defaults to including all targets. This means that with the current logic, rust-analyzer ends up processing all targets unless one a custom target is specified:
https://github.com/rust-analyzer/rust-analyzer/blob/41321d96789ed918eebda02ada76758765d19d16/crates/project_model/src/cargo_workspace.rs#L169-L171

I think rust-analyzer should probably always specify --filter-platform, and just specify the host platform if target is not specified. Cargo finds the host platform by running rustc -Vv and extracing the host: line, which rust-analyzer could probably also do pretty easily:
https://github.com/rust-lang/cargo/blob/fde8ee3ba0cba4eef75f197d42586ef4664f057b/src/cargo/util/rustc.rs#L48-L68

There's also precedent for relying on rustc to determine these kinds of things, such as using it to find the sysroot:
https://github.com/rust-analyzer/rust-analyzer/blob/48802e54d1669c68fd27b007533dd6aa5221d089/crates/project_model/src/sysroot.rs#L126-L127

One way to make this nicer would be if cargo metadata supported a "special" value for --filter-platform called host. I'll look at submitting that as a PR upstream, but the more immediate fix (and also one that would continue to work for older compiler releases) would be to just use rustc -Vv.

[steveyko]

I am using Rust toolchain 1.37.0 due to some LLVM compatibility issues with other codebases. In that version, cargo doesn't have --filter-platform. I use coc.nvim with rust-analyzer, and my nvim always complains that cargo metadata wasn't expecting --filter-platform. I thought I'd report this in case people here are interested in backward compatibility.

[chorman0773]

I'm going to second the backcompat issue. I'm currently using 1,38 in a program to ensure it can be built directly from mrustc and get the same issue. Maybe an opt-out would be useful?