Rust-analyzer doesn't recognize JSON target files

rust-analyzer version: (eg. output of "rust-analyzer: Show RA Version" command, accessible in VSCode via Ctrl/⌘+Shift+P)

rust-analyzer version: 0.3.1839-standalone

rustc version: (eg. output of rustc -V)

rustc 1.78.0-nightly (256b6fb19 2024-02-06)

relevant settings: (eg. client settings, or environment variables like CARGO, RUSTC, RUSTUP_HOME or CARGO_HOME)

Nothing out of the ordinary.

I have several targets defined by .json files. Normally RA can handle them if they are targeting the same architecture as the host, but cannot if I'm specifying them via .cargo/config.toml's build.target = [...] property. It states it can't find the target name to acquire a toolchain (name of the file sans .json) which... of course it can't. Cargo can handle them correctly because it's using the llvm-target property under the hood.

The net effect of which is that editing cross-platform stuff with custom target JSONs is not possible with RA.

Start by reproducing the issue with custom target JSON files referenced through .cargo/config.toml's build.target setting, comparing the host-target case with the cross-platform case. Trace how rust-analyzer acquires a toolchain for the target name; done means JSON targets are recognized and cross-platform editing works without the target-name lookup failure.