[rust-lang/rust] Annoying: Unknown unstable option: `json-target-spec`

### ROOT CAUSE The issue arises because `cargo` incorrectly includes the `-Zjson-target-spec` flag when invoking `rustc` with a custom target JSON file. This flag is not recognized by the `rustc` compiler, leading to a build failure. The correct approach is to pass the target JSON file directly using the `--target` option without additional unstable flags. ### CODE FIX To resolve this, we need to modify `cargo` to avoid adding the `-Zjson-target-spec` flag when a custom target JSON file is specified. The fix involves removing the problematic flag from the compiler arguments. ```toml # Cargo's build script modification (hypothetical example) # In src/cargo/src/bin/cargo/lib/rustc.rs, remove the line adding -Zjson-target-spec ``` Alternatively, if modifying `cargo` isn't feasible, users can bypass the issue by directly specifying the target without the unstable flag: ```bash rustc --target x86_64-project.json your-source.rs ``` However, the proper fix lies within `cargo`'s source code to handle custom targets correctly.