rust-analyzer @20231204: Failed to spawn proc-macro servers

[l2dy (Zero King)]

Attempting to use rust-analyzer with Eglot returns the following error.

[eglot] Server reports (type=2): Failed to spawn one or more proc-macro servers.

- cannot find proc-macro-srv, the workspace `.../cargo2port` is missing a sysroot
Failed to find sysroot for Cargo.toml file .../cargo2port/Cargo.toml. Is rust-src installed? can't load standard library from sysroot
/opt/local
(discovered via `rustc --print sysroot`)
try installing the Rust source the same way you installed rustc

rust-src seems to install source files under /opt/local/libexec/rust/src, which does not match what rustc --print sysroot returns.

See also ​https://github.com/rust-lang/rust-analyzer/issues/16001.