Editor integration

--message-format json prints Cargo's JSON messages (compiler-message, compiler-artifact, build-finished) on stdout, one object per line. Warnings of unchanged crates are replayed from the store, so a tool never loses a diagnostic because a crate was a cache hit. The output is tested against Cargo's message shape, not yet against a live rust-analyzer. To try it there:

{
  "rust-analyzer.check.overrideCommand": [
    "rune",
    "check",
    "--message-format=json",
    "--workspace",
    "--all-targets"
  ],
  "rust-analyzer.cargo.buildScripts.overrideCommand": [
    "rune",
    "check",
    "--message-format=json",
    "--workspace",
    "--all-targets"
  ]
}

The second line has rust-analyzer take build scripts' results from Rune too: the JSON includes Cargo's build-script-executed messages (each script's OUT_DIR, cfgs and environment, also when the result came from the store), so include!(concat!(env!("OUT_DIR"), …)) and script-set cfgs resolve in the editor.

rune test --message-format json adds one test-suite-finished line per test binary (outcome, counts, every test and its status). rune graph --units --format json is the build plan: every unit with its key, whether the store has it, its features and platform, and its dependencies.