Example: Getting diagnostic through rustc_interface

rustc_interface allows you to intercept diagnostics that would otherwise be printed to stderr.

Getting diagnostics

To get diagnostics from the compiler, configure rustc_interface::Config to output diagnostic to a buffer, and run TyCtxt.analysis. The following should be compiled with nightly-2021-03-28 (See here for the complete example):

let buffer = sync::Arc::new(sync::Mutex::new(Vec::new()));
let config = rustc_interface::Config {
    opts: config::Options {
        // Configure the compiler to emit diagnostics in compact JSON format.
        error_format: config::ErrorOutputType::Json {
            pretty: false,
            json_rendered: rustc_errors::emitter::HumanReadableErrorType::Default(
                rustc_errors::emitter::ColorConfig::Never,
            ),
        },
        /* other config */
    },
    // Redirect the diagnostic output of the compiler to a buffer.
    diagnostic_output: rustc_session::DiagnosticOutput::Raw(Box::from(DiagnosticSink(
        buffer.clone(),
    ))),
    /* other config */
};
rustc_interface::run_compiler(config, |compiler| {
    compiler.enter(|queries| {
        queries.global_ctxt().unwrap().take().enter(|tcx| {
            // Run the analysis phase on the local crate to trigger the type error.
            tcx.analysis(rustc_hir::def_id::LOCAL_CRATE);
        });
    });
});
// Read buffered diagnostics.
let diagnostics = String::from_utf8(buffer.lock().unwrap().clone()).unwrap();