rustc: Run the compiler in a subtask and monitor the diagnostics

Since we are no longer logging to the console it's possible for us to
hit a plain-old-fail statement and not output anything.

This adds a defensive mechanism that will monitor the emitted errors
and compare them to the result of the compiler task. If the compiler
fails without emitting an error it results in an ICE.

src/comp/driver/rustc.rs

@@ -62,7 +62,7 @@ options:
 ");
 }
 
-fn main(args: [str]) {
+fn run_compiler(args: [str], demitter: diagnostic::emitter) {
     // Don't display log spew by default. Can override with RUST_LOG.
     logging::console_off();
 
@@ -70,11 +70,6 @@ fn main(args: [str]) {
 
     if vec::len(args) == 0u { usage(binary); ret; }
 
-    let demitter = fn@(cmsp: option<(codemap::codemap, codemap::span)>,
-                       msg: str, lvl: diagnostic::level) {
-        diagnostic::emit(cmsp, msg, lvl);
-    };
-
     let match =
         alt getopts::getopts(args, opts()) {
           ok(m) { m }
@@ -118,6 +113,70 @@ fn main(args: [str]) {
     compile_input(sess, cfg, ifile, odir, ofile);
 }
 
+/*
+This is a sanity check that any failure of the compiler is performed
+through the diagnostic module and reported properly - we shouldn't be calling
+plain-old-fail on any execution path that might be taken. Since we have
+console logging off by default, hitting a plain fail statement would make the
+compiler silently exit, which would be terrible.
+
+This method wraps the compiler in a subtask and injects a function into the
+diagnostic emitter which records when we hit a fatal error. If the task
+fails without recording a fatal error then we've encountered a compiler
+bug and need to present an error.
+*/
+fn monitor(f: fn~(diagnostic::emitter)) {
+    tag monitor_msg {
+        fatal;
+        done;
+    };
+
+    let p = comm::port();
+    let ch = comm::chan(p);
+
+    alt task::try  {||
+
+        task::unsupervise();
+
+        // The 'diagnostics emitter'. Every error, warning, etc. should
+        // go through this function.
+        let demitter = fn@(cmsp: option<(codemap::codemap, codemap::span)>,
+                           msg: str, lvl: diagnostic::level) {
+            if lvl == diagnostic::fatal {
+                comm::send(ch, fatal);
+            }
+            diagnostic::emit(cmsp, msg, lvl);
+        };
+
+        resource finally(ch: comm::chan<monitor_msg>) {
+            comm::send(ch, done);
+        }
+
+        let _finally = finally(ch);
+
+        f(demitter)
+    } {
+        result::ok(_) { /* fallthrough */ }
+        result::err(_) {
+            // Task failed without emitting a fatal diagnostic
+            if comm::recv(p) == done {
+                diagnostic::emit(
+                    none,
+                    diagnostic::ice_msg("unexpected failure"),
+                    diagnostic::error);
+            }
+            // Fail so the process returns a failure code
+            fail;
+        }
+    }
+}
+
+fn main(args: [str]) {
+    monitor {|demitter|
+        run_compiler(args, demitter);
+    }
+}
+
 // Local Variables:
 // mode: rust
 // fill-column: 78;