Add help diagnostic messages

This adds ‘help’ diagnostic messages to rustc. This is used for anything that
provides help to the user, particularly the `--explain` messages that were
previously integrated into the relevant error message.

src/libsyntax/diagnostic.rs

@@ -105,6 +105,9 @@ impl SpanHandler {
     pub fn span_end_note(&self, sp: Span, msg: &str) {
         self.handler.custom_emit(&self.cm, FullSpan(sp), msg, Note);
     }
+    pub fn span_help(&self, sp: Span, msg: &str) {
+        self.handler.emit(Some((&self.cm, sp)), msg, Help);
+    }
     pub fn fileline_note(&self, sp: Span, msg: &str) {
         self.handler.custom_emit(&self.cm, FileLine(sp), msg, Note);
     }
@@ -164,6 +167,9 @@ impl Handler {
     pub fn note(&self, msg: &str) {
         self.emit.borrow_mut().emit(None, msg, None, Note);
     }
+    pub fn help(&self, msg: &str) {
+        self.emit.borrow_mut().emit(None, msg, None, Help);
+    }
     pub fn bug(&self, msg: &str) -> ! {
         self.emit.borrow_mut().emit(None, msg, None, Bug);
         fail!(ExplicitBug);
@@ -216,6 +222,7 @@ pub enum Level {
     Error,
     Warning,
     Note,
+    Help,
 }
 
 impl fmt::Show for Level {
@@ -227,6 +234,7 @@ impl fmt::Show for Level {
             Fatal | Error => "error".fmt(f),
             Warning => "warning".fmt(f),
             Note => "note".fmt(f),
+            Help => "help".fmt(f),
         }
     }
 }
@@ -236,7 +244,8 @@ impl Level {
         match self {
             Bug | Fatal | Error => term::color::BRIGHT_RED,
             Warning => term::color::BRIGHT_YELLOW,
-            Note => term::color::BRIGHT_GREEN
+            Note => term::color::BRIGHT_GREEN,
+            Help => term::color::BRIGHT_CYAN,
         }
     }
 }
@@ -293,15 +302,6 @@ fn print_diagnostic(dst: &mut EmitterWriter, topic: &str, lvl: Level,
         Some(code) => {
             let style = term::attr::ForegroundColor(term::color::BRIGHT_MAGENTA);
             try!(print_maybe_styled(dst, format!(" [{}]", code.clone()).as_slice(), style));
-            match dst.registry.as_ref().and_then(|registry| registry.find_description(code)) {
-                Some(_) => {
-                    try!(write!(&mut dst.dst,
-                        " (pass `--explain {}` to see a detailed explanation)",
-                        code
-                    ));
-                }
-                None => ()
-            }
         }
         None => ()
     }
@@ -401,7 +401,20 @@ fn emit(dst: &mut EmitterWriter, cm: &codemap::CodeMap, rsp: RenderSpan,
             try!(highlight_lines(dst, cm, sp, lvl, lines));
         }
     }
-    print_macro_backtrace(dst, cm, sp)
+    try!(print_macro_backtrace(dst, cm, sp));
+    match code {
+        Some(code) =>
+            match dst.registry.as_ref().and_then(|registry| registry.find_description(code)) {
+                Some(_) => {
+                    try!(print_diagnostic(dst, ss.as_slice(), Help,
+                                          format!("pass `--explain {}` to see a detailed \
+                                                   explanation", code).as_slice(), None));
+                }
+                None => ()
+            },
+        None => (),
+    }
+    Ok(())
 }
 
 fn highlight_lines(err: &mut EmitterWriter,

src/libsyntax/ext/base.rs

@@ -575,6 +575,10 @@ impl<'a> ExtCtxt<'a> {
         self.print_backtrace();
         self.parse_sess.span_diagnostic.span_note(sp, msg);
     }
+    pub fn span_help(&self, sp: Span, msg: &str) {
+        self.print_backtrace();
+        self.parse_sess.span_diagnostic.span_help(sp, msg);
+    }
     pub fn bug(&self, msg: &str) -> ! {
         self.print_backtrace();
         self.parse_sess.span_diagnostic.handler().bug(msg);

src/libsyntax/parse/parser.rs

@@ -959,6 +959,9 @@ impl<'a> Parser<'a> {
     pub fn span_note(&mut self, sp: Span, m: &str) {
         self.sess.span_diagnostic.span_note(sp, m)
     }
+    pub fn span_help(&mut self, sp: Span, m: &str) {
+        self.sess.span_diagnostic.span_help(sp, m)
+    }
     pub fn bug(&mut self, m: &str) -> ! {
         self.sess.span_diagnostic.span_bug(self.span, m)
     }