Add some guidelines for suggestion messages and follow them

src/librustc_borrowck/borrowck/mod.rs

@@ -1002,7 +1002,7 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
             .span_suggestion(err.span,
                              &format!("to force the closure to take ownership of {} \
                                        (and any other referenced variables), \
-                                       use the `move` keyword, as shown:",
+                                       use the `move` keyword",
                                        cmt_path_or_string),
                              suggestion)
             .emit();

src/librustc_errors/diagnostic.rs

@@ -203,6 +203,18 @@ impl Diagnostic {
 
     /// Prints out a message with a suggested edit of the code.
     ///
+    /// In case of short messages and a simple suggestion,
+    /// rustc displays it as a label like
+    ///
+    /// "try adding parentheses: `(tup.0).1`"
+    ///
+    /// The message
+    /// * should not end in any punctuation (a `:` is added automatically)
+    /// * should not be a question
+    /// * should not contain any parts like "the following", "as shown"
+    /// * may look like "to do xyz, use" or "to do xyz, use abc"
+    /// * may contain a name of a function, variable or type, but not whole expressions
+    ///
     /// See `diagnostic::CodeSuggestion` for more information.
     pub fn span_suggestion(&mut self, sp: Span, msg: &str, suggestion: String) -> &mut Self {
         self.suggestions.push(CodeSuggestion {

src/librustc_errors/emitter.rs

@@ -46,7 +46,7 @@ impl Emitter for EmitterWriter {
                // don't display multiline suggestions as labels
                sugg.substitution_parts[0].substitutions[0].find('\n').is_none() {
                 let substitution = &sugg.substitution_parts[0].substitutions[0];
-                let msg = format!("help: {} `{}`", sugg.msg, substitution);
+                let msg = format!("help: {}: `{}`", sugg.msg, substitution);
                 primary_span.push_span_label(sugg.substitution_spans().next().unwrap(), msg);
             } else {
                 // if there are multiple suggestions, print them all in full

src/librustc_resolve/lib.rs

@@ -2308,13 +2308,13 @@ impl<'a> Resolver<'a> {
                 enum_candidates.sort();
                 for (sp, variant_path, enum_path) in enum_candidates {
                     if sp == DUMMY_SP {
-                        let msg = format!("there is an enum variant `{}`,\
-                                        did you mean to use `{}`?",
+                        let msg = format!("there is an enum variant `{}`, \
+                                        try using `{}`?",
                                         variant_path,
                                         enum_path);
                         err.help(&msg);
                     } else {
-                        err.span_suggestion(span, "did you mean the variant's enum", enum_path);
+                        err.span_suggestion(span, "you can try using the variant's enum", enum_path);
                     }
                 }
             }
@@ -2323,19 +2323,19 @@ impl<'a> Resolver<'a> {
                     let self_is_available = this.self_value_is_available(path[0].ctxt, span);
                     match candidate {
                         AssocSuggestion::Field => {
-                            err.span_suggestion(span, "did you mean",
+                            err.span_suggestion(span, "try",
                                                 format!("self.{}", path_str));
                             if !self_is_available {
                                 err.span_label(span, format!("`self` value is only available in \
                                                                methods with `self` parameter"));
                             }
                         }
                         AssocSuggestion::MethodWithSelf if self_is_available => {
-                            err.span_suggestion(span, "did you mean",
+                            err.span_suggestion(span, "try",
                                                 format!("self.{}", path_str));
                         }
                         AssocSuggestion::MethodWithSelf | AssocSuggestion::AssocItem => {
-                            err.span_suggestion(span, "did you mean",
+                            err.span_suggestion(span, "try",
                                                 format!("Self::{}", path_str));
                         }
                     }

src/librustc_resolve/macros.rs

@@ -649,9 +649,9 @@ impl<'a> Resolver<'a> {
         if let Some(suggestion) = suggestion {
             if suggestion != name {
                 if let MacroKind::Bang = kind {
-                    err.span_suggestion(span, "did you mean", format!("{}!", suggestion));
+                    err.span_suggestion(span, "you could try the macro", format!("{}!", suggestion));
                 } else {
-                    err.span_suggestion(span, "did you mean", suggestion.to_string());
+                    err.span_suggestion(span, "try", suggestion.to_string());
                 }
             } else {
                 err.help("have you added the `#[macro_use]` on the module/import?");

src/librustc_typeck/check/cast.rs

@@ -276,7 +276,7 @@ impl<'a, 'gcx, 'tcx> CastCheck<'tcx> {
                     match fcx.tcx.sess.codemap().span_to_snippet(self.cast_span) {
                         Ok(s) => {
                             err.span_suggestion(self.cast_span,
-                                                "try casting to a reference instead:",
+                                                "try casting to a reference instead",
                                                 format!("&{}{}", mtstr, s));
                         }
                         Err(_) => {
@@ -295,7 +295,7 @@ impl<'a, 'gcx, 'tcx> CastCheck<'tcx> {
                 match fcx.tcx.sess.codemap().span_to_snippet(self.cast_span) {
                     Ok(s) => {
                         err.span_suggestion(self.cast_span,
-                                            "try casting to a `Box` instead:",
+                                            "try casting to a `Box` instead",
                                             format!("Box<{}>", s));
                     }
                     Err(_) => span_help!(err, self.cast_span, "did you mean `Box<{}>`?", tstr),

src/librustc_typeck/check/op.rs

@@ -290,7 +290,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                                   from a string reference. String concatenation \
                                   appends the string on the right to the string \
                                   on the left and may require reallocation. This \
-                                  requires ownership of the string on the left."), suggestion);
+                                  requires ownership of the string on the left"), suggestion);
                     is_string_addition = true;
                 }
 

src/libsyntax/parse/parser.rs

@@ -1507,7 +1507,7 @@ impl<'a> Parser<'a> {
                     s.print_bounds(" +", &bounds)?;
                     s.pclose()
                 });
-                err.span_suggestion(sum_span, "try adding parentheses:", sum_with_parens);
+                err.span_suggestion(sum_span, "try adding parentheses", sum_with_parens);
             }
             TyKind::Ptr(..) | TyKind::BareFn(..) => {
                 err.span_label(sum_span, "perhaps you forgot parentheses?");
@@ -5180,7 +5180,7 @@ impl<'a> Parser<'a> {
 `pub(in path::to::module)`: visible only on the specified path"##;
                 let path = self.parse_path(PathStyle::Mod)?;
                 let path_span = self.prev_span;
-                let help_msg = format!("make this visible only to module `{}` with `in`:", path);
+                let help_msg = format!("make this visible only to module `{}` with `in`", path);
                 self.expect(&token::CloseDelim(token::Paren))?;  // `)`
                 let mut err = self.span_fatal_help(path_span, msg, suggestion);
                 err.span_suggestion(path_span, &help_msg, format!("in {}", path));