Auto merge of #40851 - oli-obk:multisugg, r=jonathandturner

Minimize single span suggestions into a label

changes

```
14 |     println!("☃{}", tup[0]);
   |                     ^^^^^^
   |
help: to access tuple elements, use tuple indexing syntax as shown
   |     println!("☃{}", tup.0);
```

into

```
14 |     println!("☃{}", tup[0]);
   |                     ^^^^^^ to access tuple elements, use `tup.0`
```

Also makes suggestions explicit in the backend in preparation of adding multiple suggestions to a single diagnostic. Currently that's already possible, but results in a full help message + modified code snippet per suggestion, and has no rate limit (might show 100+ suggestions).

4 files changed, 39 insertions, 23 deletions

diff --git a/src/librustc_errors/diagnostic.rs b/src/librustc_errors/diagnostic.rs
index 9715ace3e2e..38fa35ecb12 100644
--- a/src/librustc_errors/diagnostic.rs
+++ b/src/librustc_errors/diagnostic.rs
@@ -11,7 +11,6 @@
 use CodeSuggestion;
 use Level;
 use RenderSpan;
-use RenderSpan::Suggestion;
 use std::fmt;
 use syntax_pos::{MultiSpan, Span};
 use snippet::Style;
@@ -24,6 +23,7 @@ pub struct Diagnostic {
     pub code: Option<String>,
     pub span: MultiSpan,
     pub children: Vec<SubDiagnostic>,
+    pub suggestion: Option<CodeSuggestion>,
 }
 
 /// For example a note attached to an error.
@@ -87,6 +87,7 @@ impl Diagnostic {
             code: code,
             span: MultiSpan::new(),
             children: vec![],
+            suggestion: None,
         }
     }
 
@@ -202,19 +203,14 @@ impl Diagnostic {
 
     /// Prints out a message with a suggested edit of the code.
     ///
-    /// See `diagnostic::RenderSpan::Suggestion` for more information.
-    pub fn span_suggestion<S: Into<MultiSpan>>(&mut self,
-                                               sp: S,
-                                               msg: &str,
-                                               suggestion: String)
-                                               -> &mut Self {
-        self.sub(Level::Help,
-                 msg,
-                 MultiSpan::new(),
-                 Some(Suggestion(CodeSuggestion {
-                     msp: sp.into(),
-                     substitutes: vec![suggestion],
-                 })));
+    /// See `diagnostic::CodeSuggestion` for more information.
+    pub fn span_suggestion(&mut self, sp: Span, msg: &str, suggestion: String) -> &mut Self {
+        assert!(self.suggestion.is_none());
+        self.suggestion = Some(CodeSuggestion {
+            msp: sp.into(),
+            substitutes: vec![suggestion],
+            msg: msg.to_owned(),
+        });
         self
     }
 
diff --git a/src/librustc_errors/diagnostic_builder.rs b/src/librustc_errors/diagnostic_builder.rs
index 7b27f13951b..9dfd47b8464 100644
--- a/src/librustc_errors/diagnostic_builder.rs
+++ b/src/librustc_errors/diagnostic_builder.rs
@@ -141,11 +141,11 @@ impl<'a> DiagnosticBuilder<'a> {
                                                   sp: S,
                                                   msg: &str)
                                                   -> &mut Self);
-    forward!(pub fn span_suggestion<S: Into<MultiSpan>>(&mut self,
-                                                        sp: S,
-                                                        msg: &str,
-                                                        suggestion: String)
-                                                        -> &mut Self);
+    forward!(pub fn span_suggestion(&mut self,
+                                    sp: Span,
+                                    msg: &str,
+                                    suggestion: String)
+                                    -> &mut Self);
     forward!(pub fn set_span<S: Into<MultiSpan>>(&mut self, sp: S) -> &mut Self);
     forward!(pub fn code(&mut self, s: String) -> &mut Self);
 
diff --git a/src/librustc_errors/emitter.rs b/src/librustc_errors/emitter.rs
index 64652bb308b..1a38018e1b3 100644
--- a/src/librustc_errors/emitter.rs
+++ b/src/librustc_errors/emitter.rs
@@ -34,6 +34,27 @@ impl Emitter for EmitterWriter {
     fn emit(&mut self, db: &DiagnosticBuilder) {
         let mut primary_span = db.span.clone();
         let mut children = db.children.clone();
+
+        if let Some(sugg) = db.suggestion.clone() {
+            assert_eq!(sugg.msp.primary_spans().len(), sugg.substitutes.len());
+            // don't display multispans as labels
+            if sugg.substitutes.len() == 1 &&
+               // don't display long messages as labels
+               sugg.msg.split_whitespace().count() < 10 &&
+               // don't display multiline suggestions as labels
+               sugg.substitutes[0].find('\n').is_none() {
+                let msg = format!("help: {} `{}`", sugg.msg, sugg.substitutes[0]);
+                primary_span.push_span_label(sugg.msp.primary_spans()[0], msg);
+            } else {
+                children.push(SubDiagnostic {
+                    level: Level::Help,
+                    message: Vec::new(),
+                    span: MultiSpan::new(),
+                    render_span: Some(Suggestion(sugg)),
+                });
+            }
+        }
+
         self.fix_multispans_in_std_macros(&mut primary_span, &mut children);
         self.emit_messages_default(&db.level,
                                    &db.styled_message(),
@@ -756,7 +777,7 @@ impl EmitterWriter {
     /// displayed, keeping the provided highlighting.
     fn msg_to_buffer(&self,
                      buffer: &mut StyledBuffer,
-                     msg: &Vec<(String, Style)>,
+                     msg: &[(String, Style)],
                      padding: usize,
                      label: &str,
                      override_style: Option<Style>) {
@@ -1022,7 +1043,6 @@ impl EmitterWriter {
     fn emit_suggestion_default(&mut self,
                                suggestion: &CodeSuggestion,
                                level: &Level,
-                               msg: &Vec<(String, Style)>,
                                max_line_num_len: usize)
                                -> io::Result<()> {
         use std::borrow::Borrow;
@@ -1034,7 +1054,7 @@ impl EmitterWriter {
             buffer.append(0, &level.to_string(), Style::Level(level.clone()));
             buffer.append(0, ": ", Style::HeaderMsg);
             self.msg_to_buffer(&mut buffer,
-                               msg,
+                               &[(suggestion.msg.to_owned(), Style::NoStyle)],
                                max_line_num_len,
                                "suggestion",
                                Some(Style::HeaderMsg));
@@ -1099,7 +1119,6 @@ impl EmitterWriter {
                         Some(Suggestion(ref cs)) => {
                             match self.emit_suggestion_default(cs,
                                                                &child.level,
-                                                               &child.styled_message(),
                                                                max_line_num_len) {
                                 Err(e) => panic!("failed to emit error: {}", e),
                                 _ => ()
diff --git a/src/librustc_errors/lib.rs b/src/librustc_errors/lib.rs
index cc8012d965a..02d8297dd46 100644
--- a/src/librustc_errors/lib.rs
+++ b/src/librustc_errors/lib.rs
@@ -67,6 +67,7 @@ pub enum RenderSpan {
 pub struct CodeSuggestion {
     pub msp: MultiSpan,
     pub substitutes: Vec<String>,
+    pub msg: String,
 }
 
 pub trait CodeMapper {