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).

src/libsyntax/json.rs

@@ -22,8 +22,9 @@
 use codemap::{CodeMap, FilePathMapping};
 use syntax_pos::{self, MacroBacktrace, Span, SpanLabel, MultiSpan};
 use errors::registry::Registry;
-use errors::{DiagnosticBuilder, SubDiagnostic, RenderSpan, CodeSuggestion, CodeMapper};
+use errors::{Level, DiagnosticBuilder, SubDiagnostic, RenderSpan, CodeSuggestion, CodeMapper};
 use errors::emitter::Emitter;
+use errors::snippet::Style;
 
 use std::rc::Rc;
 use std::io::{self, Write};
@@ -153,12 +154,21 @@ impl Diagnostic {
     fn from_diagnostic_builder(db: &DiagnosticBuilder,
                                je: &JsonEmitter)
                                -> Diagnostic {
+        let sugg = db.suggestion.as_ref().map(|sugg| {
+            SubDiagnostic {
+                level: Level::Help,
+                message: vec![(sugg.msg.clone(), Style::NoStyle)],
+                span: MultiSpan::new(),
+                render_span: Some(RenderSpan::Suggestion(sugg.clone())),
+            }
+        });
+        let sugg = sugg.as_ref();
         Diagnostic {
             message: db.message(),
             code: DiagnosticCode::map_opt_string(db.code.clone(), je),
             level: db.level.to_str(),
             spans: DiagnosticSpan::from_multispan(&db.span, je),
-            children: db.children.iter().map(|c| {
+            children: db.children.iter().chain(sugg).map(|c| {
                 Diagnostic::from_sub_diagnostic(c, je)
             }).collect(),
             rendered: None,