Teach diagnostics to have correctly padded lists

Make the suggestion list have a correct padding:

```
error[E0308]: mismatched types
 --> file.rs:3:20
  |
3 |     let x: usize = "";
  |                    ^^ expected usize, found reference
  |
  = note: expected type `usize`
  = note:    found type `&'static str`
  = help: here are some functions which might fulfill your needs:
          - .len()
          - .foo()
          - .bar()
```

3 files changed, 36 insertions, 2 deletions

diff --git a/src/librustc_errors/diagnostic.rs b/src/librustc_errors/diagnostic.rs
index 730ca8f9e2e..73a8343eafc 100644
--- a/src/librustc_errors/diagnostic.rs
+++ b/src/librustc_errors/diagnostic.rs
@@ -32,6 +32,7 @@ pub struct SubDiagnostic {
     pub message: String,
     pub span: MultiSpan,
     pub render_span: Option<RenderSpan>,
+    pub list: Vec<String>,
 }
 
 impl Diagnostic {
@@ -132,6 +133,11 @@ impl Diagnostic {
         self
     }
 
+    pub fn help_with_list(&mut self , msg: &str, list: Vec<String>) -> &mut Self {
+        self.sub_with_list(Level::Help, msg, MultiSpan::new(), None, list);
+        self
+    }
+
     pub fn span_help<S: Into<MultiSpan>>(&mut self,
                                          sp: S,
                                          msg: &str)
@@ -191,11 +197,23 @@ impl Diagnostic {
            message: &str,
            span: MultiSpan,
            render_span: Option<RenderSpan>) {
+        self.sub_with_list(level, message, span, render_span, vec![]);
+    }
+
+    /// Convenience function for internal use, clients should use one of the
+    /// public methods above.
+    fn sub_with_list(&mut self,
+           level: Level,
+           message: &str,
+           span: MultiSpan,
+           render_span: Option<RenderSpan>,
+           list: Vec<String>) {
         let sub = SubDiagnostic {
             level: level,
             message: message.to_owned(),
             span: span,
             render_span: render_span,
+            list: list,
         };
         self.children.push(sub);
     }
diff --git a/src/librustc_errors/diagnostic_builder.rs b/src/librustc_errors/diagnostic_builder.rs
index 7dfea6b8951..24f1b86739d 100644
--- a/src/librustc_errors/diagnostic_builder.rs
+++ b/src/librustc_errors/diagnostic_builder.rs
@@ -135,6 +135,7 @@ impl<'a> DiagnosticBuilder<'a> {
     forward!(pub fn warn(&mut self, msg: &str) -> &mut Self);
     forward!(pub fn span_warn<S: Into<MultiSpan>>(&mut self, sp: S, msg: &str) -> &mut Self);
     forward!(pub fn help(&mut self , msg: &str) -> &mut Self);
+    forward!(pub fn help_with_list(&mut self , msg: &str, list: Vec<String>) -> &mut Self);
     forward!(pub fn span_help<S: Into<MultiSpan>>(&mut self,
                                                   sp: S,
                                                   msg: &str)
diff --git a/src/librustc_errors/emitter.rs b/src/librustc_errors/emitter.rs
index 808fe504b95..015997211bb 100644
--- a/src/librustc_errors/emitter.rs
+++ b/src/librustc_errors/emitter.rs
@@ -699,6 +699,7 @@ impl EmitterWriter {
                     .to_string(),
                 span: MultiSpan::new(),
                 render_span: None,
+                list: vec![],
             });
         }
     }
@@ -923,14 +924,28 @@ impl EmitterWriter {
                             }
                         },
                         None => {
+                            let msg = if child.list.len() == 0 {
+                                child.message.to_owned()
+                            } else {
+                                format!("{}\n{}",
+                                        &child.message,
+                                        &child.list.iter().map(|item| {
+                                            format!("{}         - {}",
+                                                    (0..max_line_num_len)
+                                                          .map(|_| " ")
+                                                          .collect::<String>(),
+                                                    item)
+                                        }).collect::<Vec<String>>()
+                                        .join("\n"))
+                            };
                             match self.emit_message_default(&child.span,
-                                                            &child.message,
+                                                            &msg,
                                                             &None,
                                                             &child.level,
                                                             max_line_num_len,
                                                             true) {
                                 Err(e) => panic!("failed to emit error: {}", e),
-                                _ => ()
+                                _ => (),
                             }
                         }
                     }