revert "Add rustc --explain back"

This reverts commit 9b597a1. That
commit added a message informing users about the `rustc --explain`
functionality inside of a `Drop` implementation for `EmitterWriter`. In
addition to exhibiting questionable semantics (printing a
hopefully-helpful message for the user does not seem like a "cleanup"
action), this resulted in divergent behavior between humanized output
and JSON output, because the latter actually instantiates an
`EmitterWriter` for every diagnostic.

Several conflicts in this revert commit had to be fixed manually, again
owing to code-area collision between rust-lang#48449 and rust-lang#48337.

This is in the matter of rust-lang#48550.

Conflicts:
	src/librustc_errors/emitter.rs

src/librustc_errors/emitter.rs

@@ -21,7 +21,7 @@ use std::io::prelude::*;
 use std::io;
 use std::rc::Rc;
 use term;
-use std::collections::{HashMap, HashSet};
+use std::collections::HashMap;
 use std::cmp::min;
 use unicode_width;
 
@@ -109,7 +109,6 @@ pub struct EmitterWriter {
     cm: Option<Rc<CodeMapper>>,
     short_message: bool,
     teach: bool,
-    error_codes: HashSet<String>,
     ui_testing: bool,
 }
 
@@ -119,33 +118,6 @@ struct FileWithAnnotatedLines {
     multiline_depth: usize,
 }
 
-impl Drop for EmitterWriter {
-    fn drop(&mut self) {
-        if !self.short_message && !self.error_codes.is_empty() {
-            let mut error_codes = self.error_codes.clone().into_iter().collect::<Vec<_>>();
-            error_codes.sort();
-            if error_codes.len() > 1 {
-                let limit = if error_codes.len() > 9 { 9 } else { error_codes.len() };
-                writeln!(self.dst,
-                         "You've got a few errors: {}{}",
-                         error_codes[..limit].join(", "),
-                         if error_codes.len() > 9 { "..." } else { "" }
-                        ).expect("failed to give tips...");
-                writeln!(self.dst,
-                         "If you want more information on an error, try using \
-                          \"rustc --explain {}\"",
-                         &error_codes[0]).expect("failed to give tips...");
-            } else {
-                writeln!(self.dst,
-                         "If you want more information on this error, try using \
-                          \"rustc --explain {}\"",
-                         &error_codes[0]).expect("failed to give tips...");
-            }
-            self.dst.flush().expect("failed to emit errors");
-        }
-    }
-}
-
 impl EmitterWriter {
     pub fn stderr(color_config: ColorConfig,
                   code_map: Option<Rc<CodeMapper>>,
@@ -159,7 +131,6 @@ impl EmitterWriter {
                 cm: code_map,
                 short_message,
                 teach,
-                error_codes: HashSet::new(),
                 ui_testing: false,
             }
         } else {
@@ -168,7 +139,6 @@ impl EmitterWriter {
                 cm: code_map,
                 short_message,
                 teach,
-                error_codes: HashSet::new(),
                 ui_testing: false,
             }
         }
@@ -184,7 +154,6 @@ impl EmitterWriter {
             cm: code_map,
             short_message,
             teach,
-            error_codes: HashSet::new(),
             ui_testing: false,
         }
     }
@@ -1025,14 +994,12 @@ impl EmitterWriter {
             if primary_span != &&DUMMY_SP {
                 (cm.lookup_char_pos(primary_span.lo()), cm)
             } else {
-                emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message,
-                                    &mut self.error_codes)?;
+                emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message)?;
                 return Ok(());
             }
         } else {
             // If we don't have span information, emit and exit
-            emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message,
-                                &mut self.error_codes)?;
+            emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message)?;
             return Ok(());
         };
         if let Ok(pos) =
@@ -1205,8 +1172,7 @@ impl EmitterWriter {
         }
 
         // final step: take our styled buffer, render it, then output it
-        emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message,
-                            &mut self.error_codes)?;
+        emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message)?;
 
         Ok(())
 
@@ -1294,8 +1260,7 @@ impl EmitterWriter {
                 let msg = format!("and {} other candidates", suggestions.len() - MAX_SUGGESTIONS);
                 buffer.puts(row_num, 0, &msg, Style::NoStyle);
             }
-            emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message,
-                                &mut self.error_codes)?;
+            emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message)?;
         }
         Ok(())
     }
@@ -1326,7 +1291,7 @@ impl EmitterWriter {
                         draw_col_separator_no_space(&mut buffer, 0, max_line_num_len + 1);
                     }
                     match emit_to_destination(&buffer.render(), level, &mut self.dst,
-                                              self.short_message, &mut self.error_codes) {
+                                              self.short_message) {
                         Ok(()) => (),
                         Err(e) => panic!("failed to emit error: {}", e)
                     }
@@ -1419,8 +1384,7 @@ fn overlaps(a1: &Annotation, a2: &Annotation, padding: usize) -> bool {
 fn emit_to_destination(rendered_buffer: &Vec<Vec<StyledString>>,
                        lvl: &Level,
                        dst: &mut Destination,
-                       short_message: bool,
-                       error_codes: &mut HashSet<String>)
+                       short_message: bool)
                        -> io::Result<()> {
     use lock;
 
@@ -1441,9 +1405,6 @@ fn emit_to_destination(rendered_buffer: &Vec<Vec<StyledString>>,
         for part in line {
             dst.apply_style(lvl.clone(), part.style)?;
             write!(dst, "{}", part.text)?;
-            if !short_message && part.text.len() == 12 && part.text.starts_with("error[E") {
-                error_codes.insert(part.text[6..11].to_owned());
-            }
             dst.reset_attrs()?;
         }
         if !short_message {