Auto merge of #48337 - GuillaumeGomez:rustc-explain, r=estebank

Rustc explain

Fixes #48041.

To make the review easier, I separated tests update to code update. Also, I used this script to generate new ui tests stderr:

```python
from os import listdir
from os.path import isdir, isfile, join

PATH = "src/test/ui"

def do_something(path):
    files = [join(path, f) for f in listdir(path)]

    for f in files:
        if isdir(f):
            do_something(f)
            continue
        if not isfile(f) or not f.endswith(".stderr"):
            continue
        x = open(f, "r")
        content = x.read().strip()
        if "error[E" not in content:
            continue
        errors = dict()
        for y in content.splitlines():
            if y.startswith("error[E"):
                errors[y[6:11]] = True
        errors = sorted(errors.keys())
        if len(errors) < 1:
            print("weird... {}".format(f))
            continue
        if len(errors) > 1:
            content += "\n\nYou've got a few errors: {}".format(", ".join(errors))
            content += "\nIf you want more information on an error, try using \"rustc --explain {}\"".format(errors[0])
        else:
            content += "\n\nIf you want more information on this error, try using \"rustc --explain {}\"".format(errors[0])
        content += "\n"
        x = open(f, "w")
        x.write(content)

do_something(PATH)
```

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;
+use std::collections::{HashMap, HashSet};
 use std::cmp::min;
 use unicode_width;
 
@@ -107,6 +107,7 @@ pub struct EmitterWriter {
     cm: Option<Rc<CodeMapper>>,
     short_message: bool,
     teach: bool,
+    error_codes: HashSet<String>,
 }
 
 struct FileWithAnnotatedLines {
@@ -115,6 +116,33 @@ 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>>,
@@ -128,13 +156,15 @@ impl EmitterWriter {
                 cm: code_map,
                 short_message,
                 teach,
+                error_codes: HashSet::new(),
             }
         } else {
             EmitterWriter {
                 dst: Raw(Box::new(io::stderr())),
                 cm: code_map,
                 short_message,
                 teach,
+                error_codes: HashSet::new(),
             }
         }
     }
@@ -149,6 +179,7 @@ impl EmitterWriter {
             cm: code_map,
             short_message,
             teach,
+            error_codes: HashSet::new(),
         }
     }
 
@@ -975,12 +1006,14 @@ 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)?;
+                emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message,
+                                    &mut self.error_codes)?;
                 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)?;
+            emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message,
+                                &mut self.error_codes)?;
             return Ok(());
         };
         if let Ok(pos) =
@@ -1153,7 +1186,8 @@ 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)?;
+        emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message,
+                            &mut self.error_codes)?;
 
         Ok(())
 
@@ -1241,7 +1275,8 @@ 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)?;
+            emit_to_destination(&buffer.render(), level, &mut self.dst, self.short_message,
+                                &mut self.error_codes)?;
         }
         Ok(())
     }
@@ -1269,7 +1304,7 @@ 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) {
+                                              self.short_message, &mut self.error_codes) {
                         Ok(()) => (),
                         Err(e) => panic!("failed to emit error: {}", e)
                     }
@@ -1362,7 +1397,8 @@ 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)
+                       short_message: bool,
+                       error_codes: &mut HashSet<String>)
                        -> io::Result<()> {
     use lock;
 
@@ -1383,6 +1419,9 @@ 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 {

src/test/ui-fulldeps/lint-plugin-forbid-attrs.stderr

@@ -21,3 +21,4 @@ error[E0453]: allow(test_lint) overruled by outer forbid(test_lint)
 
 error: aborting due to 2 previous errors
 
+If you want more information on this error, try using "rustc --explain E0453"

src/test/ui-fulldeps/proc-macro/signature.stderr

@@ -12,3 +12,4 @@ error[E0308]: mismatched types
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0308"

src/test/ui/anonymous-higher-ranked-lifetime.stderr

@@ -154,3 +154,4 @@ note: required by `h2`
 
 error: aborting due to 11 previous errors
 
+If you want more information on this error, try using "rustc --explain E0631"

src/test/ui/arbitrary-self-types-not-object-safe.stderr

@@ -17,3 +17,4 @@ error[E0038]: the trait `Foo` cannot be made into an object
 
 error: aborting due to 2 previous errors
 
+If you want more information on this error, try using "rustc --explain E0038"

src/test/ui/asm-out-assign-imm.stderr

@@ -9,3 +9,4 @@ error[E0384]: cannot assign twice to immutable variable `x`
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0384"

src/test/ui/associated-const-impl-wrong-lifetime.stderr

@@ -15,3 +15,4 @@ note: the lifetime 'a as defined on the impl at 17:1...
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0308"

src/test/ui/associated-const-impl-wrong-type.stderr

@@ -9,3 +9,4 @@ error[E0326]: implemented const `BAR` has an incompatible type for trait
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0326"

src/test/ui/associated-type-projection-from-multiple-supertraits.stderr

@@ -42,3 +42,5 @@ error[E0221]: ambiguous associated type `Color` in bounds of `C`
 
 error: aborting due to 4 previous errors
 
+You've got a few errors: E0191, E0221
+If you want more information on an error, try using "rustc --explain E0191"

src/test/ui/associated-types-ICE-when-projecting-out-of-err.stderr

@@ -6,3 +6,4 @@ error[E0277]: the trait bound `(): Add<A>` is not satisfied
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0277"

src/test/ui/associated-types-in-ambiguous-context.stderr

@@ -24,3 +24,4 @@ error[E0223]: ambiguous associated type
 
 error: aborting due to 3 previous errors
 
+If you want more information on this error, try using "rustc --explain E0223"

src/test/ui/attr-usage-repr.stderr

@@ -40,3 +40,4 @@ error[E0517]: attribute should be applied to struct
 
 error: aborting due to 5 previous errors
 
+If you want more information on this error, try using "rustc --explain E0517"

src/test/ui/augmented-assignments.stderr

@@ -20,3 +20,5 @@ error[E0382]: use of moved value: `x`
 
 error: aborting due to 2 previous errors
 
+You've got a few errors: E0382, E0596
+If you want more information on an error, try using "rustc --explain E0382"

src/test/ui/binary-op-on-double-ref.stderr

@@ -9,3 +9,4 @@ error[E0369]: binary operation `%` cannot be applied to type `&&{integer}`
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0369"

src/test/ui/blind-item-item-shadow.stderr

@@ -15,3 +15,4 @@ help: You can use `as` to change the binding name of the import
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0255"

src/test/ui/block-result/block-must-not-have-result-do.stderr

@@ -9,3 +9,4 @@ error[E0308]: mismatched types
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0308"

src/test/ui/block-result/block-must-not-have-result-res.stderr

@@ -11,3 +11,4 @@ error[E0308]: mismatched types
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0308"

src/test/ui/block-result/block-must-not-have-result-while.stderr

@@ -9,3 +9,4 @@ error[E0308]: mismatched types
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0308"

src/test/ui/block-result/consider-removing-last-semi.stderr

@@ -28,3 +28,4 @@ error[E0308]: mismatched types
 
 error: aborting due to 2 previous errors
 
+If you want more information on this error, try using "rustc --explain E0308"

src/test/ui/block-result/issue-11714.stderr

@@ -15,3 +15,4 @@ error[E0308]: mismatched types
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0308"

src/test/ui/block-result/issue-13428.stderr

@@ -31,3 +31,4 @@ error[E0308]: mismatched types
 
 error: aborting due to 2 previous errors
 
+If you want more information on this error, try using "rustc --explain E0308"

src/test/ui/block-result/issue-13624.stderr

@@ -20,3 +20,4 @@ error[E0308]: mismatched types
 
 error: aborting due to 2 previous errors
 
+If you want more information on this error, try using "rustc --explain E0308"

src/test/ui/block-result/issue-20862.stderr

@@ -17,3 +17,5 @@ error[E0618]: expected function, found `()`
 
 error: aborting due to 2 previous errors
 
+You've got a few errors: E0308, E0618
+If you want more information on an error, try using "rustc --explain E0308"

src/test/ui/block-result/issue-22645.stderr

@@ -22,3 +22,5 @@ error[E0308]: mismatched types
 
 error: aborting due to 2 previous errors
 
+You've got a few errors: E0277, E0308
+If you want more information on an error, try using "rustc --explain E0277"

src/test/ui/block-result/issue-3563.stderr

@@ -8,3 +8,4 @@ error[E0599]: no method named `b` found for type `&Self` in the current scope
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0599"

src/test/ui/block-result/issue-5500.stderr

@@ -11,3 +11,4 @@ error[E0308]: mismatched types
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0308"

src/test/ui/block-result/unexpected-return-on-unit.stderr

@@ -17,3 +17,4 @@ help: try adding a return type
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0308"

src/test/ui/bogus-tag.stderr

@@ -9,3 +9,4 @@ error[E0599]: no variant named `hsl` found for type `color` in the current scope
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0599"

src/test/ui/borrowck/borrowck-box-insensitivity.stderr

@@ -161,3 +161,5 @@ error[E0502]: cannot borrow `a.y` as mutable because `a.x.x` is also borrowed as
 
 error: aborting due to 16 previous errors
 
+You've got a few errors: E0382, E0502, E0503, E0505
+If you want more information on an error, try using "rustc --explain E0382"

src/test/ui/borrowck/borrowck-closures-two-mut.stderr

@@ -150,3 +150,4 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time (Mir)
 
 error: aborting due to 10 previous errors
 
+If you want more information on this error, try using "rustc --explain E0499"

src/test/ui/borrowck/borrowck-escaping-closure-error-1.stderr

@@ -12,3 +12,4 @@ help: to force the closure to take ownership of `books` (and any other reference
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0373"

src/test/ui/borrowck/borrowck-escaping-closure-error-2.stderr

@@ -12,3 +12,4 @@ help: to force the closure to take ownership of `books` (and any other reference
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0373"

src/test/ui/borrowck/borrowck-in-static.stderr

@@ -8,3 +8,4 @@ error[E0507]: cannot move out of captured outer variable in an `Fn` closure
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0507"

src/test/ui/borrowck/borrowck-move-error-with-note.stderr

@@ -34,3 +34,5 @@ error[E0507]: cannot move out of borrowed content
 
 error: aborting due to 3 previous errors
 
+You've got a few errors: E0507, E0509
+If you want more information on an error, try using "rustc --explain E0507"

src/test/ui/borrowck/borrowck-move-out-of-vec-tail.stderr

@@ -15,3 +15,4 @@ error[E0508]: cannot move out of type `[Foo]`, a non-copy slice
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0508"

src/test/ui/borrowck/borrowck-reinit.stderr

@@ -20,3 +20,4 @@ error[E0382]: use of moved value: `x` (Mir)
 
 error: aborting due to 2 previous errors
 
+If you want more information on this error, try using "rustc --explain E0382"

src/test/ui/borrowck/borrowck-report-with-custom-diagnostic.stderr

@@ -36,3 +36,5 @@ error[E0499]: cannot borrow `x` as mutable more than once at a time
 
 error: aborting due to 3 previous errors
 
+You've got a few errors: E0499, E0502
+If you want more information on an error, try using "rustc --explain E0499"

src/test/ui/borrowck/borrowck-vec-pattern-nesting.stderr

@@ -80,3 +80,5 @@ error[E0508]: cannot move out of type `[std::boxed::Box<isize>]`, a non-copy sli
 
 error: aborting due to 8 previous errors
 
+You've got a few errors: E0506, E0508
+If you want more information on an error, try using "rustc --explain E0506"

src/test/ui/borrowck/immutable-arg.stderr

@@ -16,3 +16,4 @@ error[E0384]: cannot assign to immutable argument `_x` (Mir)
 
 error: aborting due to 2 previous errors
 
+If you want more information on this error, try using "rustc --explain E0384"

src/test/ui/borrowck/issue-41962.stderr

@@ -54,3 +54,4 @@ error[E0382]: use of moved value: `maybe.0` (Mir)
 
 error: aborting due to 5 previous errors
 
+If you want more information on this error, try using "rustc --explain E0382"

src/test/ui/borrowck/mut-borrow-in-loop.stderr

@@ -27,3 +27,4 @@ error[E0499]: cannot borrow `*arg` as mutable more than once at a time
 
 error: aborting due to 3 previous errors
 
+If you want more information on this error, try using "rustc --explain E0499"

src/test/ui/borrowck/mut-borrow-outside-loop.stderr

@@ -21,3 +21,4 @@ error[E0499]: cannot borrow `inner_void` as mutable more than once at a time
 
 error: aborting due to 2 previous errors
 
+If you want more information on this error, try using "rustc --explain E0499"

src/test/ui/borrowck/regions-bound-missing-bound-in-impl.stderr

@@ -46,3 +46,5 @@ error[E0276]: impl has stricter requirements than trait
 
 error: aborting due to 4 previous errors
 
+You've got a few errors: E0195, E0276, E0308
+If you want more information on an error, try using "rustc --explain E0195"

src/test/ui/borrowck/unboxed-closures-move-upvar-from-non-once-ref-closure.stderr

@@ -9,3 +9,4 @@ error[E0507]: cannot move out of captured outer variable in an `Fn` closure
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0507"

src/test/ui/cast-as-bool.stderr

@@ -8,3 +8,4 @@ error[E0054]: cannot cast as `bool`
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0054"

src/test/ui/cast-errors-issue-43825.stderr

@@ -6,3 +6,4 @@ error[E0425]: cannot find value `error` in this scope
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0425"

src/test/ui/cast-rfc0401-2.stderr

@@ -8,3 +8,4 @@ error[E0054]: cannot cast as `bool`
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0054"

src/test/ui/cast-to-unsized-trait-object-suggestion.stderr

@@ -16,3 +16,4 @@ error[E0620]: cast to unsized type: `std::boxed::Box<{integer}>` as `std::marker
 
 error: aborting due to 2 previous errors
 
+If you want more information on this error, try using "rustc --explain E0620"

src/test/ui/casts-differing-anon.stderr

@@ -8,3 +8,4 @@ error[E0606]: casting `*mut impl std::fmt::Debug+?Sized` as `*mut impl std::fmt:
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0606"

src/test/ui/casts-issue-46365.stderr

@@ -6,3 +6,4 @@ error[E0412]: cannot find type `Ipsum` in this scope
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0412"

src/test/ui/changing-crates.stderr

@@ -11,3 +11,4 @@ error[E0460]: found possibly newer version of crate `a` which `b` depends on
 
 error: aborting due to previous error
 
+If you want more information on this error, try using "rustc --explain E0460"