Provide more context for unenclosed delimiters

 * When encountering EOF, point at the last opening brace that does not
   have the same indentation level as its close delimiter.
 * When encountering the wrong type of close delimiter, point at the
   likely correct open delimiter to give a better idea of what went
   wrong.

src/libsyntax/parse/lexer/mod.rs

@@ -66,6 +66,10 @@ pub struct StringReader<'a> {
     /// The raw source span which *does not* take `override_span` into account
     span_src_raw: Span,
     open_braces: Vec<(token::DelimToken, Span)>,
+    /// The type and spans for all braces that have different indentation.
+    ///
+    /// Used only for error recovery when arriving to EOF with mismatched braces.
+    suspicious_open_spans: Vec<(token::DelimToken, Span, Span)>,
     crate override_span: Option<Span>,
     last_unclosed_found_span: Option<Span>,
 }
@@ -216,6 +220,7 @@ impl<'a> StringReader<'a> {
             span: syntax_pos::DUMMY_SP,
             span_src_raw: syntax_pos::DUMMY_SP,
             open_braces: Vec::new(),
+            suspicious_open_spans: Vec::new(),
             override_span,
             last_unclosed_found_span: None,
         }

src/libsyntax/parse/lexer/tokentrees.rs

@@ -52,6 +52,20 @@ impl<'a> StringReader<'a> {
                     err.span_label(sp, "un-closed delimiter");
                 }
 
+                if let Some((delim, _)) = self.open_braces.last() {
+                    if let Some((d, open_sp, close_sp)) = self.suspicious_open_spans.iter()
+                        .filter(|(d, _, _)| delim == d)
+                        .next()  // these are in reverse order as they get inserted on close, but
+                    {            // we want the last open/first close
+                        if d == delim {
+                            err.span_label(*open_sp, "this might be the culprit...");
+                            err.span_label(
+                                *close_sp,
+                                "...as it matches this but it has different indentation",
+                            );
+                        }
+                    }
+                }
                 Err(err)
             },
             token::OpenDelim(delim) => {
@@ -70,11 +84,20 @@ impl<'a> StringReader<'a> {
                 // Expand to cover the entire delimited token tree
                 let span = pre_span.with_hi(self.span.hi());
 
+                let sm = self.sess.source_map();
                 match self.token {
                     // Correct delimiter.
                     token::CloseDelim(d) if d == delim => {
-                        self.open_braces.pop().unwrap();
-
+                        let (open_brace, open_brace_span) = self.open_braces.pop().unwrap();
+                        if let Some(current_padding) = sm.span_to_margin(self.span) {
+                            if let Some(padding) = sm.span_to_margin(open_brace_span) {
+                                if current_padding != padding {
+                                    self.suspicious_open_spans.push(
+                                        (open_brace, open_brace_span, self.span),
+                                    );
+                                }
+                            }
+                        }
                         // Parse the close delimiter.
                         self.real_token();
                     }
@@ -96,6 +119,19 @@ impl<'a> StringReader<'a> {
                             if let Some(&(_, sp)) = self.open_braces.last() {
                                 err.span_label(sp, "un-closed delimiter");
                             };
+                            if let Some(current_padding) = sm.span_to_margin(self.span) {
+                                for (brace, brace_span) in &self.open_braces {
+                                    if let Some(padding) = sm.span_to_margin(*brace_span) {
+                                        // high likelihood of these two corresponding
+                                        if current_padding == padding && brace == &other {
+                                            err.span_label(
+                                                *brace_span,
+                                                "close delimiter possibly meant for this",
+                                            );
+                                        }
+                                    }
+                                }
+                            }
                             err.emit();
                         }
                         self.open_braces.pop().unwrap();

src/libsyntax/source_map.rs

@@ -251,17 +251,18 @@ impl SourceMap {
     /// crate. The source code of such an "imported source_file" is not available,
     /// but we still know enough to generate accurate debuginfo location
     /// information for things inlined from other crates.
-    pub fn new_imported_source_file(&self,
-                                filename: FileName,
-                                name_was_remapped: bool,
-                                crate_of_origin: u32,
-                                src_hash: u128,
-                                name_hash: u128,
-                                source_len: usize,
-                                mut file_local_lines: Vec<BytePos>,
-                                mut file_local_multibyte_chars: Vec<MultiByteChar>,
-                                mut file_local_non_narrow_chars: Vec<NonNarrowChar>)
-                                -> Lrc<SourceFile> {
+    pub fn new_imported_source_file(
+        &self,
+        filename: FileName,
+        name_was_remapped: bool,
+        crate_of_origin: u32,
+        src_hash: u128,
+        name_hash: u128,
+        source_len: usize,
+        mut file_local_lines: Vec<BytePos>,
+        mut file_local_multibyte_chars: Vec<MultiByteChar>,
+        mut file_local_non_narrow_chars: Vec<NonNarrowChar>,
+    ) -> Lrc<SourceFile> {
         let start_pos = self.next_start_pos();
 
         let end_pos = Pos::from_usize(start_pos + source_len);
@@ -578,6 +579,15 @@ impl SourceMap {
                                                                 .to_string())
     }
 
+    pub fn span_to_margin(&self, sp: Span) -> Option<usize> {
+        match self.span_to_prev_source(sp) {
+            Err(_) => None,
+            Ok(source) => source.split('\n').last().map(|last_line| {
+                last_line.len() - last_line.trim_left().len()
+            })
+        }
+    }
+
     /// Return the source snippet as `String` before the given `Span`
     pub fn span_to_prev_source(&self, sp: Span) -> Result<String, SpanSnippetError> {
         self.span_to_source(sp, |src, start_index, _| src[..start_index].to_string())

src/test/ui/parser-recovery-1.stderr

@@ -3,6 +3,11 @@ error: this file contains an un-closed delimiter
    |
 LL | trait Foo {
    |           - un-closed delimiter
+LL |     fn bar() {
+   |              - this might be the culprit...
+...
+LL | }
+   | - ...as it matches this but it has different indentation
 ...
 LL | } //~ ERROR this file contains an un-closed delimiter
    |                                                       ^

src/test/ui/parser/unclosed-braces.rs

@@ -0,0 +1,32 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+struct S {
+    x: [usize; 3],
+}
+
+fn foo() {
+    {
+        {
+            println!("hi");
+        }
+    }
+}
+
+fn main() {
+//~^ NOTE un-closed delimiter
+    {
+        {
+        //~^ NOTE this might be the culprit...
+            foo();
+    }
+    //~^ NOTE ...as it matches this but it has different indentation
+}
+//~ ERROR this file contains an un-closed delimiter

src/test/ui/parser/unclosed-braces.stderr

@@ -0,0 +1,17 @@
+error: this file contains an un-closed delimiter
+  --> $DIR/unclosed-braces.rs:32:53
+   |
+LL | fn main() {
+   |           - un-closed delimiter
+...
+LL |         {
+   |         - this might be the culprit...
+...
+LL |     }
+   |     - ...as it matches this but it has different indentation
+...
+LL | //~ ERROR this file contains an un-closed delimiter
+   |                                                     ^
+
+error: aborting due to previous error
+

src/test/ui/resolve/token-error-correct-3.stderr

@@ -1,6 +1,8 @@
 error: incorrect close delimiter: `}`
   --> $DIR/token-error-correct-3.rs:30:9
    |
+LL |         if !is_directory(path.as_ref()) { //~ ERROR: cannot find function `is_directory`
+   |                                         - close delimiter possibly meant for this
 LL |             callback(path.as_ref(); //~ ERROR expected one of
    |                     - un-closed delimiter
 ...

src/test/ui/resolve/token-error-correct.stderr

@@ -1,6 +1,8 @@
 error: incorrect close delimiter: `}`
   --> $DIR/token-error-correct.rs:16:1
    |
+LL | fn main() {
+   |           - close delimiter possibly meant for this
 LL |     foo(bar(;
    |            - un-closed delimiter
 LL |     //~^ ERROR: expected expression, found `;`

src/test/ui/token/issue-10636-2.stderr

@@ -1,6 +1,8 @@
 error: incorrect close delimiter: `}`
   --> $DIR/issue-10636-2.rs:18:1
    |
+LL | pub fn trace_option(option: Option<isize>) {
+   |                                            - close delimiter possibly meant for this
 LL |     option.map(|some| 42;
    |               - un-closed delimiter
 ...