Auto merge of #53902 - dtolnay:group, r=petrochenkov

proc_macro::Group::span_open and span_close

Before this addition, every delimited group like `(`...`)` `[`...`]` `{`...`}` has only a single Span that covers the full source location from opening delimiter to closing delimiter. This makes it impossible for a procedural macro to trigger an error pointing to just the opening or closing delimiter. The Rust compiler does not seem to have the same limitation:

```rust
mod m {
    type T =
}
```

```console
error: expected type, found `}`
 --> src/main.rs:3:1
  |
3 | }
  | ^
```

On that same input, a procedural macro would be forced to trigger the error on the last token inside the block, on the entire block, or on the next token after the block, none of which is really what you want for an error like above.

This commit adds `group.span_open()` and `group.span_close()` which access the Span associated with just the opening delimiter and just the closing delimiter of the group. Relevant to Syn as we implement real error messages for when parsing fails in a procedural macro: https://github.com/dtolnay/syn/issues/476.

```diff
  impl Group {
      fn span(&self) -> Span;
+     fn span_open(&self) -> Span;
+     fn span_close(&self) -> Span;
  }
```

Fixes #48187
r? @alexcrichton

src/libsyntax/parse/parser.rs

@@ -54,7 +54,7 @@ use print::pprust;
 use ptr::P;
 use parse::PResult;
 use ThinVec;
-use tokenstream::{self, Delimited, ThinTokenStream, TokenTree, TokenStream};
+use tokenstream::{self, Delimited, DelimSpan, ThinTokenStream, TokenTree, TokenStream};
 use symbol::{Symbol, keywords};
 
 use std::borrow::Cow;
@@ -262,7 +262,7 @@ struct TokenCursor {
 #[derive(Clone)]
 struct TokenCursorFrame {
     delim: token::DelimToken,
-    span: Span,
+    span: DelimSpan,
     open_delim: bool,
     tree_cursor: tokenstream::Cursor,
     close_delim: bool,
@@ -293,7 +293,7 @@ enum LastToken {
 }
 
 impl TokenCursorFrame {
-    fn new(sp: Span, delimited: &Delimited) -> Self {
+    fn new(sp: DelimSpan, delimited: &Delimited) -> Self {
         TokenCursorFrame {
             delim: delimited.delim,
             span: sp,
@@ -311,13 +311,13 @@ impl TokenCursor {
             let tree = if !self.frame.open_delim {
                 self.frame.open_delim = true;
                 Delimited { delim: self.frame.delim, tts: TokenStream::empty().into() }
-                    .open_tt(self.frame.span)
+                    .open_tt(self.frame.span.open)
             } else if let Some(tree) = self.frame.tree_cursor.next() {
                 tree
             } else if !self.frame.close_delim {
                 self.frame.close_delim = true;
                 Delimited { delim: self.frame.delim, tts: TokenStream::empty().into() }
-                    .close_tt(self.frame.span)
+                    .close_tt(self.frame.span.close)
             } else if let Some(frame) = self.stack.pop() {
                 self.frame = frame;
                 continue
@@ -361,7 +361,8 @@ impl TokenCursor {
             num_of_hashes = cmp::max(num_of_hashes, count);
         }
 
-        let body = TokenTree::Delimited(sp, Delimited {
+        let delim_span = DelimSpan::from_single(sp);
+        let body = TokenTree::Delimited(delim_span, Delimited {
             delim: token::Bracket,
             tts: [TokenTree::Token(sp, token::Ident(ast::Ident::from_str("doc"), false)),
                   TokenTree::Token(sp, token::Eq),
@@ -370,7 +371,7 @@ impl TokenCursor {
                 .iter().cloned().collect::<TokenStream>().into(),
         });
 
-        self.stack.push(mem::replace(&mut self.frame, TokenCursorFrame::new(sp, &Delimited {
+        self.stack.push(mem::replace(&mut self.frame, TokenCursorFrame::new(delim_span, &Delimited {
             delim: token::NoDelim,
             tts: if doc_comment_style(&name.as_str()) == AttrStyle::Inner {
                 [TokenTree::Token(sp, token::Pound), TokenTree::Token(sp, token::Not), body]
@@ -560,7 +561,7 @@ impl<'a> Parser<'a> {
             root_module_name: None,
             expected_tokens: Vec::new(),
             token_cursor: TokenCursor {
-                frame: TokenCursorFrame::new(syntax_pos::DUMMY_SP, &Delimited {
+                frame: TokenCursorFrame::new(DelimSpan::dummy(), &Delimited {
                     delim: token::NoDelim,
                     tts: tokens.into(),
                 }),
@@ -1227,7 +1228,8 @@ impl<'a> Parser<'a> {
         }
 
         match self.token_cursor.frame.tree_cursor.look_ahead(dist - 1) {
-            Some(TokenTree::Token(span, _)) | Some(TokenTree::Delimited(span, _)) => span,
+            Some(TokenTree::Token(span, _)) => span,
+            Some(TokenTree::Delimited(span, _)) => span.entire(),
             None => self.look_ahead_span(dist - 1),
         }
     }
@@ -2788,7 +2790,7 @@ impl<'a> Parser<'a> {
             token::OpenDelim(..) => {
                 let frame = mem::replace(&mut self.token_cursor.frame,
                                          self.token_cursor.stack.pop().unwrap());
-                self.span = frame.span;
+                self.span = frame.span.entire();
                 self.bump();
                 TokenTree::Delimited(frame.span, Delimited {
                     delim: frame.delim,