Remove TreeAndSpacing.

A `TokenStream` contains a `Lrc<Vec<(TokenTree, Spacing)>>`. But this is
not quite right. `Spacing` makes sense for `TokenTree::Token`, but does
not make sense for `TokenTree::Delimited`, because a
`TokenTree::Delimited` cannot be joined with another `TokenTree`.

This commit fixes this problem, by adding `Spacing` to `TokenTree::Token`,
changing `TokenStream` to contain a `Lrc<Vec<TokenTree>>`, and removing the
`TreeAndSpacing` typedef.

The commit removes these two impls:
- `impl From<TokenTree> for TokenStream`
- `impl From<TokenTree> for TreeAndSpacing`

These were useful, but also resulted in code with many `.into()` calls
that was hard to read, particularly for anyone not highly familiar with
the relevant types. This commit makes some other changes to compensate:
- `TokenTree::token()` becomes `TokenTree::token_{alone,joint}()`.
- `TokenStream::token_{alone,joint}()` are added.
- `TokenStream::delimited` is added.

This results in things like this:
```rust
TokenTree::token(token::Semi, stmt.span).into()
```
changing to this:
```rust
TokenStream::token_alone(token::Semi, stmt.span)
```
This makes the type of the result, and its spacing, clearer.

These changes also simplifies `Cursor` and `CursorRef`, because they no longer
need to distinguish between `next` and `next_with_spacing`.

compiler/rustc_parse/src/parser/mod.rs

@@ -268,13 +268,13 @@ impl TokenCursor {
             // FIXME: we currently don't return `Delimiter` open/close delims. To fix #67062 we will
             // need to, whereupon the `delim != Delimiter::Invisible` conditions below can be
             // removed.
-            if let Some((tree, spacing)) = self.frame.tree_cursor.next_with_spacing_ref() {
+            if let Some(tree) = self.frame.tree_cursor.next_ref() {
                 match tree {
-                    &TokenTree::Token(ref token) => match (desugar_doc_comments, token) {
+                    &TokenTree::Token(ref token, spacing) => match (desugar_doc_comments, token) {
                         (true, &Token { kind: token::DocComment(_, attr_style, data), span }) => {
                             return self.desugar(attr_style, data, span);
                         }
-                        _ => return (token.clone(), *spacing),
+                        _ => return (token.clone(), spacing),
                     },
                     &TokenTree::Delimited(sp, delim, ref tts) => {
                         // Set `open_delim` to true here because we deal with it immediately.
@@ -318,12 +318,14 @@ impl TokenCursor {
             delim_span,
             Delimiter::Bracket,
             [
-                TokenTree::token(token::Ident(sym::doc, false), span),
-                TokenTree::token(token::Eq, span),
-                TokenTree::token(TokenKind::lit(token::StrRaw(num_of_hashes), data, None), span),
+                TokenTree::token_alone(token::Ident(sym::doc, false), span),
+                TokenTree::token_alone(token::Eq, span),
+                TokenTree::token_alone(
+                    TokenKind::lit(token::StrRaw(num_of_hashes), data, None),
+                    span,
+                ),
             ]
-            .iter()
-            .cloned()
+            .into_iter()
             .collect::<TokenStream>(),
         );
 
@@ -332,14 +334,16 @@ impl TokenCursor {
             TokenCursorFrame::new(
                 None,
                 if attr_style == AttrStyle::Inner {
-                    [TokenTree::token(token::Pound, span), TokenTree::token(token::Not, span), body]
-                        .iter()
-                        .cloned()
-                        .collect::<TokenStream>()
+                    [
+                        TokenTree::token_alone(token::Pound, span),
+                        TokenTree::token_alone(token::Not, span),
+                        body,
+                    ]
+                    .into_iter()
+                    .collect::<TokenStream>()
                 } else {
-                    [TokenTree::token(token::Pound, span), body]
-                        .iter()
-                        .cloned()
+                    [TokenTree::token_alone(token::Pound, span), body]
+                        .into_iter()
                         .collect::<TokenStream>()
                 },
             ),
@@ -1042,7 +1046,7 @@ impl<'a> Parser<'a> {
             if all_normal {
                 return match frame.tree_cursor.look_ahead(dist - 1) {
                     Some(tree) => match tree {
-                        TokenTree::Token(token) => looker(token),
+                        TokenTree::Token(token, _) => looker(token),
                         TokenTree::Delimited(dspan, delim, _) => {
                             looker(&Token::new(token::OpenDelim(*delim), dspan.open))
                         }
@@ -1226,7 +1230,7 @@ impl<'a> Parser<'a> {
             token::CloseDelim(_) | token::Eof => unreachable!(),
             _ => {
                 self.bump();
-                TokenTree::Token(self.prev_token.clone())
+                TokenTree::Token(self.prev_token.clone(), Spacing::Alone)
             }
         }
     }
@@ -1245,7 +1249,7 @@ impl<'a> Parser<'a> {
         loop {
             match self.token.kind {
                 token::Eof | token::CloseDelim(..) => break,
-                _ => result.push(self.parse_token_tree().into()),
+                _ => result.push(self.parse_token_tree()),
             }
         }
         TokenStream::new(result)