Simplify RefTokenTreeCursor::look_ahead.

It's only ever used with a lookahead of 0, so this commit removes the
lookahead and renames it `peek`.

compiler/rustc_ast/src/tokenstream.rs

@@ -678,8 +678,8 @@ impl<'t> RefTokenTreeCursor<'t> {
         RefTokenTreeCursor { stream, index: 0 }
     }
 
-    pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
+    pub fn peek(&self) -> Option<&TokenTree> {
-        self.stream.0.get(self.index + n)
+        self.stream.0.get(self.index)
     }
 }
 

compiler/rustc_expand/src/mbe/metavar_expr.rs

@@ -74,7 +74,7 @@ impl MetaVarExpr {
                         }
                     };
                     result.push(element);
-                    if iter.look_ahead(0).is_none() {
+                    if iter.peek().is_none() {
                         break;
                     }
                     if !try_eat_comma(&mut iter) {
@@ -166,7 +166,7 @@ fn parse_count<'psess>(
     eat_dollar(iter, psess, span)?;
     let ident = parse_ident(iter, psess, span)?;
     let depth = if try_eat_comma(iter) {
-        if iter.look_ahead(0).is_none() {
+        if iter.peek().is_none() {
             return Err(psess.dcx().struct_span_err(
                 span,
                 "`count` followed by a comma must have an associated index indicating its depth",
@@ -252,7 +252,7 @@ fn parse_token<'psess, 't>(
 /// Tries to move the iterator forward returning `true` if there is a comma. If not, then the
 /// iterator is not modified and the result is `false`.
 fn try_eat_comma(iter: &mut RefTokenTreeCursor<'_>) -> bool {
-    if let Some(TokenTree::Token(Token { kind: token::Comma, .. }, _)) = iter.look_ahead(0) {
+    if let Some(TokenTree::Token(Token { kind: token::Comma, .. }, _)) = iter.peek() {
         let _ = iter.next();
         return true;
     }
@@ -262,7 +262,7 @@ fn try_eat_comma(iter: &mut RefTokenTreeCursor<'_>) -> bool {
 /// Tries to move the iterator forward returning `true` if there is a dollar sign. If not, then the
 /// iterator is not modified and the result is `false`.
 fn try_eat_dollar(iter: &mut RefTokenTreeCursor<'_>) -> bool {
-    if let Some(TokenTree::Token(Token { kind: token::Dollar, .. }, _)) = iter.look_ahead(0) {
+    if let Some(TokenTree::Token(Token { kind: token::Dollar, .. }, _)) = iter.peek() {
         let _ = iter.next();
         return true;
     }
@@ -275,7 +275,7 @@ fn eat_dollar<'psess>(
     psess: &'psess ParseSess,
     span: Span,
 ) -> PResult<'psess, ()> {
-    if let Some(TokenTree::Token(Token { kind: token::Dollar, .. }, _)) = iter.look_ahead(0) {
+    if let Some(TokenTree::Token(Token { kind: token::Dollar, .. }, _)) = iter.peek() {
         let _ = iter.next();
         return Ok(());
     }

src/tools/clippy/clippy_lints/src/crate_in_macro_def.rs

@@ -86,7 +86,7 @@ fn contains_unhygienic_crate_reference(tts: &TokenStream) -> Option<Span> {
     while let Some(curr) = cursor.next() {
         if !prev_is_dollar
             && let Some(span) = is_crate_keyword(curr)
-            && let Some(next) = cursor.look_ahead(0)
+            && let Some(next) = cursor.peek()
             && is_token(next, &TokenKind::PathSep)
         {
             return Some(span);

src/tools/rustfmt/src/macros.rs

@@ -1190,7 +1190,7 @@ impl<'a> MacroParser<'a> {
     // (`(` ... `)` `=>` `{` ... `}`)*
     fn parse(&mut self) -> Option<Macro> {
         let mut branches = vec![];
-        while self.toks.look_ahead(0).is_some() {
+        while self.toks.peek().is_some() {
             branches.push(self.parse_branch()?);
         }
 
@@ -1237,7 +1237,7 @@ impl<'a> MacroParser<'a> {
                 span,
             },
             _,
-        )) = self.toks.look_ahead(0)
+        )) = self.toks.peek()
         {
             hi = span.hi();
             self.toks.next();