Adopt let else in more places

compiler/rustc_parse/src/parser/expr.rs

@@ -1667,9 +1667,8 @@ impl<'a> Parser<'a> {
             Err(LitError::NotLiteral) => None,
             Err(err) => {
                 let span = token.span;
-                let lit = match token.kind {
-                    token::Literal(lit) => lit,
-                    _ => unreachable!(),
+                let token::Literal(lit) = token.kind else {
+                    unreachable!();
                 };
                 self.bump();
                 self.report_lit_error(err, lit, span);

compiler/rustc_parse/src/parser/item.rs

@@ -439,9 +439,8 @@ impl<'a> Parser<'a> {
 
     /// Recover if we parsed attributes and expected an item but there was none.
     fn recover_attrs_no_item(&mut self, attrs: &[Attribute]) -> PResult<'a, ()> {
-        let (start, end) = match attrs {
-            [] => return Ok(()),
-            [x0 @ xn] | [x0, .., xn] => (x0, xn),
+        let ([start @ end] | [start, .., end]) = attrs else {
+            return Ok(());
         };
         let msg = if end.is_doc_comment() {
             "expected item after doc comment"

compiler/rustc_parse/src/parser/stmt.rs

@@ -524,9 +524,8 @@ impl<'a> Parser<'a> {
         // Skip looking for a trailing semicolon when we have an interpolated statement.
         maybe_whole!(self, NtStmt, |x| Some(x));
 
-        let mut stmt = match self.parse_stmt_without_recovery(true, ForceCollect::No)? {
-            Some(stmt) => stmt,
-            None => return Ok(None),
+        let Some(mut stmt) = self.parse_stmt_without_recovery(true, ForceCollect::No)? else {
+            return Ok(None);
         };
 
         let mut eat_semi = true;