Fix some more clippy warnings

compiler/rustc_expand/src/mbe.rs

@@ -102,10 +102,7 @@ impl TokenTree {
 
     /// Returns `true` if the given token tree is delimited.
     fn is_delimited(&self) -> bool {
-        match *self {
-            TokenTree::Delimited(..) => true,
-            _ => false,
-        }
+        matches!(*self, TokenTree::Delimited(..))
     }
 
     /// Returns `true` if the given token tree is a token of the given kind.

compiler/rustc_expand/src/mbe/macro_check.rs

@@ -134,10 +134,7 @@ enum Stack<'a, T> {
 impl<'a, T> Stack<'a, T> {
     /// Returns whether a stack is empty.
     fn is_empty(&self) -> bool {
-        match *self {
-            Stack::Empty => true,
-            _ => false,
-        }
+        matches!(*self, Stack::Empty)
     }
 
     /// Returns a new stack with an element of top.

compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -1036,17 +1036,16 @@ fn token_can_be_followed_by_any(tok: &mbe::TokenTree) -> bool {
 /// a fragment specifier (indeed, these fragments can be followed by
 /// ANYTHING without fear of future compatibility hazards).
 fn frag_can_be_followed_by_any(kind: NonterminalKind) -> bool {
-    match kind {
+    matches!(
+        kind,
         NonterminalKind::Item           // always terminated by `}` or `;`
         | NonterminalKind::Block        // exactly one token tree
         | NonterminalKind::Ident        // exactly one token tree
         | NonterminalKind::Literal      // exactly one token tree
         | NonterminalKind::Meta         // exactly one token tree
         | NonterminalKind::Lifetime     // exactly one token tree
-        | NonterminalKind::TT => true,  // exactly one token tree
-
-        _ => false,
-    }
+        | NonterminalKind::TT // exactly one token tree
+    )
 }
 
 enum IsInFollow {

compiler/rustc_expand/src/placeholders.rs

@@ -345,10 +345,10 @@ impl<'a, 'b> MutVisitor for PlaceholderExpander<'a, 'b> {
 
     fn visit_mod(&mut self, module: &mut ast::Mod) {
         noop_visit_mod(module, self);
-        module.items.retain(|item| match item.kind {
-            ast::ItemKind::MacCall(_) if !self.cx.ecfg.keep_macs => false, // remove macro definitions
-            _ => true,
-        });
+        // remove macro definitions
+        module.items.retain(
+            |item| !matches!(item.kind, ast::ItemKind::MacCall(_) if !self.cx.ecfg.keep_macs),
+        );
     }
 
     fn visit_mac(&mut self, _mac: &mut ast::MacCall) {