prefer `if let` to match with `None => {}` arm in some places

This is a spiritual succesor to #34268/8531d581, in which we replaced a
number of matches of None to the unit value with `if let` conditionals
where it was judged that this made for clearer/simpler code (as would be
recommended by Manishearth/rust-clippy's `single_match` lint). The same
rationale applies to matches of None to the empty block.

3 files changed, 16 insertions, 26 deletions

diff --git a/src/libsyntax/parse/attr.rs b/src/libsyntax/parse/attr.rs
index f6e94b7caea..15344cef1db 100644
--- a/src/libsyntax/parse/attr.rs
+++ b/src/libsyntax/parse/attr.rs
@@ -160,12 +160,9 @@ impl<'a> Parser<'a> {
             _ => None,
         };
 
-        match nt_meta {
-            Some(meta) => {
-                self.bump();
-                return Ok(meta);
-            }
-            None => {}
+        if let Some(meta) = nt_meta {
+            self.bump();
+            return Ok(meta);
         }
 
         let lo = self.span.lo;
diff --git a/src/libsyntax/parse/lexer/mod.rs b/src/libsyntax/parse/lexer/mod.rs
index 809f4daa361..77b5c10899a 100644
--- a/src/libsyntax/parse/lexer/mod.rs
+++ b/src/libsyntax/parse/lexer/mod.rs
@@ -470,15 +470,12 @@ impl<'a> StringReader<'a> {
     /// PRECONDITION: self.curr is not whitespace
     /// Eats any kind of comment.
     fn scan_comment(&mut self) -> Option<TokenAndSpan> {
-        match self.curr {
-            Some(c) => {
-                if c.is_whitespace() {
-                    self.span_diagnostic.span_err(syntax_pos::mk_sp(self.last_pos, self.last_pos),
-                                                  "called consume_any_line_comment, but there \
-                                                   was whitespace");
-                }
+        if let Some(c) = self.curr {
+            if c.is_whitespace() {
+                self.span_diagnostic.span_err(syntax_pos::mk_sp(self.last_pos, self.last_pos),
+                                              "called consume_any_line_comment, but there \
+                                               was whitespace");
             }
-            None => {}
         }
 
         if self.curr_is('/') {
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 20a54228d01..6fa95afd9fb 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -2752,9 +2752,8 @@ impl<'a> Parser<'a> {
             }
         };
 
-        match parse_kleene_op(self)? {
-            Some(kleene_op) => return Ok((None, kleene_op)),
-            None => {}
+        if let Some(kleene_op) = parse_kleene_op(self)? {
+            return Ok((None, kleene_op));
         }
 
         let separator = self.bump_and_get();
@@ -5691,15 +5690,12 @@ impl<'a> Parser<'a> {
             }
             _ => None
         };
-        match nt_item {
-            Some(mut item) => {
-                self.bump();
-                let mut attrs = attrs;
-                mem::swap(&mut item.attrs, &mut attrs);
-                item.attrs.extend(attrs);
-                return Ok(Some(P(item)));
-            }
-            None => {}
+        if let Some(mut item) = nt_item {
+            self.bump();
+            let mut attrs = attrs;
+            mem::swap(&mut item.attrs, &mut attrs);
+            item.attrs.extend(attrs);
+            return Ok(Some(P(item)));
         }
 
         let lo = self.span.lo;