libstd: Prefer `Option::map`/etc over `match` where applicable

src/libstd/net/parser.rs

@@ -53,10 +53,7 @@ fn read_till_eof<T, F>(&mut self, cb: F) -> Option<T> where
         F: FnOnce(&mut Parser) -> Option<T>,
     {
         self.read_atomically(move |p| {
-            match cb(p) {
-                Some(x) => if p.is_eof() {Some(x)} else {None},
-                None => None,
-            }
+            cb(p).filter(|_| p.is_eof())
         })
     }
 

src/libstd/path.rs

@@ -1065,10 +1065,7 @@ impl<'a> Iterator for Ancestors<'a> {
 
     fn next(&mut self) -> Option<Self::Item> {
         let next = self.next;
-        self.next = match next {
-            Some(path) => path.parent(),
-            None => None,
-        };
+        self.next = next.and_then(Path::parent);
         next
     }
 }

src/libstd/sys_common/backtrace.rs

@@ -156,16 +156,15 @@ pub fn log_enabled() -> Option<PrintFormat> {
         _ => return Some(PrintFormat::Full),
     }
 
-    let val = match env::var_os("RUST_BACKTRACE") {
-        Some(x) => if &x == "0" {
+    let val = env::var_os("RUST_BACKTRACE").and_then(|x|
+        if &x == "0" {
             None
         } else if &x == "full" {
             Some(PrintFormat::Full)
         } else {
             Some(PrintFormat::Short)
-        },
-        None => None,
-    };
+        }
+    );
     ENABLED.store(match val {
         Some(v) => v as isize,
         None => 1,