Fix clippy::len_zero warnings

src/liballoc/collections/btree/map.rs

@@ -200,7 +200,7 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
             }
         }
 
-        if self.len() == 0 {
+        if self.is_empty() {
             // Ideally we'd call `BTreeMap::new` here, but that has the `K:
             // Ord` constraint, which this method lacks.
             BTreeMap {
@@ -759,12 +759,12 @@ impl<K: Ord, V> BTreeMap<K, V> {
     #[stable(feature = "btree_append", since = "1.11.0")]
     pub fn append(&mut self, other: &mut Self) {
         // Do we have to append anything at all?
-        if other.len() == 0 {
+        if other.is_empty() {
             return;
         }
 
         // We can just swap `self` and `other` if `self` is empty.
-        if self.len() == 0 {
+        if self.is_empty() {
             mem::swap(self, other);
             return;
         }

src/libcore/str/lossy.rs

@@ -46,7 +46,7 @@ impl<'a> Iterator for Utf8LossyChunksIter<'a> {
     type Item = Utf8LossyChunk<'a>;
 
     fn next(&mut self) -> Option<Utf8LossyChunk<'a>> {
-        if self.source.len() == 0 {
+        if self.source.is_empty() {
             return None;
         }
 
@@ -141,7 +141,7 @@ impl fmt::Display for Utf8Lossy {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         // If we're the empty string then our iterator won't actually yield
         // anything, so perform the formatting manually
-        if self.bytes.len() == 0 {
+        if self.bytes.is_empty() {
             return "".fmt(f)
         }
 

src/libstd/io/mod.rs

@@ -1923,7 +1923,7 @@ impl<T: Read, U: Read> Read for Chain<T, U> {
     fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
         if !self.done_first {
             match self.first.read(buf)? {
-                0 if buf.len() != 0 => self.done_first = true,
+                0 if !buf.is_empty() => self.done_first = true,
                 n => return Ok(n),
             }
         }
@@ -1955,7 +1955,7 @@ impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
     fn fill_buf(&mut self) -> Result<&[u8]> {
         if !self.done_first {
             match self.first.fill_buf()? {
-                buf if buf.len() == 0 => { self.done_first = true; }
+                buf if buf.is_empty() => { self.done_first = true; }
                 buf => return Ok(buf),
             }
         }

src/libstd/sys/unix/process/process_unix.rs

@@ -277,7 +277,7 @@ impl Command {
         if self.get_gid().is_some() ||
             self.get_uid().is_some() ||
             self.env_saw_path() ||
-            self.get_closures().len() != 0 {
+            !self.get_closures().is_empty() {
             return Ok(None)
         }