Auto merge of #31608 - frewsxcv:osstring-simple-functions, r=alexcrichton

https://github.com/rust-lang/rust/issues/29453

src/libstd/sys/common/wtf8.rs

@@ -178,6 +178,10 @@ impl Wtf8Buf {
         Wtf8Buf { bytes: <[_]>::to_vec(str.as_bytes()) }
     }
 
+    pub fn clear(&mut self) {
+        self.bytes.clear()
+    }
+
     /// Creates a WTF-8 string from a potentially ill-formed UTF-16 slice of 16-bit code units.
     ///
     /// This is lossless: calling `.encode_wide()` on the resulting string
@@ -234,6 +238,11 @@ impl Wtf8Buf {
         self.bytes.reserve(additional)
     }
 
+    #[inline]
+    pub fn reserve_exact(&mut self, additional: usize) {
+        self.bytes.reserve_exact(additional)
+    }
+
     /// Returns the number of bytes that this string buffer can hold without reallocating.
     #[inline]
     pub fn capacity(&self) -> usize {
@@ -443,6 +452,11 @@ impl Wtf8 {
         self.bytes.len()
     }
 
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.bytes.is_empty()
+    }
+
     /// Returns the code point at `position` if it is in the ASCII range,
     /// or `b'\xFF' otherwise.
     ///

src/libstd/sys/unix/os_str.rs

@@ -17,6 +17,7 @@ use vec::Vec;
 use str;
 use string::String;
 use mem;
+use sys_common::{AsInner, IntoInner};
 
 #[derive(Clone, Hash)]
 pub struct Buf {
@@ -39,11 +40,51 @@ impl Debug for Buf {
     }
 }
 
+impl IntoInner<Vec<u8>> for Buf {
+    fn into_inner(self) -> Vec<u8> {
+        self.inner
+    }
+}
+
+impl AsInner<[u8]> for Buf {
+    fn as_inner(&self) -> &[u8] {
+        &self.inner
+    }
+}
+
+
 impl Buf {
     pub fn from_string(s: String) -> Buf {
         Buf { inner: s.into_bytes() }
     }
 
+    #[inline]
+    pub fn with_capacity(capacity: usize) -> Buf {
+        Buf {
+            inner: Vec::with_capacity(capacity)
+        }
+    }
+
+    #[inline]
+    pub fn clear(&mut self) {
+        self.inner.clear()
+    }
+
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.inner.capacity()
+    }
+
+    #[inline]
+    pub fn reserve(&mut self, additional: usize) {
+        self.inner.reserve(additional)
+    }
+
+    #[inline]
+    pub fn reserve_exact(&mut self, additional: usize) {
+        self.inner.reserve_exact(additional)
+    }
+
     pub fn as_slice(&self) -> &Slice {
         unsafe { mem::transmute(&*self.inner) }
     }

src/libstd/sys/windows/os_str.rs

@@ -18,12 +18,25 @@ use string::String;
 use result::Result;
 use option::Option;
 use mem;
+use sys_common::{AsInner, IntoInner};
 
 #[derive(Clone, Hash)]
 pub struct Buf {
     pub inner: Wtf8Buf
 }
 
+impl IntoInner<Wtf8Buf> for Buf {
+    fn into_inner(self) -> Wtf8Buf {
+        self.inner
+    }
+}
+
+impl AsInner<Wtf8> for Buf {
+    fn as_inner(&self) -> &Wtf8 {
+        &self.inner
+    }
+}
+
 impl Debug for Buf {
     fn fmt(&self, formatter: &mut fmt::Formatter) -> Result<(), fmt::Error> {
         self.as_slice().fmt(formatter)
@@ -41,6 +54,20 @@ impl Debug for Slice {
 }
 
 impl Buf {
+    pub fn with_capacity(capacity: usize) -> Buf {
+        Buf {
+            inner: Wtf8Buf::with_capacity(capacity)
+        }
+    }
+
+    pub fn clear(&mut self) {
+        self.inner.clear()
+    }
+
+    pub fn capacity(&self) -> usize {
+        self.inner.capacity()
+    }
+
     pub fn from_string(s: String) -> Buf {
         Buf { inner: Wtf8Buf::from_string(s) }
     }
@@ -56,6 +83,14 @@ impl Buf {
     pub fn push_slice(&mut self, s: &Slice) {
         self.inner.push_wtf8(&s.inner)
     }
+
+    pub fn reserve(&mut self, additional: usize) {
+        self.inner.reserve(additional)
+    }
+
+    pub fn reserve_exact(&mut self, additional: usize) {
+        self.inner.reserve_exact(additional)
+    }
 }
 
 impl Slice {