Rollup merge of #50170 - burtonageo:more_cow_from, r=alexcrichton

Implement From for more types on Cow

This is basically https://github.com/rust-lang/rust/pull/48191, except that it should be implemented in a way that doesn't break third party crates.

src/liballoc/string.rs

@@ -2240,6 +2240,14 @@ impl<'a> From<String> for Cow<'a, str> {
     }
 }
 
+#[stable(feature = "cow_from_string_ref", since = "1.28.0")]
+impl<'a> From<&'a String> for Cow<'a, str> {
+    #[inline]
+    fn from(s: &'a String) -> Cow<'a, str> {
+        Cow::Borrowed(s.as_str())
+    }
+}
+
 #[stable(feature = "cow_str_from_iter", since = "1.12.0")]
 impl<'a> FromIterator<char> for Cow<'a, str> {
     fn from_iter<I: IntoIterator<Item = char>>(it: I) -> Cow<'a, str> {

src/liballoc/vec.rs

@@ -2286,6 +2286,13 @@ impl<'a, T: Clone> From<Vec<T>> for Cow<'a, [T]> {
     }
 }
 
+#[stable(feature = "cow_from_vec_ref", since = "1.28.0")]
+impl<'a, T: Clone> From<&'a Vec<T>> for Cow<'a, [T]> {
+    fn from(v: &'a Vec<T>) -> Cow<'a, [T]> {
+        Cow::Borrowed(v.as_slice())
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> FromIterator<T> for Cow<'a, [T]> where T: Clone {
     fn from_iter<I: IntoIterator<Item = T>>(it: I) -> Cow<'a, [T]> {

src/libstd/ffi/c_str.rs

@@ -682,6 +682,14 @@ impl Borrow<CStr> for CString {
     fn borrow(&self) -> &CStr { self }
 }
 
+#[stable(feature = "cstring_from_cow_cstr", since = "1.28.0")]
+impl<'a> From<Cow<'a, CStr>> for CString {
+    #[inline]
+    fn from(s: Cow<'a, CStr>) -> Self {
+        s.into_owned()
+    }
+}
+
 #[stable(feature = "box_from_c_str", since = "1.17.0")]
 impl<'a> From<&'a CStr> for Box<CStr> {
     fn from(s: &'a CStr) -> Box<CStr> {
@@ -706,6 +714,30 @@ impl From<CString> for Box<CStr> {
     }
 }
 
+#[stable(feature = "cow_from_cstr", since = "1.28.0")]
+impl<'a> From<CString> for Cow<'a, CStr> {
+    #[inline]
+    fn from(s: CString) -> Cow<'a, CStr> {
+        Cow::Owned(s)
+    }
+}
+
+#[stable(feature = "cow_from_cstr", since = "1.28.0")]
+impl<'a> From<&'a CStr> for Cow<'a, CStr> {
+    #[inline]
+    fn from(s: &'a CStr) -> Cow<'a, CStr> {
+        Cow::Borrowed(s)
+    }
+}
+
+#[stable(feature = "cow_from_cstr", since = "1.28.0")]
+impl<'a> From<&'a CString> for Cow<'a, CStr> {
+    #[inline]
+    fn from(s: &'a CString) -> Cow<'a, CStr> {
+        Cow::Borrowed(s.as_c_str())
+    }
+}
+
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<CString> for Arc<CStr> {
     #[inline]

src/libstd/ffi/os_str.rs

@@ -664,6 +664,38 @@ impl<'a> From<&'a OsStr> for Rc<OsStr> {
     }
 }
 
+#[stable(feature = "cow_from_osstr", since = "1.28.0")]
+impl<'a> From<OsString> for Cow<'a, OsStr> {
+    #[inline]
+    fn from(s: OsString) -> Cow<'a, OsStr> {
+        Cow::Owned(s)
+    }
+}
+
+#[stable(feature = "cow_from_osstr", since = "1.28.0")]
+impl<'a> From<&'a OsStr> for Cow<'a, OsStr> {
+    #[inline]
+    fn from(s: &'a OsStr) -> Cow<'a, OsStr> {
+        Cow::Borrowed(s)
+    }
+}
+
+#[stable(feature = "cow_from_osstr", since = "1.28.0")]
+impl<'a> From<&'a OsString> for Cow<'a, OsStr> {
+    #[inline]
+    fn from(s: &'a OsString) -> Cow<'a, OsStr> {
+        Cow::Borrowed(s.as_os_str())
+    }
+}
+
+#[stable(feature = "osstring_from_cow_osstr", since = "1.28.0")]
+impl<'a> From<Cow<'a, OsStr>> for OsString {
+    #[inline]
+    fn from(s: Cow<'a, OsStr>) -> Self {
+        s.into_owned()
+    }
+}
+
 #[stable(feature = "box_default_extra", since = "1.17.0")]
 impl Default for Box<OsStr> {
     fn default() -> Box<OsStr> {

src/libstd/path.rs

@@ -1504,6 +1504,22 @@ impl<'a> From<PathBuf> for Cow<'a, Path> {
     }
 }
 
+#[stable(feature = "cow_from_pathbuf_ref", since = "1.28.0")]
+impl<'a> From<&'a PathBuf> for Cow<'a, Path> {
+    #[inline]
+    fn from(p: &'a PathBuf) -> Cow<'a, Path> {
+        Cow::Borrowed(p.as_path())
+    }
+}
+
+#[stable(feature = "pathbuf_from_cow_path", since = "1.28.0")]
+impl<'a> From<Cow<'a, Path>> for PathBuf {
+    #[inline]
+    fn from(p: Cow<'a, Path>) -> Self {
+        p.into_owned()
+    }
+}
+
 #[stable(feature = "shared_from_slice2", since = "1.24.0")]
 impl From<PathBuf> for Arc<Path> {
     #[inline]