Implement FromStr for PathBuf

Initially landed in rust-lang#48292
and reverted in rust-lang#50401.
This time, use `std::string::ParseError` as suggested in
rust-lang#44431 (comment)

src/libstd/path.rs

@@ -87,6 +87,8 @@ use io;
 use iter::{self, FusedIterator};
 use ops::{self, Deref};
 use rc::Rc;
+use str::FromStr;
+use string::ParseError;
 use sync::Arc;
 
 use ffi::{OsStr, OsString};
@@ -1446,6 +1448,15 @@ impl From<String> for PathBuf {
     }
 }
 
+#[stable(feature = "path_from_str", since = "1.26.0")]
+impl FromStr for PathBuf {
+    type Err = ParseError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(PathBuf::from(s))
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<P: AsRef<Path>> iter::FromIterator<P> for PathBuf {
     fn from_iter<I: IntoIterator<Item = P>>(iter: I) -> PathBuf {