Auto merge of #24517 - erickt:str, r=alexcrichton

This implementation is currently about 3-4 times faster than using the `.to_string()` based approach.

I would also suggest we deprecate `String::from_str` since it's redundant with the stable `String::from` method, but I'll leave that for a future PR.

1 files changed, 12 insertions, 1 deletions

diff --git a/src/libcollections/string.rs b/src/libcollections/string.rs
index 74af5783fa8..81710286fde 100644
--- a/src/libcollections/string.rs
+++ b/src/libcollections/string.rs
@@ -1013,9 +1013,20 @@ impl AsRef<str> for String {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a> From<&'a str> for String {
+    #[cfg(not(test))]
     #[inline]
     fn from(s: &'a str) -> String {
-        s.to_string()
+        String { vec: <[_]>::to_vec(s.as_bytes()) }
+    }
+
+    // HACK(japaric): with cfg(test) the inherent `[T]::to_vec` method, which is
+    // required for this method definition, is not available. Since we don't
+    // require this method for testing purposes, I'll just stub it
+    // NB see the slice::hack module in slice.rs for more information
+    #[inline]
+    #[cfg(test)]
+    fn from(_: &str) -> String {
+        panic!("not available with cfg(test)");
     }
 }