Change dynamic_library::open_external to take ToCStr

`std::unstable::dynamic_library::open_external` currently takes a
`Path`, but because `Paths` produce normalized strings, this can
change the semantics of lookups in a given environment. This patch
generalizes the function to take a `ToCStr`-bounded type, which
includes both `Path`s and `str`s.

Closes rust-lang#11650.

src/libstd/unstable/dynamic_lib.rs

@@ -42,12 +42,22 @@ impl Drop for DynamicLibrary {
 }
 
 impl DynamicLibrary {
+    // FIXME (#12938): Until DST lands, we cannot decompose &str into
+    // & and str, so we cannot usefully take ToCStr arguments by
+    // reference (without forcing an additional & around &str). So we
+    // are instead temporarily adding an instance for &Path, so that
+    // we can take ToCStr as owned. When DST lands, the &Path instance
+    // should be removed, and arguments bound by ToCStr should be
+    // passed by reference. (Here: in the `open` method.)
+
     /// Lazily open a dynamic library. When passed None it gives a
     /// handle to the calling process
-    pub fn open(filename: Option<&path::Path>) -> Result<DynamicLibrary, ~str> {
+    pub fn open<T: ToCStr>(filename: Option<T>)
+                        -> Result<DynamicLibrary, ~str> {
         unsafe {
+            let mut filename = filename;
             let maybe_library = dl::check_for_errors_in(|| {
-                match filename {
+                match filename.take() {
                     Some(name) => dl::open_external(name),
                     None => dl::open_internal()
                 }
@@ -109,7 +119,8 @@ mod test {
     fn test_loading_cosine() {
         // The math library does not need to be loaded since it is already
         // statically linked in
-        let libm = match DynamicLibrary::open(None) {
+        let none: Option<Path> = None; // appease the typechecker
+        let libm = match DynamicLibrary::open(none) {
             Err(error) => fail!("Could not load self as module: {}", error),
             Ok(libm) => libm
         };
@@ -137,7 +148,7 @@ mod test {
     fn test_errors_do_not_crash() {
         // Open /dev/null as a library to get an error, and make sure
         // that only causes an error, and not a crash.
-        let path = GenericPath::new("/dev/null");
+        let path = Path::new("/dev/null");
         match DynamicLibrary::open(Some(&path)) {
             Err(_) => {}
             Ok(_) => fail!("Successfully opened the empty library.")
@@ -152,12 +163,11 @@ mod test {
 pub mod dl {
     use c_str::ToCStr;
     use libc;
-    use path;
     use ptr;
     use str;
     use result::*;
 
-    pub unsafe fn open_external(filename: &path::Path) -> *u8 {
+    pub unsafe fn open_external<T: ToCStr>(filename: T) -> *u8 {
         filename.with_c_str(|raw_name| {
             dlopen(raw_name, Lazy as libc::c_int) as *u8
         })