See https://github.com/thepowersgang/mrustc/archive/v0.8.0.tar.gz

--- rustc-1.19.0-src-orig/src/libcore/intrinsics.rs
+++ rustc-1.19.0-src/src/libcore/intrinsics.rs
@@ -678,5 +678,9 @@
     pub fn min_align_of_val<T: ?Sized>(_: &T) -> usize;
 
+    /// Obtain the length of a slice pointer
+    #[cfg(rust_compiler="mrustc")]
+    pub fn mrustc_slice_len<T>(pointer: *const [T]) -> usize;
+
     /// Gets a static string slice containing the name of a type.
     pub fn type_name<T: ?Sized>() -> &'static str;

--- rustc-1.19.0-src-orig/src/libcore/slice/mod.rs
+++ rustc-1.19.0-src/src/libcore/slice/mod.rs
@@ -413,6 +413,8 @@
     #[inline]
     fn len(&self) -> usize {
-        unsafe {
-            mem::transmute::<&[T], Repr<T>>(self).len
-        }
+        #[cfg(not(rust_compiler="mrustc"))]
+        let rv = unsafe { mem::transmute::<&[T], Repr<T>>(self).len };
+        #[cfg(rust_compiler="mrustc")]
+        let rv = unsafe { ::intrinsics::mrustc_slice_len(self) };
+        rv
     }
