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
     }
jtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=aeec4a8fc1dce690b5d6aeb163871aca93d50111'>refs</a><a class='active' href='/guix/log/gnu/packages/autogen.scm?showmsg=1'>log</a><a href='/guix/tree/gnu/packages/autogen.scm?id=aeec4a8fc1dce690b5d6aeb163871aca93d50111'>tree</a><a href='/guix/commit/gnu/packages/autogen.scm?id=aeec4a8fc1dce690b5d6aeb163871aca93d50111'>commit</a><a href='/guix/diff/gnu/packages/autogen.scm?id=aeec4a8fc1dce690b5d6aeb163871aca93d50111'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages/autogen.scm'>
<input type='hidden' name='id' value='aeec4a8fc1dce690b5d6aeb163871aca93d50111'/><input type='hidden' name='showmsg' value='1'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=aeec4a8fc1dce690b5d6aeb163871aca93d50111&amp;showmsg=1'>root</a>/<a href='/guix/log/gnu?id=aeec4a8fc1dce690b5d6aeb163871aca93d50111&amp;showmsg=1'>gnu</a>/<a href='/guix/log/gnu/packages?id=aeec4a8fc1dce690b5d6aeb163871aca93d50111&amp;showmsg=1'>packages</a>/<a href='/guix/log/gnu/packages/autogen.scm?id=aeec4a8fc1dce690b5d6aeb163871aca93d50111&amp;showmsg=1'>autogen.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/packages/autogen.scm?id=aeec4a8fc1dce690b5d6aeb163871aca93d50111'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2018-08-09 02:17:24 +0200'>2018-08-09</span></td><td class='logsubject'><a href='/guix/commit/gnu/packages/autogen.scm?id=11c8a6491249e63e98d902d7396b9ac0a2bca947'>gnu: autogen: Update to 5.18.14.</a></td><td>Tobias Geerinckx-Rice</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* gnu/packages/autogen.scm (autogen): Update to 5.18.14.
[inputs]: Move perl from native-inputs to here...
[native-inputs]: ...and which from inputs to here.
[arguments]: Remove unnecessary ‘/usr/bin/tr’ substitution.


</td></tr>