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
     }
r></table>
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<tr><td><span title='2020-07-12 23:08:15 +0200'>2020-07-12</span></td><td><a href='/guix/commit/gnu/packages/jupyter.scm?id=b0e7b6992f3f845e83cfbca4d700b51dba50b4d5'>gnu: Remove ".git" from "https://github/…/….git".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-01-15 00:24:28 +0100'>2020-01-15</span></td><td><a href='/guix/commit/gnu/packages/jupyter.scm?id=bb93042c8beb881ab907fa802eefb3c4091039a2'>gnu: util-linux: Move libraries to separate output.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-12-04 20:10:34 +0100'>2019-12-04</span></td><td><a href='/guix/commit/gnu/packages/jupyter.scm?id=936aa05209402dd399869ffae188db5dd5f67d69'>gnu: Remove duplicate nlohmann-json-cpp package.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-10-12 18:39:58 +0200'>2019-10-12</span></td><td><a href='/guix/commit/gnu/packages/jupyter.scm?id=9f294736b247c55c65bb1a7053e1c98d9bfa161c'>gnu: python-jupyter-kernel-test: Use PYTHON-VERSION.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2019-09-18 23:08:29 +0200'>2019-09-18</span></td><td><a href='/guix/commit/gnu/packages/jupyter.scm?id=4bb7d14bbe959c884dfe3a370927211b0593e6d0'>gnu: Add Xeus.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-18 23:08:29 +0200'>2019-09-18</span></td><td><a href='/guix/commit/gnu/packages/jupyter.scm?id=a52d8cc6c79941de3380ed763d46b38404094758'>gnu: Add python-jupyter-kernel-test.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-18 23:08:29 +0200'>2019-09-18</span></td><td><a href='/guix/commit/gnu/packages/jupyter.scm?id=2835fb07bacd5e99e99120eb9f3b7004a872f574'>gnu: Add python-jupyter-kernel-mgmt.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-18 23:08:29 +0200'>2019-09-18</span></td><td><a href='/guix/commit/gnu/packages/jupyter.scm?id=2cde4a967d162dc46cc1f6967ed21b196c14c338'>gnu: Add python-jupyter-protocol.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>