Use the official implementation of type_name.

This makes the `type_name` intrinsic call the implementation in
`librustc_mir::interpret::type_name` instead of `Ty::to_string()`.

src/intrinsic.rs

@@ -1,5 +1,5 @@
 use rustc::mir;
-use rustc::mir::interpret::{EvalResult, PointerArithmetic};
+use rustc::mir::interpret::{ConstValue, EvalResult, PointerArithmetic};
 use rustc::ty::layout::{self, LayoutOf, Size};
 use rustc::ty;
 
@@ -400,10 +400,17 @@ pub trait EvalContextExt<'a, 'mir, 'tcx: 'a+'mir>: crate::MiriEvalContextExt<'a,
 
             "type_name" => {
                 let ty = substs.type_at(0);
-                let ty_name = ty.to_string();
-                let ptr = this.memory_mut().allocate_static_bytes(ty_name.as_bytes(), MiriMemoryKind::Static.into());
-                let value = Immediate::new_slice(Scalar::Ptr(ptr), ty_name.len() as u64, this);
-                this.write_immediate(value, dest)?;
+                let ty_name = rustc_mir::interpret::type_name(*tcx, ty);
+
+                if let ConstValue::Slice { data, start, end } = ty_name.val {
+                    let ptr = this.memory_mut().allocate_with(data.clone(), MiriMemoryKind::Static.into());
+                    let ptr = ptr.offset(Size::from_bytes(start as u64), this)?;
+                    let len = end - start;
+                    let value = Immediate::new_slice(Scalar::Ptr(ptr), len as u64, this);
+                    this.write_immediate(value, dest)?;
+                } else {
+                    unreachable!("Result of `type_name` must be a `ConstValue::Slice`");
+                }
             }
 
             "unchecked_div" => {

tests/run-pass/intrinsics.rs

@@ -1,3 +1,6 @@
+#![feature(core_intrinsics)]
+
+use std::intrinsics::type_name;
 use std::mem::{size_of, size_of_val};
 
 fn main() {
@@ -7,4 +10,6 @@ fn main() {
     assert_eq!(size_of_val(&[] as &[i32]), 0);
     assert_eq!(size_of_val(&[1, 2, 3] as &[i32]), 12);
     assert_eq!(size_of_val("foobar"), 6);
+
+    assert_eq!(unsafe { type_name::<Option<i32>>() }, "core::option::Option<i32>");
 }