Add specific diagnostic for transmuting between equal associated types

Author: varkor

src/librustc/middle/intrinsicck.rs

@@ -113,8 +113,12 @@ impl<'a, 'tcx> ExprVisitor<'a, 'tcx> {
         let mut err = struct_span_err!(self.tcx.sess, span, E0512,
                                        "cannot transmute between types of different sizes, \
                                         or dependently-sized types");
+        if from == to {
+            err.note(&format!("`{}` does not have a fixed size", from));
+        } else {
             err.note(&format!("source type: `{}` ({})", from, skeleton_string(from, sk_from)))
                 .note(&format!("target type: `{}` ({})", to, skeleton_string(to, sk_to)));
+        }
         err.emit()
     }
 }

src/test/ui/transmute-equal-assoc-types.rs

@@ -0,0 +1,9 @@
+trait Foo {
+    type Bar;
+}
+
+unsafe fn noop<F: Foo>(foo: F::Bar) -> F::Bar {
+    ::std::mem::transmute(foo) //~ ERROR cannot transmute between types of different sizes
+}
+
+fn main() {}

src/test/ui/transmute-equal-assoc-types.stderr

@@ -0,0 +1,11 @@
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute-equal-assoc-types.rs:6:5
+   |
+LL |     ::std::mem::transmute(foo) //~ ERROR cannot transmute between types of different sizes
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `<F as Foo>::Bar` does not have a fixed size
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0512`.