Allow transmuting generic pattern types to and from their base

compiler/rustc_middle/src/ty/layout.rs

@@ -504,6 +504,9 @@ impl<'tcx> SizeSkeleton<'tcx> {
                 }
             }
 
+            // Pattern types are always the same size as their base.
+            ty::Pat(base, _) => SizeSkeleton::compute(base, tcx, typing_env),
+
             _ => Err(err),
         }
     }

tests/ui/type/pattern_types/transmute.rs

@@ -0,0 +1,32 @@
+#![feature(pattern_types)]
+#![feature(pattern_type_macro)]
+
+use std::pat::pattern_type;
+
+// ok
+fn create<const S: u32, const E: u32>(x: u32) -> pattern_type!(u32 is S..=E) {
+    unsafe { std::mem::transmute(x) }
+}
+
+// ok
+fn unwrap<const S: u32, const E: u32>(x: pattern_type!(u32 is S..=E)) -> u32 {
+    unsafe { std::mem::transmute(x) }
+}
+
+// bad, only when S != u32::MIN or E != u32::MAX will this ok
+fn non_base_ty_transmute<const S: u32, const E: u32>(
+    x: Option<pattern_type!(u32 is S..=E)>,
+) -> u32 {
+    unsafe { std::mem::transmute(x) }
+    //~^ ERROR types of different sizes
+}
+
+// bad, only when S = u32::MIN and E = u32::MAX will this ok
+fn wrapped_transmute<const S: u32, const E: u32>(
+    x: Option<pattern_type!(u32 is S..=E)>,
+) -> Option<u32> {
+    unsafe { std::mem::transmute(x) }
+    //~^ ERROR types of different sizes
+}
+
+fn main() {}

tests/ui/type/pattern_types/transmute.stderr

@@ -0,0 +1,21 @@
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute.rs:20:14
+   |
+LL |     unsafe { std::mem::transmute(x) }
+   |              ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `Option<(u32) is S..=E>` (size can vary because of u32)
+   = note: target type: `u32` (32 bits)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute.rs:28:14
+   |
+LL |     unsafe { std::mem::transmute(x) }
+   |              ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `Option<(u32) is S..=E>` (size can vary because of u32)
+   = note: target type: `Option<u32>` (64 bits)
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0512`.