Allow transmuting generic pattern types to and from their base

Author: oli-obk

compiler/rustc_middle/src/ty/layout.rs

@@ -498,6 +498,9 @@ 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

@@ -1,16 +1,19 @@
+//! Ensure we can transmute between pattern types and their base types
+//! in generic contexts.
+
+//@ check-pass
+
 #![feature(pattern_types)]
 #![feature(pattern_type_macro)]
 
 use std::pat::pattern_type;
 
 fn create<const S: u32, const E: u32>(x: u32) -> pattern_type!(u32 is S..=E) {
     unsafe { std::mem::transmute(x) }
-    //~^ ERROR types of different sizes
 }
 
 fn unwrap<const S: u32, const E: u32>(x: pattern_type!(u32 is S..=E)) -> u32 {
     unsafe { std::mem::transmute(x) }
-    //~^ ERROR types of different sizes
 }
 
 fn main() {}