Allow transmuting generic pattern types to and from their base

Author: oli-obk

Diff for: 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),
         }
     }

Diff for: tests/ui/type/pattern_types/transmute.rs

@@ -6,13 +6,11 @@ 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) }
-    //~^ ERROR types of different sizes
 }
 
 // ok
 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
 }
 
 // bad, only when S != u32::MIN or E != u32::MAX will this ok

Diff for: tests/ui/type/pattern_types/transmute.stderr

@@ -1,23 +1,5 @@
 error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
-  --> $DIR/transmute.rs:8:14
-   |
-LL |     unsafe { std::mem::transmute(x) }
-   |              ^^^^^^^^^^^^^^^^^^^
-   |
-   = note: source type: `u32` (32 bits)
-   = note: target type: `(u32) is S..=E` (size can vary because of u32)
-
-error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
-  --> $DIR/transmute.rs:14:14
-   |
-LL |     unsafe { std::mem::transmute(x) }
-   |              ^^^^^^^^^^^^^^^^^^^
-   |
-   = note: source type: `(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:22:14
+  --> $DIR/transmute.rs:20:14
    |
 LL |     unsafe { std::mem::transmute(x) }
    |              ^^^^^^^^^^^^^^^^^^^
@@ -26,14 +8,14 @@ LL |     unsafe { std::mem::transmute(x) }
    = note: target type: `u32` (32 bits)
 
 error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
-  --> $DIR/transmute.rs:30:14
+  --> $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 4 previous errors
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0512`.