Rollup merge of rust-lang#136179 - oli-obk:push-vxvyttorquxw, r=BoxyUwU

Allow transmuting generic pattern types to and from their base

Pattern types always have the same size as their base type, so we can just ignore the pattern and look at the base type for figuring out whether transmuting is possible.

Author: matthiaskrgr

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

@@ -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() {}

Diff for: 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`.