compiler: Do not ice in layout_of_union on invalid reprs

Delay the bug so layouts can fail to compute on incorrectly-written enums.

compiler/rustc_abi/src/layout.rs

@@ -54,6 +54,9 @@ pub enum LayoutCalculatorError<F> {
 
     /// A union had no fields.
     EmptyUnion,
+
+    /// The fields or variants have irreconcilable reprs
+    ReprConflict,
 }
 
 type LayoutCalculatorResult<FieldIdx, VariantIdx, F> =
@@ -307,12 +310,11 @@ impl<Cx: HasDataLayout> LayoutCalculator<Cx> {
                         common_non_zst_abi_and_align = Err(AbiMismatch);
                     } else {
                         // Fields with the same non-Aggregate ABI should also
-                        // have the same alignment
-                        if !matches!(common_abi, Abi::Aggregate { .. }) {
-                            assert_eq!(
-                                common_align, field.align.abi,
-                                "non-Aggregate field with matching ABI but differing alignment"
-                            );
+                        // have the same alignment, but one may have an invalid repr
+                        if !matches!(common_abi, Abi::Aggregate { .. })
+                            && common_align != field.align.abi
+                        {
+                            return Err(LayoutCalculatorError::ReprConflict);
                         }
                     }
                 } else {

compiler/rustc_ty_utils/src/layout.rs

@@ -115,6 +115,11 @@ fn map_error<'tcx>(
             cx.tcx().dcx().delayed_bug(format!("computed layout of empty union: {ty:?}"));
             LayoutError::Unknown(ty)
         }
+        LayoutCalculatorError::ReprConflict => {
+            // packed enums are the only known trigger of this, but others might arise
+            cx.tcx().dcx().delayed_bug(format!("computed impossible repr (packed enum): {ty:?}"));
+            LayoutError::Unknown(ty)
+        }
     };
     error(cx, err)
 }

tests/ui/layout/thaw-transmute-invalid-enum.rs

@@ -1,10 +1,14 @@
-//@ known-bug: rust-lang/rust#126966
+#![crate_type = "lib"]
+
 mod assert {
     use std::mem::{Assume, TransmuteFrom};
+    //~^ ERROR: use of unstable library feature 'transmutability'
+    //~| ERROR: use of unstable library feature 'transmutability'
 
     pub fn is_transmutable<Src, Dst>()
     where
-        Dst: TransmuteFrom<Src>,
+        Dst: TransmuteFrom<Src>, //~ ERROR: use of unstable library feature 'transmutability'
+
     {
     }
 }
@@ -14,16 +18,16 @@ enum Ox00 {
     V = 0x00,
 }
 
-#[repr(C, packed(2))]
+#[repr(C, packed(2))] //~ ERROR: attribute should be applied to a struct
 enum OxFF {
     V = 0xFF,
 }
 
 fn test() {
     union Superset {
-        a: Ox00,
+        a: Ox00, //~ ERROR: field must implement `Copy`
         b: OxFF,
     }
 
-    assert::is_transmutable::<Superset, Subset>();
+    assert::is_transmutable::<Superset, Subset>(); //~ ERROR: cannot find type `Subset`
 }

tests/ui/layout/thaw-transmute-invalid-enum.stderr

@@ -0,0 +1,67 @@
+error[E0412]: cannot find type `Subset` in this scope
+  --> $DIR/thaw-transmute-invalid-enum.rs:32:41
+   |
+LL |     assert::is_transmutable::<Superset, Subset>();
+   |                                         ^^^^^^ not found in this scope
+   |
+help: you might be missing a type parameter
+   |
+LL | fn test<Subset>() {
+   |        ++++++++
+
+error[E0517]: attribute should be applied to a struct or union
+  --> $DIR/thaw-transmute-invalid-enum.rs:21:11
+   |
+LL |   #[repr(C, packed(2))]
+   |             ^^^^^^^^^
+LL | / enum OxFF {
+LL | |     V = 0xFF,
+LL | | }
+   | |_- not a struct or union
+
+error[E0658]: use of unstable library feature 'transmutability'
+  --> $DIR/thaw-transmute-invalid-enum.rs:4:20
+   |
+LL |     use std::mem::{Assume, TransmuteFrom};
+   |                    ^^^^^^
+   |
+   = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
+   = help: add `#![feature(transmutability)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature 'transmutability'
+  --> $DIR/thaw-transmute-invalid-enum.rs:4:28
+   |
+LL |     use std::mem::{Assume, TransmuteFrom};
+   |                            ^^^^^^^^^^^^^
+   |
+   = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
+   = help: add `#![feature(transmutability)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature 'transmutability'
+  --> $DIR/thaw-transmute-invalid-enum.rs:10:14
+   |
+LL |         Dst: TransmuteFrom<Src>,
+   |              ^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #99571 <https://github.com/rust-lang/rust/issues/99571> for more information
+   = help: add `#![feature(transmutability)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0740]: field must implement `Copy` or be wrapped in `ManuallyDrop<...>` to be used in a union
+  --> $DIR/thaw-transmute-invalid-enum.rs:28:9
+   |
+LL |         a: Ox00,
+   |         ^^^^^^^
+   |
+   = note: union fields must not have drop side-effects, which is currently enforced via either `Copy` or `ManuallyDrop<...>`
+help: wrap the field type in `ManuallyDrop<...>`
+   |
+LL |         a: std::mem::ManuallyDrop<Ox00>,
+   |            +++++++++++++++++++++++    +
+
+error: aborting due to 6 previous errors
+
+Some errors have detailed explanations: E0412, E0517, E0658, E0740.
+For more information about an error, try `rustc --explain E0412`.

tests/ui/layout/thaw-validate-invalid-enum.rs

@@ -1,7 +1,6 @@
-//@ known-bug: rust-lang/rust#128870
 //@ compile-flags: -Zvalidate-mir
 
-#[repr(packed)]
+#[repr(packed)] //~ ERROR: attribute should be applied to a struct
 #[repr(u32)]
 enum E {
     A,
@@ -12,7 +11,7 @@ enum E {
 fn main() {
     union InvalidTag {
         int: u32,
-        e: E,
+        e: E, //~ ERROR: field must implement `Copy`
     }
     let _invalid_tag = InvalidTag { int: 4 };
 }

tests/ui/layout/thaw-validate-invalid-enum.stderr

@@ -0,0 +1,29 @@
+error[E0517]: attribute should be applied to a struct or union
+  --> $DIR/thaw-validate-invalid-enum.rs:3:8
+   |
+LL |   #[repr(packed)]
+   |          ^^^^^^
+LL |   #[repr(u32)]
+LL | / enum E {
+LL | |     A,
+LL | |     B,
+LL | |     C,
+LL | | }
+   | |_- not a struct or union
+
+error[E0740]: field must implement `Copy` or be wrapped in `ManuallyDrop<...>` to be used in a union
+  --> $DIR/thaw-validate-invalid-enum.rs:14:9
+   |
+LL |         e: E,
+   |         ^^^^
+   |
+   = note: union fields must not have drop side-effects, which is currently enforced via either `Copy` or `ManuallyDrop<...>`
+help: wrap the field type in `ManuallyDrop<...>`
+   |
+LL |         e: std::mem::ManuallyDrop<E>,
+   |            +++++++++++++++++++++++ +
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0517, E0740.
+For more information about an error, try `rustc --explain E0517`.