Fix ICE when validating transmuting ZST to inhabited enum

MIR validation attempts to determine the number of bytes needed to
represent the size of the source type to compute the discriminant for
the inhabited target enum. For a ZST source, there is no source data to
use as a discriminant so no proper runtime check can be generated.

Since that should never be possible, insert a delayed bug to ensure the
problem has been properly reported to the user by the type checker.

Author: samueltardieu

compiler/rustc_mir_transform/src/check_enums.rs

@@ -48,6 +48,21 @@ impl<'tcx> crate::MirPass<'tcx> for CheckEnums {
                     let new_block = split_block(basic_blocks, location);
 
                     match check {
+                        EnumCheckType::Direct { op_size, .. }
+                        | EnumCheckType::WithNiche { op_size, .. }
+                            if op_size.bytes() == 0 =>
+                        {
+                            // It is never valid to use a ZST as a discriminant for an inhabited enum, but that will
+                            // have been caught by the type checker. Do nothing but ensure that a bug has been signaled.
+                            tcx.dcx().span_delayed_bug(
+                                source_info.span,
+                                "cannot build enum discriminant from zero-sized type",
+                            );
+                            basic_blocks[block].terminator = Some(Terminator {
+                                source_info,
+                                kind: TerminatorKind::Goto { target: new_block },
+                            });
+                        }
                         EnumCheckType::Direct { source_op, discr, op_size, valid_discrs } => {
                             insert_direct_enum_check(
                                 tcx,

tests/ui/mir/validate/ice-zst-as-discr-145786.rs

@@ -0,0 +1,14 @@
+// Do not attempt to take the discriminant as the source
+// converted to a `u128`, that won't work for ZST.
+//
+//@ compile-flags: -Zvalidate-mir
+
+enum A {
+    B,
+    C,
+}
+
+fn main() {
+    let _: A = unsafe { std::mem::transmute(()) };
+    //~^ ERROR cannot transmute between types of different sizes, or dependently-sized types
+}

tests/ui/mir/validate/ice-zst-as-discr-145786.stderr

@@ -0,0 +1,12 @@
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/ice-zst-as-discr-145786.rs:12:25
+   |
+LL |     let _: A = unsafe { std::mem::transmute(()) };
+   |                         ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `()` (0 bits)
+   = note: target type: `A` (8 bits)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0512`.