Fix ICE for repr simd on non struct

compiler/rustc_hir_typeck/src/inline_asm.rs

@@ -93,6 +93,14 @@ impl<'a, 'tcx> InlineAsmCtxt<'a, 'tcx> {
                 }
             }
             ty::Adt(adt, args) if adt.repr().simd() => {
+                if !adt.is_struct() {
+                    self.fcx.dcx().span_delayed_bug(
+                        span,
+                        format!("repr(simd) should only be used on structs, got {}", adt.descr()),
+                    );
+                    return Err(NonAsmTypeReason::Invalid(ty));
+                }
+
                 let fields = &adt.non_enum_variant().fields;
                 if fields.is_empty() {
                     return Err(NonAsmTypeReason::EmptySIMDArray(ty));

tests/ui/asm/invalid-repr-simd-on-enum-148634.rs

@@ -0,0 +1,16 @@
+#![feature(repr_simd)]
+
+use std::arch::asm;
+
+#[repr(simd)]
+//~^ ERROR attribute should be applied to a struct
+//~| ERROR unsupported representation for zero-variant enum
+enum Es {}
+
+fn main() {
+    unsafe {
+        let mut x: Es;
+        asm!("{}", out(reg) x);
+        //~^ ERROR cannot use value of type `Es` for inline assembly
+    }
+}

tests/ui/asm/invalid-repr-simd-on-enum-148634.stderr

@@ -0,0 +1,30 @@
+error[E0517]: attribute should be applied to a struct
+  --> $DIR/invalid-repr-simd-on-enum-148634.rs:5:8
+   |
+LL | #[repr(simd)]
+   |        ^^^^
+...
+LL | enum Es {}
+   | ---------- not a struct
+
+error[E0084]: unsupported representation for zero-variant enum
+  --> $DIR/invalid-repr-simd-on-enum-148634.rs:5:8
+   |
+LL | #[repr(simd)]
+   |        ^^^^
+...
+LL | enum Es {}
+   | ------- zero-variant enum
+
+error: cannot use value of type `Es` for inline assembly
+  --> $DIR/invalid-repr-simd-on-enum-148634.rs:13:29
+   |
+LL |         asm!("{}", out(reg) x);
+   |                             ^
+   |
+   = note: only integers, floats, SIMD vectors, pointers and function pointers can be used as arguments for inline assembly
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0084, E0517.
+For more information about an error, try `rustc --explain E0084`.