hir_analysis: enums return `None` in `find_field`

Unnamed union fields with enums are checked for, but if `find_field`
causes an ICE then the compiler won't get to that point.

Signed-off-by: David Wood <david@davidtw.co>

compiler/rustc_hir_analysis/src/collect.rs

@@ -791,7 +791,12 @@ fn convert_enum_variant_types(tcx: TyCtxt<'_>, def_id: DefId) {
 }
 
 fn find_field(tcx: TyCtxt<'_>, (def_id, ident): (DefId, Ident)) -> Option<FieldIdx> {
-    tcx.adt_def(def_id).non_enum_variant().fields.iter_enumerated().find_map(|(idx, field)| {
+    let adt = tcx.adt_def(def_id);
+    if adt.is_enum() {
+        return None;
+    }
+
+    adt.non_enum_variant().fields.iter_enumerated().find_map(|(idx, field)| {
         if field.is_unnamed() {
             let field_ty = tcx.type_of(field.did).instantiate_identity();
             let adt_def = field_ty.ty_adt_def().expect("expect Adt for unnamed field");

tests/ui/union/unnamed-fields/unnamed-enum-field-issue-121757.rs

@@ -0,0 +1,25 @@
+type NodeId = u32;
+struct Type<'a>(std::marker::PhantomData::<&'a ()>);
+
+type Ast<'ast> = &'ast AstStructure<'ast>;
+
+struct AstStructure<'ast> {
+//~^ ERROR struct with unnamed fields must have `#[repr(C)]` representation
+    id: NodeId,
+    _: AstKind<'ast>
+//~^ ERROR unnamed fields are not yet fully implemented [E0658]
+//~^^ ERROR unnamed fields can only have struct or union types
+}
+
+enum AstKind<'ast> {
+    ExprInt,
+    ExprLambda(Ast<'ast>),
+}
+
+fn compute_types<'tcx,'ast>(ast: Ast<'ast>) -> Type<'tcx>
+{
+    match ast.kind {}
+//~^ ERROR no field `kind` on type `&'ast AstStructure<'ast>` [E0609]
+}
+
+fn main() {}

tests/ui/union/unnamed-fields/unnamed-enum-field-issue-121757.stderr

@@ -0,0 +1,45 @@
+error[E0658]: unnamed fields are not yet fully implemented
+  --> $DIR/unnamed-enum-field-issue-121757.rs:9:5
+   |
+LL |     _: AstKind<'ast>
+   |     ^
+   |
+   = note: see issue #49804 <https://github.com/rust-lang/rust/issues/49804> for more information
+   = help: add `#![feature(unnamed_fields)]` 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+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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: struct with unnamed fields must have `#[repr(C)]` representation
+  --> $DIR/unnamed-enum-field-issue-121757.rs:6:1
+   |
+LL | struct AstStructure<'ast> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ struct `AstStructure` defined here
+   |
+note: unnamed field defined here
+  --> $DIR/unnamed-enum-field-issue-121757.rs:9:5
+   |
+LL |     _: AstKind<'ast>
+   |     ^^^^^^^^^^^^^^^^
+help: add `#[repr(C)]` to this struct
+   |
+LL + #[repr(C)]
+LL | struct AstStructure<'ast> {
+   |
+
+error: unnamed fields can only have struct or union types
+  --> $DIR/unnamed-enum-field-issue-121757.rs:9:5
+   |
+LL |     _: AstKind<'ast>
+   |     ^^^^^^^^^^^^^^^^
+
+error[E0609]: no field `kind` on type `&'ast AstStructure<'ast>`
+  --> $DIR/unnamed-enum-field-issue-121757.rs:21:15
+   |
+LL |     match ast.kind {}
+   |               ^^^^ unknown field
+   |
+   = note: available fields are: `id`, `_`
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0609, E0658.
+For more information about an error, try `rustc --explain E0609`.