Improve some error messages

Closes #12547: explain why arrays aren't allowed in packed structs.
Closes #13832: improve error message for a common mistake on the LHS of
the binary `!` operator.

src/Sema.zig

@@ -8666,16 +8666,20 @@ fn zirErrorUnionType(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileEr
     const extra = sema.code.extraData(Zir.Inst.Bin, inst_data.payload_index).data;
     const lhs_src = block.src(.{ .node_offset_bin_lhs = inst_data.src_node });
     const rhs_src = block.src(.{ .node_offset_bin_rhs = inst_data.src_node });
-    const error_set = try sema.resolveType(block, lhs_src, extra.lhs);
+    // Do not resolve as type yet, so that we can give a more useful message
+    // when someone writes `error.XYZ` instead of `error{XYZ}` as the LHS.
+    const error_set = try sema.resolveConstDefinedValue(block, lhs_src, try sema.resolveInst(extra.lhs), .{
+        .needed_comptime_reason = "types must be comptime-known",
+    });
     const payload = try sema.resolveType(block, rhs_src, extra.rhs);
 
-    if (error_set.zigTypeTag(mod) != .ErrorSet) {
+    if (error_set.typeOf(mod).ip_index != .type_type or error_set.toType().zigTypeTag(mod) != .ErrorSet) {
         return sema.fail(block, lhs_src, "expected error set type, found '{}'", .{
-            error_set.fmt(pt),
+            error_set.fmtValue(pt, sema),
         });
     }
     try sema.validateErrorUnionPayloadType(block, payload, rhs_src);
-    const err_union_ty = try pt.errorUnionType(error_set, payload);
+    const err_union_ty = try pt.errorUnionType(error_set.toType(), payload);
     return Air.internedToRef(err_union_ty.toIntern());
 }
 
@@ -27071,8 +27075,8 @@ fn explainWhyTypeIsNotPacked(
         .ErrorSet,
         .AnyFrame,
         .Optional,
-        .Array,
         => try sema.errNote(src_loc, msg, "type has no guaranteed in-memory representation", .{}),
+        .Array => try sema.errNote(src_loc, msg, "packed fields are ordered according to machine endianness, array elements are not", .{}),
         .Pointer => if (ty.isSlice(mod)) {
             try sema.errNote(src_loc, msg, "slices have no guaranteed in-memory representation", .{});
         } else {

test/cases/compile_errors/error_union_operator_with_non_error_set_LHS.zig

@@ -1,3 +1,8 @@
+comptime {
+    const z = error.Foo!i32;
+    const x: z = undefined;
+    _ = x;
+}
 comptime {
     const z = i32!i32;
     const x: z = undefined;
@@ -8,4 +13,5 @@ comptime {
 // backend=stage2
 // target=native
 //
-// :2:15: error: expected error set type, found 'i32'
+// :2:15: error: expected error set type, found 'error.Foo'
+// :3:15: error: expected error set type, found 'i32'

test/cases/compile_errors/packed_struct_with_fields_of_not_allowed_types.zig

@@ -84,7 +84,7 @@ export fn entry14() void {
 // :3:12: error: packed structs cannot contain fields of type 'anyerror'
 // :3:12: note: type has no guaranteed in-memory representation
 // :8:12: error: packed structs cannot contain fields of type '[2]u24'
-// :8:12: note: type has no guaranteed in-memory representation
+// :8:12: note: packed fields are ordered according to machine endianness, array elements are not
 // :13:20: error: packed structs cannot contain fields of type 'anyerror!u32'
 // :13:20: note: type has no guaranteed in-memory representation
 // :18:12: error: packed structs cannot contain fields of type 'tmp.S'