errorSetDecl

fn errorSetDecl(gz: *GenZir, ri: ResultInfo, node: Ast.Node.Index) InnerError!Zir.Inst.Ref {
    const astgen = gz.astgen;
    const gpa = astgen.gpa;
    const tree = astgen.tree;

    const payload_index = try reserveExtra(astgen, @typeInfo(Zir.Inst.ErrorSetDecl).@"struct".field_names.len);
    var fields_len: usize = 0;
    {
        var idents: std.AutoHashMapUnmanaged(Zir.NullTerminatedString, Ast.TokenIndex) = .empty;
        defer idents.deinit(gpa);

        const lbrace, const rbrace = tree.nodeData(node).token_and_token;
        for (lbrace + 1..rbrace) |i| {
            const tok_i: Ast.TokenIndex = @intCast(i);
            switch (tree.tokenTag(tok_i)) {
                .doc_comment, .comma => {},
                .identifier => {
                    const str_index = try astgen.identAsString(tok_i);
                    const gop = try idents.getOrPut(gpa, str_index);
                    if (gop.found_existing) {
                        const name = try gpa.dupe(u8, mem.span(astgen.nullTerminatedString(str_index)));
                        defer gpa.free(name);
                        return astgen.failTokNotes(
                            tok_i,
                            "duplicate error set field '{s}'",
                            .{name},
                            &[_]u32{
                                try astgen.errNoteTok(
                                    gop.value_ptr.*,
                                    "previous declaration here",
                                    .{},
                                ),
                            },
                        );
                    }
                    gop.value_ptr.* = tok_i;

                    try astgen.extra.append(gpa, @backingInt(str_index));
                    fields_len += 1;
                },
                else => unreachable,
            }
        }
    }

    setExtra(astgen, payload_index, Zir.Inst.ErrorSetDecl{
        .fields_len = @intCast(fields_len),
    });
    const result = try gz.addPlNodePayloadIndex(.error_set_decl, node, payload_index);
    return rvalue(gz, ri, result, node);
}