firstToken

pub fn firstToken(tree: Ast, node: Node.Index) TokenIndex {
    var end_offset: u32 = 0;
    var n = node;
    while (true) switch (tree.nodeTag(n)) {
        .root => return 0,

        .test_decl,
        .@"errdefer",
        .@"defer",
        .bool_not,
        .negation,
        .bit_not,
        .negation_wrap,
        .address_of,
        .@"try",
        .optional_type,
        .@"switch",
        .switch_comma,
        .if_simple,
        .@"if",
        .@"suspend",
        .@"resume",
        .@"continue",
        .@"break",
        .@"return",
        .anyframe_type,
        .identifier,
        .anyframe_literal,
        .char_literal,
        .number_literal,
        .unreachable_literal,
        .string_literal,
        .multiline_string_literal,
        .grouped_expression,
        .builtin_call_two,
        .builtin_call_two_comma,
        .builtin_call,
        .builtin_call_comma,
        .error_set_decl,
        .@"comptime",
        .@"nosuspend",
        .asm_simple,
        .@"asm",
        .asm_legacy,
        .array_type,
        .array_type_sentinel,
        .error_value,
        => return tree.nodeMainToken(n) - end_offset,

        .array_init_dot,
        .array_init_dot_comma,
        .array_init_dot_two,
        .array_init_dot_two_comma,
        .struct_init_dot,
        .struct_init_dot_comma,
        .struct_init_dot_two,
        .struct_init_dot_two_comma,
        .enum_literal,
        => return tree.nodeMainToken(n) - 1 - end_offset,

        .@"catch",
        .equal_equal,
        .bang_equal,
        .less_than,
        .greater_than,
        .less_or_equal,
        .greater_or_equal,
        .assign_mul,
        .assign_div,
        .assign_mod,
        .assign_add,
        .assign_sub,
        .assign_shl,
        .assign_shl_sat,
        .assign_shr,
        .assign_bit_and,
        .assign_bit_xor,
        .assign_bit_or,
        .assign_mul_wrap,
        .assign_add_wrap,
        .assign_sub_wrap,
        .assign_mul_sat,
        .assign_add_sat,
        .assign_sub_sat,
        .assign,
        .merge_error_sets,
        .mul,
        .div,
        .mod,
        .array_mult,
        .mul_wrap,
        .mul_sat,
        .add,
        .sub,
        .array_cat,
        .add_wrap,
        .sub_wrap,
        .add_sat,
        .sub_sat,
        .shl,
        .shl_sat,
        .shr,
        .bit_and,
        .bit_xor,
        .bit_or,
        .@"orelse",
        .bool_and,
        .bool_or,
        .slice_open,
        .array_access,
        .array_init_one,
        .array_init_one_comma,
        .switch_range,
        .error_union,
        => n = tree.nodeData(n).node_and_node[0],

        .for_range,
        .call_one,
        .call_one_comma,
        .struct_init_one,
        .struct_init_one_comma,
        => n = tree.nodeData(n).node_and_opt_node[0],

        .field_access,
        .unwrap_optional,
        => n = tree.nodeData(n).node_and_token[0],

        .slice,
        .slice_sentinel,
        .array_init,
        .array_init_comma,
        .struct_init,
        .struct_init_comma,
        .call,
        .call_comma,
        => n = tree.nodeData(n).node_and_extra[0],

        .deref => n = tree.nodeData(n).node,

        .assign_destructure => n = tree.assignDestructure(n).ast.variables[0],

        .fn_decl,
        .fn_proto_simple,
        .fn_proto_multi,
        .fn_proto_one,
        .fn_proto,
        => {
            var i = tree.nodeMainToken(n); // fn token
            while (i > 0) {
                i -= 1;
                switch (tree.tokenTag(i)) {
                    .keyword_extern,
                    .keyword_export,
                    .keyword_pub,
                    .keyword_inline,
                    .keyword_noinline,
                    .string_literal,
                    => continue,

                    else => return i + 1 - end_offset,
                }
            }
            return i - end_offset;
        },

        .container_field_init,
        .container_field_align,
        .container_field,
        => {
            const name_token = tree.nodeMainToken(n);
            const has_comptime_token = tree.isTokenPrecededByTags(name_token, &.{.keyword_comptime});
            end_offset += @intFromBool(has_comptime_token);
            return name_token - end_offset;
        },

        .global_var_decl,
        .local_var_decl,
        .simple_var_decl,
        .aligned_var_decl,
        => {
            var i = tree.nodeMainToken(n); // mut token
            while (i > 0) {
                i -= 1;
                switch (tree.tokenTag(i)) {
                    .keyword_extern,
                    .keyword_export,
                    .keyword_comptime,
                    .keyword_pub,
                    .keyword_threadlocal,
                    .string_literal,
                    => continue,

                    else => return i + 1 - end_offset,
                }
            }
            return i - end_offset;
        },

        .block,
        .block_semicolon,
        .block_two,
        .block_two_semicolon,
        => {
            // Look for a label.
            const lbrace = tree.nodeMainToken(n);
            if (tree.isTokenPrecededByTags(lbrace, &.{ .identifier, .colon })) {
                end_offset += 2;
            }
            return lbrace - end_offset;
        },

        .container_decl,
        .container_decl_trailing,
        .container_decl_two,
        .container_decl_two_trailing,
        .container_decl_arg,
        .container_decl_arg_trailing,
        .tagged_union,
        .tagged_union_trailing,
        .tagged_union_two,
        .tagged_union_two_trailing,
        .tagged_union_enum_tag,
        .tagged_union_enum_tag_trailing,
        => {
            const main_token = tree.nodeMainToken(n);
            switch (tree.tokenTag(main_token -| 1)) {
                .keyword_packed, .keyword_extern => end_offset += 1,
                else => {},
            }
            return main_token - end_offset;
        },

        .ptr_type_aligned,
        .ptr_type_sentinel,
        .ptr_type,
        .ptr_type_bit_range,
        => return tree.nodeMainToken(n) - end_offset,

        .switch_case_one,
        .switch_case_inline_one,
        .switch_case,
        .switch_case_inline,
        => {
            const full_switch = tree.fullSwitchCase(n).?;
            if (full_switch.inline_token) |inline_token| {
                return inline_token;
            } else if (full_switch.ast.values.len == 0) {
                return full_switch.ast.arrow_token - 1 - end_offset; // else token
            } else {
                n = full_switch.ast.values[0];
            }
        },

        .asm_output, .asm_input => {
            assert(tree.tokenTag(tree.nodeMainToken(n) - 1) == .l_bracket);
            return tree.nodeMainToken(n) - 1 - end_offset;
        },

        .while_simple,
        .while_cont,
        .@"while",
        .for_simple,
        .@"for",
        => {
            // Look for a label and inline.
            const main_token = tree.nodeMainToken(n);
            var result = main_token;
            if (tree.isTokenPrecededByTags(result, &.{.keyword_inline})) {
                result = result - 1;
            }
            if (tree.isTokenPrecededByTags(result, &.{ .identifier, .colon })) {
                result = result - 2;
            }
            return result - end_offset;
        },
    };
}