aboutsummaryrefslogtreecommitdiff
path: root/libbuild2/build/script/parser.cxx
blob: 372c6223607fe3ec640f0e546d18490dfc6f2157 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
// file      : libbuild2/build/script/parser.cxx -*- C++ -*-
// license   : MIT; see accompanying LICENSE file

#include <libbuild2/build/script/parser.hxx>

#include <libbutl/builtin.mxx>

#include <libbuild2/function.hxx>
#include <libbuild2/algorithm.hxx>

#include <libbuild2/build/script/lexer.hxx>
#include <libbuild2/build/script/runner.hxx>

using namespace std;
using namespace butl;

namespace build2
{
  namespace build
  {
    namespace script
    {
      using type = token_type;

      //
      // Pre-parse.
      //

      script parser::
      pre_parse (const target& tg, const adhoc_actions& as,
                 istream& is, const path_name& pn, uint64_t line,
                 optional<string> diag, const location& diag_loc)
      {
        path_ = &pn;

        pre_parse_ = true;

        lexer l (is, *path_, line, lexer_mode::command_line);
        set_lexer (&l);

        // The script shouldn't be able to modify the target/scopes.
        //
        target_  = const_cast<target*> (&tg);
        actions_ = &as;
        scope_   = const_cast<scope*> (&tg.base_scope ());
        root_    = scope_->root_scope ();

        pbase_  = scope_->src_path_;

        perform_update_ = find (as.begin (), as.end (), perform_update_id) !=
                          as.end ();

        script s;
        script_ = &s;
        runner_ = nullptr;
        environment_ = nullptr;

        if (diag)
        {
          diag_name_   = make_pair (move (*diag), diag_loc);
          diag_weight_ = 4;
        }

        s.start_loc = location (*path_, line, 1);

        token t (pre_parse_script ());

        assert (t.type == type::eos);

        s.end_loc = get_location (t);

        // Diagnose impure function calls.
        //
        if (impure_func_)
          fail (impure_func_->second)
            << "call to impure function " << impure_func_->first << " is "
            << "only allowed in depdb preamble" <<
            info << "consider using 'depdb' builtin to track its result "
                 << "changes";

        // Diagnose absent/ambigous script name.
        //
        {
          diag_record dr;

          if (!diag_name_ && !diag_line_)
          {
            dr << fail (s.start_loc)
               << "unable to deduce low-verbosity script diagnostics name";
          }
          else if (diag_name2_)
          {
            assert (diag_name_);

            dr << fail (s.start_loc)
               << "low-verbosity script diagnostics name is ambiguous" <<
              info (diag_name_->second) << "could be '" << diag_name_->first
               << "'" <<
              info (diag_name2_->second) << "could be '" << diag_name2_->first
               << "'";
          }

          if (!dr.empty ())
          {
            dr << info << "consider specifying it explicitly with the 'diag' "
               << "recipe attribute";

            dr << info << "or provide custom low-verbosity diagnostics with "
               << "the 'diag' builtin";
          }
        }

        // Save the script name or custom diagnostics line.
        //
        assert (diag_name_.has_value () != diag_line_.has_value ());

        if (diag_name_)
          s.diag_name = move (diag_name_->first);
        else
          s.diag_line = move (diag_line_->first);

        // Save the custom dependency change tracking lines, if present.
        //
        s.depdb_clear = depdb_clear_.has_value ();
        s.depdb_preamble = move (depdb_preamble_);

        return s;
      }

      token parser::
      pre_parse_script ()
      {
        // enter: next token is first token of the script
        // leave: eos (returned)

        token t;
        type tt;

        // Parse lines until we see eos.
        //
        for (;;)
        {
          // Start lexing each line.
          //
          tt = peek (lexer_mode::first_token);

          // Determine the line type by peeking at the first token.
          //
          switch (tt)
          {
          case type::eos:
            {
              next (t, tt);
              return t;
            }
          default:
            {
              pre_parse_line (t, tt);
              assert (tt == type::newline);
              break;
            }
          }
        }
      }

      void parser::
      pre_parse_line (token& t, type& tt, bool if_line)
      {
        // Determine the line type/start token.
        //
        line_type lt (
          pre_parse_line_start (t, tt, lexer_mode::second_token));

        line ln;

        // Indicates that the parsed line should by default be appended to the
        // script.
        //
        save_line_ = &ln;

        switch (lt)
        {
        case line_type::var:
          {
            // Check if we are trying to modify any of the special variables.
            //
            if (special_variable (t.value))
              fail (t) << "attempt to set '" << t.value << "' special "
                       << "variable";

            // We don't pre-enter variables.
            //
            ln.var = nullptr;

            next (t, tt); // Assignment kind.

            mode (lexer_mode::variable_line);
            parse_variable_line (t, tt);

            if (tt != type::newline)
              fail (t) << "expected newline instead of " << t;

            break;
          }
        case line_type::cmd_elif:
        case line_type::cmd_elifn:
        case line_type::cmd_else:
        case line_type::cmd_end:
          {
            if (!if_line)
            {
              fail (t) << lt << " without preceding 'if'";
            }
          }
          // Fall through.
        case line_type::cmd_if:
        case line_type::cmd_ifn:
          next (t, tt); // Skip to start of command.

          if (lt == line_type::cmd_if || lt == line_type::cmd_ifn)
            ++level_;
          else if (lt == line_type::cmd_end)
            --level_;

          // Fall through.
        case line_type::cmd:
          {
            pair<command_expr, here_docs> p;

            if (lt != line_type::cmd_else && lt != line_type::cmd_end)
              p = parse_command_expr (t, tt, lexer::redirect_aliases);

            if (tt != type::newline)
              fail (t) << "expected newline instead of " << t;

            parse_here_documents (t, tt, p);
            break;
          }
        }

        assert (tt == type::newline);

        ln.type = lt;
        ln.tokens = replay_data ();

        if (save_line_ != nullptr)
        {
          if (save_line_ == &ln)
            script_->body.push_back (move (ln));
          else
            *save_line_ = move (ln);
        }

        if (lt == line_type::cmd_if || lt == line_type::cmd_ifn)
        {
          tt = peek (lexer_mode::first_token);

          pre_parse_if_else (t, tt);
        }
      }

      void parser::
      pre_parse_if_else (token& t, type& tt)
      {
        // enter: peeked first token of next line (type in tt)
        // leave: newline

        // Parse lines until we see closing 'end'. Nested if-else blocks are
        // handled recursively.
        //
        for (line_type bt (line_type::cmd_if); // Current block.
             ;
             tt = peek (lexer_mode::first_token))
        {
          const location ll (get_location (peeked ()));

          if (tt == type::eos)
            fail (ll) << "expected closing 'end'";

          // Parse one line. Note that this one line can still be multiple
          // lines in case of if-else. In this case we want to view it as
          // cmd_if, not cmd_end. Thus remember the start position of the
          // next logical line.
          //
          size_t i (script_->body.size ());

          pre_parse_line (t, tt, true /* if_line */);
          assert (tt == type::newline);

          line_type lt (script_->body[i].type);

          // First take care of 'end'.
          //
          if (lt == line_type::cmd_end)
            return;

          // Check if-else block sequencing.
          //
          if (bt == line_type::cmd_else)
          {
            if (lt == line_type::cmd_else ||
                lt == line_type::cmd_elif ||
                lt == line_type::cmd_elifn)
              fail (ll) << lt << " after " << bt;
          }

          // Update current if-else block.
          //
          switch (lt)
          {
          case line_type::cmd_elif:
          case line_type::cmd_elifn: bt = line_type::cmd_elif; break;
          case line_type::cmd_else:  bt = line_type::cmd_else; break;
          default: break;
          }
        }
      }

      command_expr parser::
      parse_command_line (token& t, type& tt)
      {
        // enter: first token of the command line
        // leave: <newline>

        // Note: this one is only used during execution.
        //
        assert (!pre_parse_);

        pair<command_expr, here_docs> p (
          parse_command_expr (t, tt, lexer::redirect_aliases));

        assert (tt == type::newline);

        parse_here_documents (t, tt, p);
        assert (tt == type::newline);

        // @@ Note that currently running programs via a runner (e.g., see
        //    test.runner) needs to be handled explicitly in ad hoc recipes.
        //    We could potentially run them via the runner implicitly, similar
        //    to how we do in the testscript. We would need then to match the
        //    command program path against the recipe target ad hoc member
        //    paths (test programs), to detect if it must be run via the
        //    runner. The runner path/options would need to be optionally
        //    passed to the environment constructor, similar to passing the
        //    script deadline.
        //
        return move (p.first);
      }

      //
      // Execute.
      //

      optional<process_path> parser::
      parse_program (token& t, build2::script::token_type& tt,
                     bool first,
                     bool env,
                     names& ns)
      {
        const location l (get_location (t));

        // Set the current script name if it is not set or its weight is less
        // than the new name weight, skipping names with the zero weight. If
        // the weight is the same but the name is different then record this
        // ambiguity, unless one is already recorded. This ambiguity will be
        // reported at the end of the script pre-parsing, unless discarded by
        // the name with a greater weight.
        //
        auto set_diag = [&l, this] (string d, uint8_t w)
        {
          if (diag_weight_ < w)
          {
            diag_name_   = make_pair (move (d), l);
            diag_weight_ = w;
            diag_name2_  = nullopt;
          }
          else if (w != 0                 &&
                   w == diag_weight_      &&
                   d != diag_name_->first &&
                   !diag_name2_)
            diag_name2_ = make_pair (move (d), l);
        };

        // Handle special builtins.
        //
        // NOTE: update line dumping (script.cxx:dump()) if adding a special
        // builtin. Also review the non-script-local variables tracking while
        // executing a single line in lookup_variable().
        //
        if (pre_parse_ && tt == type::word)
        {
          const string& v (t.value);

          // Verify that the special builtin is not called inside an improper
          // context (flow control construct or complex expression).
          //
          auto verify = [first, env, &v, &l, this] ()
          {
            if (level_ != 0)
              fail (l) << "'" << v << "' call inside flow control construct";

            if (!first)
              fail (l) << "'" << v << "' call must be the only command";

            if (env)
              fail (l) << "'" << v << "' call via 'env' builtin";
          };

          if (v == "diag")
          {
            verify ();

            // Check for ambiguity.
            //
            if (diag_weight_ == 4)
            {
              if (diag_name_) // Script name.
              {
                fail (l) << "both low-verbosity script diagnostics name "
                         << "and 'diag' builtin call" <<
                  info (diag_name_->second) << "script name specified here";
              }
              else           // Custom diagnostics.
              {
                assert (diag_line_);

                fail (l) << "multiple 'diag' builtin calls" <<
                  info (diag_line_->second) << "previous call is here";
              }
            }

            // Instruct the parser to save the diag builtin line separately
            // from the script lines, when it is fully parsed. Note that it
            // will be executed prior to the script body execution to obtain
            // the custom diagnostics.
            //
            diag_line_   = make_pair (line (), l);
            save_line_   = &diag_line_->first;
            diag_weight_ = 4;

            diag_name_  = nullopt;
            diag_name2_ = nullopt;

            // Note that the rest of the line contains the builtin argument to
            // be printed, thus we parse it in the value lexer mode.
            //
            mode (lexer_mode::value);
            parse_names (t, tt, pattern_mode::ignore);
            return nullopt;
          }
          else if (v == "depdb")
          {
            verify ();

            // Verify that depdb is not used for anything other than
            // performing update.
            //
            assert (actions_ != nullptr);

            for (const action& a: *actions_)
            {
              if (a != perform_update_id)
                fail (l) << "'depdb' builtin cannot be used to "
                         << ctx.meta_operation_table[a.meta_operation ()].name
                         << ' ' << ctx.operation_table[a.operation ()];
            }

            if (diag_line_)
              fail (diag_line_->second)
                << "'diag' builtin call before 'depdb' call" <<
                info (l) << "'depdb' call is here";

            // Note that the rest of the line contains the builtin command
            // name, potentially followed by the arguments to be hashed/saved.
            // Thus, we parse it in the value lexer mode.
            //
            mode (lexer_mode::value);

            // Obtain and validate the depdb builtin command name.
            //
            next (t, tt);

            if (tt != type::word ||
                (v != "clear" && v != "hash" && v != "string" && v != "env"))
            {
              fail (get_location (t))
                << "expected 'depdb' builtin command instead of " << t;
            }

            if (v == "clear")
            {
              // Make sure the clear depdb command comes first.
              //
              if (depdb_clear_)
                fail (l) << "multiple 'depdb clear' builtin calls" <<
                  info (*depdb_clear_) << "previous call is here";

              if (!depdb_preamble_.empty ())
              {
                diag_record dr (fail (l));
                dr << "'depdb clear' should be the first 'depdb' builtin call";

                // Print the first depdb call location.
                //
                for (const line& l: depdb_preamble_)
                {
                  const replay_tokens& rt (l.tokens);
                  assert (!rt.empty ());

                  const token& t (rt[0].token);
                  if (t.type == type::word && t.value == "depdb")
                  {
                    dr << info (rt[0].location ())
                       << "first 'depdb' call is here";
                    break;
                  }
                }
              }

              // Save the builtin location, cancel the line saving, and clear
              // the referenced variable list, since it won't be used.
              //
              depdb_clear_ = l;
              save_line_   = nullptr;

              script_->vars.clear ();
            }
            else
            {
              // Move the script body to the end of the depdb preamble.
              //
              // Note that at this (pre-parsing) stage we cannot evaluate if
              // all the script body lines are allowed for depdb preamble.
              // That, in particular, would require to analyze pipelines to
              // see if they are terminated with the set builtin, but this
              // information is only available at the execution stage. Thus,
              // we move into the preamble whatever is there and delay the
              // check until the execution.
              //
              lines& ls (script_->body);
              depdb_preamble_.insert (depdb_preamble_.end (),
                                      make_move_iterator (ls.begin ()),
                                      make_move_iterator (ls.end ()));
              ls.clear ();

              // Also move the body_temp_dir flag, if it is true.
              //
              if (script_->body_temp_dir)
              {
                script_->depdb_preamble_temp_dir = true;
                script_->body_temp_dir = false;
              }

              // Reset the impure function call info since it's valid for the
              // depdb preamble.
              //
              impure_func_ = nullopt;

              // Instruct the parser to save the depdb builtin line separately
              // from the script lines, when it is fully parsed. Note that the
              // builtin command arguments will be validated during execution,
              // when expanded.
              //
              depdb_preamble_.push_back (line ());
              save_line_ = &depdb_preamble_.back ();
            }

            // Parse the rest of the line and bail out.
            //
            parse_names (t, tt, pattern_mode::ignore);
            return nullopt;
          }
        }

        auto suggest_diag = [this] (const diag_record& dr)
        {
          dr << info << "consider specifying it explicitly with "
             << "the 'diag' recipe attribute";

          dr << info << "or provide custom low-verbosity diagnostics "
             << "with the 'diag' builtin";
        };

        parse_names_result pr;
        {
          // During pre-parse, if the script name is not set manually, we
          // suspend pre-parse, parse the command names for real and try to
          // deduce the script name from the result. Otherwise, we continue to
          // pre-parse and bail out after parsing the names.
          //
          // Note that the later is not just an optimization since expansion
          // that wouldn't fail during execution may fail in this special
          // mode, for example:
          //
          // ...
          // {{
          //    x = true
          //    ba($x ? r : z)
          // }}
          //
          // v = a b
          // ...
          // {{
          //    v = o
          //    fo$v
          // }}
          //
          // This is also the reason why we add a diag frame.
          //
          if (pre_parse_ && diag_weight_ != 4)
          {
            pre_parse_ = false; // Make parse_names() perform expansions.
            pre_parse_suspended_ = true;
          }

          auto df = make_diag_frame (
            [&l, &suggest_diag, this] (const diag_record& dr)
            {
              if (pre_parse_suspended_)
              {
                dr << info (l)
                   << "while deducing low-verbosity script diagnostics name";

                suggest_diag (dr);
              }
            });

          pr = parse_names (t, tt,
                            ns,
                            pattern_mode::ignore,
                            true /* chunk */,
                            "command line",
                            nullptr);

          if (pre_parse_suspended_)
          {
            pre_parse_suspended_ = false;
            pre_parse_ = true;
          }

          if (pre_parse_ && diag_weight_ == 4)
            return nullopt;
        }

        // Try to translate names into a process path, unless there is nothing
        // to translate.
        //
        // We only end up here in the pre-parse mode if we are still searching
        // for the script name.
        //
        if (!pr.not_null || ns.empty ())
        {
          if (pre_parse_)
          {
            diag_record dr (fail (l));
            dr << "unable to deduce low-verbosity script diagnostics name";
            suggest_diag (dr);
          }

          return nullopt;
        }

        // We have to handle process_path[_ex] and executable target. The
        // process_path[_ex] we may have to recognize syntactically because
        // of the loss of type, for example:
        //
        // c = $cxx.path --version
        //
        // {{
        //    $c ...
        // }}
        //
        // This is further complicated by the fact that the first name in
        // process_path[_ex] may or may not be a pair (it's not a pair if
        // recall and effective paths are the same). If it's not a pair and we
        // are dealing with process_path, then we don't need to do anything
        // extra -- it will just be treated as normal program path. However,
        // if it's process_path_ex, then we may end up with something along
        // these lines:
        //
        // /usr/bin/g++ name@c++ checksum@... env-checksum@... --version
        //
        // Which is a bit harder to recognize syntactically. So what we are
        // going to do is have a separate first pass which reduces the
        // syntactic cases to the typed ones.
        //
        names pp_ns;
        if (pr.type == &value_traits<process_path>::value_type ||
            pr.type == &value_traits<process_path_ex>::value_type)
        {
          pp_ns = move (ns);
          ns.clear ();
        }
        else if (ns[0].file ())
        {
          // Find the end of the value.
          //
          auto b (ns.begin ());
          auto i (value_traits<process_path_ex>::find_end (ns));

          if (b->pair || i != b + 1) // First is a pair or pairs after.
          {
            pp_ns = names (make_move_iterator (b), make_move_iterator (i));

            ns.erase (b, i);

            pr.type = i != b + 1
                      ? &value_traits<process_path_ex>::value_type
                      : &value_traits<process_path>::value_type;
          }
        }

        // Handle process_path[_ex], for example:
        //
        // {{
        //    $cxx.path ...
        // }}
        //
        if (pr.type == &value_traits<process_path>::value_type)
        {
          auto pp (convert<process_path> (move (pp_ns)));

          if (pre_parse_)
          {
            diag_record dr (fail (l));
            dr << "unable to deduce low-verbosity script diagnostics name "
               << "from process path " << pp;
            suggest_diag (dr);
          }
          else
            return optional<process_path> (move (pp));
        }
        else if (pr.type == &value_traits<process_path_ex>::value_type)
        {
          auto pp (convert<process_path_ex> (move (pp_ns)));

          if (pre_parse_)
          {
            if (pp.name)
            {
              set_diag (move (*pp.name), 3);
              return nullopt;
            }

            diag_record dr (fail (l));
            dr << "unable to deduce low-verbosity script diagnostics name "
               << "from process path " << pp;
            suggest_diag (dr);
          }
          else
            return optional<process_path> (move (pp));
        }
        //
        // Handle the executable target, for example:
        //
        // import! [metadata] cli = cli%exe{cli}
        // ...
        // {{
        //    $cli ...
        // }}
        //
        else if (!ns[0].simple ())
        {
          if (const target* t = search_existing (
                ns[0], *scope_, ns[0].pair ? ns[1].dir : empty_dir_path))
          {
            if (const auto* et = t->is_a<exe> ())
            {
              if (pre_parse_)
              {
                if (auto* n = et->lookup_metadata<string> ("name"))
                {
                  set_diag (*n, 3);
                  return nullopt;
                }
                // Fall through.
              }
              else
              {
                process_path pp (et->process_path ());

                if (pp.empty ())
                  fail (l) << "target " << *et << " is out of date" <<
                    info << "consider specifying it as a prerequisite of "
                           << environment_->target;

                ns.erase (ns.begin (), ns.begin () + (ns[0].pair ? 2 : 1));
                return optional<process_path> (move (pp));
              }
            }

            if (pre_parse_)
            {
              diag_record dr (fail (l));
              dr << "unable to deduce low-verbosity script diagnostics name "
                 << "from target " << *t;
              suggest_diag (dr);
            }
          }

          if (pre_parse_)
          {
            diag_record dr (fail (l));
            dr << "unable to deduce low-verbosity script diagnostics name "
               << "from " << ns;
            suggest_diag (dr);
          }
          else
            return nullopt;
        }
        else if (pre_parse_)
        {
          // If we are here, the name is simple and is not part of a pair.
          //
          string& v (ns[0].value);

          // Try to interpret the name as a builtin.
          //
          const builtin_info* bi (builtins.find (v));

          if (bi != nullptr)
          {
            set_diag (move (v), bi->weight);
            return nullopt;
          }
          //
          // Try to interpret the name as a pseudo-builtin.
          //
          // Note that both of them has the zero weight and cannot be picked
          // up as a script name.
          //
          else if (v == "set" || v == "exit")
          {
            return nullopt;
          }

          diag_record dr (fail (l));
          dr << "unable to deduce low-verbosity script diagnostics name "
             << "for program " << ns[0];
          suggest_diag (dr);
        }

        return nullopt;
      }

      void parser::
      execute_body (const scope& rs, const scope& bs,
                    environment& e, const script& s, runner& r,
                    bool enter, bool leave)
      {
        pre_exec (rs, bs, e, &s, &r);

        if (enter)
          runner_->enter (e, s.start_loc);

        // Note that we rely on "small function object" optimization here.
        //
        auto exec_cmd = [this] (token& t, build2::script::token_type& tt,
                                size_t li,
                                bool single,
                                const location& ll)
        {
          // We use the 0 index to signal that this is the only command.
          //
          if (single)
            li = 0;

          command_expr ce (
            parse_command_line (t, static_cast<token_type&> (tt)));

          runner_->run (*environment_, ce, li, ll);
        };

        exec_lines (s.body, exec_cmd);

        if (leave)
          runner_->leave (e, s.end_loc);
      }

      void parser::
      execute_depdb_preamble (const scope& rs, const scope& bs,
                              environment& e, const script& s, runner& r,
                              depdb& dd)
      {
        tracer trace ("execute_depdb_preamble");

        // The only valid lines in the depdb preamble are the depdb builtin
        // itself as well as the variable assignments, including via the set
        // builtin.

        pre_exec (rs, bs, e, &s, &r);

        // Let's "wrap up" the objects we operate upon into the single object
        // to rely on "small function object" optimization.
        //
        struct
        {
          environment& env;
          const script& scr;
          depdb& dd;
          tracer& trace;
        } ctx {e, s, dd, trace};

        auto exec_cmd = [&ctx, this]
                        (token& t,
                         build2::script::token_type& tt,
                         size_t li,
                         bool /* single */,
                         const location& ll)
        {
          if (tt == type::word && t.value == "depdb")
          {
            names ns (exec_special (t, tt));

            // This should have been enforced during pre-parsing.
            //
            assert (!ns.empty ()); // <cmd> ... <newline>

            const string& cmd (ns[0].value);

            if (cmd == "hash")
            {
              sha256 cs;
              for (auto i (ns.begin () + 1); i != ns.end (); ++i) // Skip <cmd>.
                to_checksum (cs, *i);

              if (ctx.dd.expect (cs.string ()) != nullptr)
                l4 ([&] {
                    ctx.trace (ll)
                      << "'depdb hash' argument change forcing update of "
                      << ctx.env.target;});
            }
            else if (cmd == "string")
            {
              string s;
              try
              {
                s = convert<string> (
                  names (make_move_iterator (ns.begin () + 1),
                         make_move_iterator (ns.end ())));
              }
              catch (const invalid_argument& e)
              {
                fail (ll) << "invalid 'depdb string' argument: " << e;
              }

              if (ctx.dd.expect (s) != nullptr)
                l4 ([&] {
                    ctx.trace (ll)
                      << "'depdb string' argument change forcing update of "
                      << ctx.env.target;});
            }
            else if (cmd == "env")
            {
              sha256 cs;
              const char* pf ("invalid 'depdb env' argument: ");

              try
              {
                // Skip <cmd>.
                //
                for (auto i (ns.begin () + 1); i != ns.end (); ++i)
                {
                  string vn (convert<string> (move (*i)));
                  build2::script::verify_environment_var_name (vn, pf, ll);
                  hash_environment (cs, vn);
                }
              }
              catch (const invalid_argument& e)
              {
                fail (ll) << pf << e;
              }

              if (ctx.dd.expect (cs.string ()) != nullptr)
                l4 ([&] {
                    ctx.trace (ll)
                      << "'depdb env' environment change forcing update of "
                      << ctx.env.target;});
            }
            else
              assert (false);
          }
          else
          {
            // Note that we don't reset the line index to zero (as we do in
            // execute_body()) assuming that there are some script body
            // commands to follow.
            //
            command_expr ce (
              parse_command_line (t, static_cast<token_type&> (tt)));

            // Verify that this expression executes the set builtin.
            //
            if (find_if (ce.begin (), ce.end (),
                         [] (const expr_term& et)
                         {
                           const process_path& p (et.pipe.back ().program);
                           return p.initial == nullptr &&
                                  p.recall.string () == "set";
                         }) == ce.end ())
            {
              const replay_tokens& rt (ctx.scr.depdb_preamble.back ().tokens);
              assert (!rt.empty ());

              fail (ll) << "disallowed command in depdb preamble" <<
                info << "only variable assignments are allowed in "
                     << "depdb preamble" <<
                info (rt[0].location ()) << "depdb preamble ends here";
            }

            runner_->run (*environment_, ce, li, ll);
          }
        };

        exec_lines (s.depdb_preamble, exec_cmd);
      }

      void parser::
      pre_exec (const scope& rs, const scope& bs,
                environment& e, const script* s, runner* r)
      {
        path_ = nullptr; // Set by replays.

        pre_parse_ = false;

        set_lexer (nullptr);

        actions_ = nullptr;

        // The script shouldn't be able to modify the scopes.
        //
        // Note that for now we don't set target_ since it's not clear what
        // it could be used for (we need scope_ for calling functions such as
        // $target.path()).
        //
        root_ = const_cast<scope*> (&rs);
        scope_ = const_cast<scope*> (&bs);
        pbase_ = scope_->src_path_;

        script_ = const_cast<script*> (s);
        runner_ = r;
        environment_ = &e;
      }

      void parser::
      exec_lines (const lines& lns,
                  const function<exec_cmd_function>& exec_cmd)
      {
        // Note that we rely on "small function object" optimization for the
        // exec_*() lambdas.
        //
        auto exec_set = [this] (const variable& var,
                                token& t, build2::script::token_type& tt,
                                const location&)
        {
          next (t, tt);
          type kind (tt); // Assignment kind.

          mode (lexer_mode::variable_line);
          value rhs (parse_variable_line (t, tt));

          assert (tt == type::newline);

          // Assign.
          //
          value& lhs (kind == type::assign
                      ? environment_->assign (var)
                      : environment_->append (var));

          apply_value_attributes (&var, lhs, move (rhs), kind);
        };

        auto exec_if = [this] (token& t, build2::script::token_type& tt,
                               size_t li,
                               const location& ll)
        {
          command_expr ce (
            parse_command_line (t, static_cast<token_type&> (tt)));

          // Assume if-else always involves multiple commands.
          //
          return runner_->run_if (*environment_, ce, li, ll);
        };

        build2::script::parser::exec_lines (lns.begin (), lns.end (),
                                            exec_set, exec_cmd, exec_if,
                                            environment_->exec_line,
                                            &environment_->var_pool);
      }

      names parser::
      exec_special (token& t, build2::script::token_type& tt,
                    bool omit_builtin)
      {
        if (omit_builtin)
        {
          assert (tt != type::newline && tt != type::eos);

          next (t, tt);
        }

        return tt != type::newline && tt != type::eos
               ? parse_names (t, tt, pattern_mode::expand)
               : names ();
      }

      names parser::
      execute_special (const scope& rs, const scope& bs,
                       environment& e,
                       const line& ln,
                       bool omit_builtin)
      {
        pre_exec (rs, bs, e, nullptr /* script */, nullptr /* runner */);

        // Copy the tokens and start playing.
        //
        replay_data (replay_tokens (ln.tokens));

        token t;
        build2::script::token_type tt;
        next (t, tt);

        names r (exec_special (t, tt, omit_builtin));

        replay_stop ();
        return r;
      }

      // When add a special variable don't forget to update lexer::word().
      //
      bool parser::
      special_variable (const string& n) noexcept
      {
        return n == ">" || n == "<" || n == "~";
      }

      lookup parser::
      lookup_variable (name&& qual, string&& name, const location& loc)
      {
        // In the pre-parse mode collect the referenced variable names for the
        // script semantics change tracking.
        //
        if (pre_parse_ || pre_parse_suspended_)
        {
          lookup r;

          // Add the variable name skipping special variables and suppressing
          // duplicates, unless the default variables change tracking is
          // canceled with `depdb clear`. While at it, check if the script
          // temporary directory is referenced and set the flag, if that's the
          // case.
          //
          if (special_variable (name))
          {
            if (name == "~")
              script_->body_temp_dir = true;
          }
          else if (!name.empty ())
          {
            if (pre_parse_suspended_)
            {
              const variable* pvar (scope_->ctx.var_pool.find (name));

              if (pvar != nullptr)
                r = (*target_)[*pvar];
            }

            if (!depdb_clear_)
            {
              auto& vars (script_->vars);

              if (find (vars.begin (), vars.end (), name) == vars.end ())
                vars.push_back (move (name));
            }
          }

          return r;
        }

        if (!qual.empty ())
          fail (loc) << "qualified variable name";

        lookup r (environment_->lookup (name));

        // Fail if non-script-local variable with an untracked name.
        //
        // Note that we don't check for untracked variables when executing a
        // single line with execute_special() (script_ is NULL), since the
        // diag builtin argument change (which can be affected by such a
        // variable expansion) doesn't affect the script semantics and the
        // depdb argument is specifically used for the script semantics change
        // tracking. We also omit this check it the depdb builtin is used in
        // the script, assuming that such variables are tracked manually, if
        // required.
        //
        if (script_ != nullptr    &&
            !script_->depdb_clear &&
            script_->depdb_preamble.empty ())
        {
          if (r.defined () && !r.belongs (*environment_))
          {
            const auto& vars (script_->vars);

            if (find (vars.begin (), vars.end (), name) == vars.end ())
              fail (loc) << "use of untracked variable '" << name << "'" <<
                info << "use the 'depdb' builtin to manually track it";
          }
        }

        return r;
      }

      void parser::
      lookup_function (string&& name, const location& loc)
      {
        if (perform_update_ && !impure_func_)
        {
          const function_overloads* f (ctx.functions.find (name));

          if (f != nullptr && !f->pure)
            impure_func_ = make_pair (move (name), loc);
        }
      }
    }
  }
}