Add multi-inputs tool

The 'multi-inputs' option will list all <target> + <inputs> for
the given targets.

Run:
ninja -t multi-inputs <target1> <target2> <target3>

Ninja will then output:
<target1> <input_x>
<target1> <input_y>
<target2> <input_x>
<target2> <input_z>
<target3> <input_y>

doc/manual.asciidoc

@@ -265,6 +265,37 @@ output files are out of date.
 rebuild those targets.
 _Available since Ninja 1.11._
 
+`multi-inputs`:: print one or more sets of inputs required to build targets.
+Each line will consist of a target, a delimiter, an input and a terminator character.
+The list produced by the tool can be helpful if one would like to know which targets
+that are affected by a certain input.
++
+The output will be a series of lines with the following elements:
++
+----
+<target> <delimiter> <input> <terminator>
+----
++
+Example usage of the `multi-inputs` tool:
++
+----
+ninja -t multi-inputs target1 target2 target3
+----
++
+Example of produced output from the `multi-inputs` tool:
++
+----
+target1 file1.c
+target2 file1.c
+target2 file2.c
+target3 file1.c
+target3 file2.c
+target3 file3.c
+----
++
+_Note that a given input may appear for several targets if it is used by more
+than one targets._
+
 `clean`:: remove built files. By default, it removes all built files
 except for those created by the generator.  Adding the `-g` flag also
 removes built files created by the generator (see <<ref_rule,the rule

misc/output_test.py

@@ -449,6 +449,40 @@ def test_tool_compdb_targets(self) -> None:
             self.assertEqual(expected, actual)
 
 
+    def test_tool_multi_inputs(self) -> None:
+        plan = '''
+rule cat
+  command = cat $in $out
+build out1 : cat in1
+build out2 : cat in1 in2
+build out3 : cat in1 in2 in3
+'''
+        self.assertEqual(run(plan, flags='-t multi-inputs out1'),
+'''out1<TAB>in1
+'''.replace("<TAB>", "\t"))
+
+        self.assertEqual(run(plan, flags='-t multi-inputs out1 out2 out3'),
+'''out1<TAB>in1
+out2<TAB>in1
+out2<TAB>in2
+out3<TAB>in1
+out3<TAB>in2
+out3<TAB>in3
+'''.replace("<TAB>", "\t"))
+
+        self.assertEqual(run(plan, flags='-t multi-inputs -d: out1'),
+'''out1:in1
+''')
+
+        self.assertEqual(
+          run(
+            plan,
+            flags='-t multi-inputs -d, --print0 out1 out2'
+          ),
+          '''out1,in1\0out2,in1\0out2,in2\0'''
+        )
+
+
     def test_explain_output(self):
         b = BuildDir('''\
             build .FORCE: phony

src/ninja.cc

@@ -130,6 +130,7 @@ struct NinjaMain : public BuildLogUser {
   int ToolTargets(const Options* options, int argc, char* argv[]);
   int ToolCommands(const Options* options, int argc, char* argv[]);
   int ToolInputs(const Options* options, int argc, char* argv[]);
+  int ToolMultiInputs(const Options* options, int argc, char* argv[]);
   int ToolClean(const Options* options, int argc, char* argv[]);
   int ToolCleanDead(const Options* options, int argc, char* argv[]);
   int ToolCompilationDatabase(const Options* options, int argc, char* argv[]);
@@ -845,6 +846,74 @@ int NinjaMain::ToolInputs(const Options* options, int argc, char* argv[]) {
   return 0;
 }
 
+int NinjaMain::ToolMultiInputs(const Options* options, int argc, char* argv[]) {
+  // The inputs tool uses getopt, and expects argv[0] to contain the name of
+  // the tool, i.e. "inputs".
+  argc++;
+  argv--;
+
+  optind = 1;
+  int opt;
+  char terminator = '\n';
+  const char* delimiter = "\t";
+  const option kLongOptions[] = { { "help", no_argument, NULL, 'h' },
+                                  { "delimiter", required_argument, NULL,
+                                    'd' },
+                                  { "print0", no_argument, NULL, '0' },
+                                  { NULL, 0, NULL, 0 } };
+  while ((opt = getopt_long(argc, argv, "d:h0", kLongOptions, NULL)) != -1) {
+    switch (opt) {
+    case 'd':
+      delimiter = optarg;
+      break;
+    case '0':
+      terminator = '\0';
+      break;
+    case 'h':
+    default:
+      // clang-format off
+      printf(
+"Usage '-t multi-inputs [options] [targets]\n"
+"\n"
+"Print one or more sets of inputs required to build targets, sorted in dependency order.\n"
+"The tool works like inputs tool but with addition of the target for each line.\n"
+"The output will be a series of lines with the following elements:\n"
+"<target> <delimiter> <input> <terminator>\n"
+"Note that a given input may appear for several targets if it is used by more than one targets.\n"
+"Options:\n"
+"  -h, --help                   Print this message.\n"
+"  -d  --delimiter=DELIM        Use DELIM instead of TAB for field delimiter.\n"
+"  -0, --print0                 Use \\0, instead of \\n as a line terminator.\n"
+      );
+      // clang-format on
+      return 1;
+    }
+  }
+  argv += optind;
+  argc -= optind;
+
+  std::vector<Node*> nodes;
+  std::string err;
+  if (!CollectTargetsFromArgs(argc, argv, &nodes, &err)) {
+    Error("%s", err.c_str());
+    return 1;
+  }
+
+  for (const Node* node : nodes) {
+    InputsCollector collector;
+
+    collector.VisitNode(node);
+    std::vector<std::string> inputs = collector.GetInputsAsStrings();
+
+    for (const std::string& input : inputs) {
+      printf("%s%s%s", node->path().c_str(), delimiter, input.c_str());
+      fputc(terminator, stdout);
+    }
+  }
+
+  return 0;
+}
+
 int NinjaMain::ToolClean(const Options* options, int argc, char* argv[]) {
   // The clean tool uses getopt, and expects argv[0] to contain the name of
   // the tool, i.e. "clean".
@@ -1234,6 +1303,8 @@ const Tool* ChooseTool(const string& tool_name) {
       Tool::RUN_AFTER_LOAD, &NinjaMain::ToolCommands },
     { "inputs", "list all inputs required to rebuild given targets",
       Tool::RUN_AFTER_LOAD, &NinjaMain::ToolInputs},
+    { "multi-inputs", "print one or more sets of inputs required to build targets",
+      Tool::RUN_AFTER_LOAD, &NinjaMain::ToolMultiInputs},
     { "deps", "show dependencies stored in the deps log",
       Tool::RUN_AFTER_LOGS, &NinjaMain::ToolDeps },
     { "missingdeps", "check deps log dependencies on generated files",