Feat: Add ability to compile from stdin (#1327)

* Implement ability to compile directly from stdin

* Add doc comment to process_cpp2 function

* Move choice between file or stdin to source.load()

* Add a little comment above source loading decision

* Don't file-open `stdin`

* Fixed logic, and added default to stdout if stdin is used

Also:
- re-ran self-build and regression tests
- suppressed non-error output when stdout is used, so piping is easier

---------

Co-authored-by: Herb Sutter <herb.sutter@gmail.com>

Author: vanceism7

source/common.h

@@ -799,7 +799,9 @@ class cmdline_processor
             [](auto& a, auto& b){ return a.group < b.group || (a.group == b.group && a.name < b.name); }
         );
 
-        print("\nUsage: cppfront [options] file ...\n\nOptions:\n");
+        print("\nUsage: cppfront [options] file ...\n");
+        print("\n  file                    source file(s) (can be 'stdin')\n");
+        print("\nOptions: \n");
         int last_group = -1;
         for (auto& flag : flags) {
             //  Skip hidden flags

source/cppfront.cpp

@@ -79,7 +79,12 @@ auto main(
 
         auto& out = flag_cpp1_filename != "stdout" ? std::cout : std::cerr;
 
-        if (!flag_quiet) {
+        if (
+            !flag_quiet
+            && arg.text != "stdin"
+            && flag_cpp1_filename != "stdout"
+            ) 
+        {
             out << arg.text << "...";
         }
 
@@ -92,7 +97,10 @@ auto main(
         //  If there were no errors, say so and generate Cpp1
         if (c.had_no_errors())
         {
-            if (!flag_quiet)
+            if (
+                !flag_quiet
+                && flag_cpp1_filename != "stdout"
+                )
             {
                 if (!c.has_cpp1()) {
                     out << " ok (all Cpp2, passes safety checks)\n";

source/io.h

@@ -882,11 +882,17 @@ class source
     )
         -> bool
     {
-        std::ifstream in{ filename };
-        if (!in.is_open()) {
-            return false;
+        //  If filename is stdin, we read from stdin, otherwise we try to read the file
+        //
+        auto is_stdin = filename == "stdin";
+        std::ifstream fss;
+        if (!is_stdin) 
+        {
+            fss.open(filename);
+            if( !fss.is_open()) { return false; }
         }
-
+        std::istream& in = is_stdin ? std::cin : fss;
+    
         auto in_comment            = false;
         auto in_string_literal     = false;
         auto in_raw_string_literal = false;

source/to_cpp1.h

@@ -1179,6 +1179,7 @@ class cppfront
         if (
             !sourcefile.ends_with(".cpp2")
             && !sourcefile.ends_with(".h2")
+            && sourcefile != "stdin"
             )
         {
             errors.emplace_back(
@@ -1257,14 +1258,18 @@ class cppfront
         }
 
         //  Now we'll open the Cpp1 file
-        auto cpp1_filename = sourcefile.substr(0, std::ssize(sourcefile) - 1);
+        //  Default to stdout if input is stdin
+        auto cpp1_filename = std::string{"stdout"};
+        if (sourcefile != "stdin") {
+            assert(sourcefile.ends_with("2"));
+            cpp1_filename = sourcefile.substr(0, std::ssize(sourcefile) - 1);
+        }
 
-        //  Use explicit filename override if present,
-        //  otherwise strip leading path
+        //  Use explicit filename override if present, otherwise strip leading path
         if (!flag_cpp1_filename.empty()) {
             cpp1_filename = flag_cpp1_filename;
         }
-        else {
+        else if (cpp1_filename != "stdout") {
             cpp1_filename = std::filesystem::path(cpp1_filename).filename().string();
         }