Improve heuristics for capnpc -o param on Windows. Fixes #722.

c++/src/capnp/compiler/capnp.c++

@@ -382,10 +382,14 @@ public:
       kj::StringPtr dir = spec.slice(*split + 1);
       auto plugin = spec.slice(0, *split);
 
-      KJ_IF_MAYBE(split2, dir.findFirst(':')) {
-        // Grr, there are two colons. Might this be a Windows path? Let's do some heuristics.
       if (*split == 1 && (dir.startsWith("/") || dir.startsWith("\\"))) {
-          // So, the first ':' was the second char, and was followed by '/' or '\', e.g.:
+        // The colon is the second character and is immediately followed by a slash or backslash.
+        // So, the user passed something like `-o c:/foo`. Is this a request to run the C plugin
+        // and output to `/foo`? Or are we on Windows, and is this a request to run the plugin
+        // `c:/foo`?
+        KJ_IF_MAYBE(split2, dir.findFirst(':')) {
+          // There are two colons. The first ':' was the second char, and was followed by '/' or
+          // '\', e.g.:
           //     capnp compile -o c:/foo.exe:bar
           //
           // In this case we can conclude that the second colon is actually meant to be the
@@ -407,18 +411,24 @@ public:
           //   -> CONTRADICTION
           //
           // We therefore conclude that the *second* colon is in fact the plugin/location separator.
-          //
-          // Note that there is still an ambiguous case:
-          //     capnp compile -o c:/foo
-          //
-          // In this unfortunate case, we have no way to tell if the user meant "use the 'c' plugin
-          // and output to /foo" or "use the plugin c:/foo and output to the default location". We
-          // prefer the former interpretation, because the latter is Windows-specific and such
-          // users can always explicitly specify the output location like:
-          //     capnp compile -o c:/foo:.
 
           dir = dir.slice(*split2 + 1);
           plugin = spec.slice(0, *split2 + 2);
+#if _WIN32
+        } else {
+          // The user wrote something like:
+          //
+          //     capnp compile -o c:/foo/bar
+          //
+          // What does this mean? It depends on what system we're on. On a Unix system, the above
+          // clearly is a request to run the `capnpc-c` plugin (perhaps to output C code) and write
+          // to the directory /foo/bar. But on Windows, absolute paths do not start with '/', and
+          // the above is actually a request to run the plugin `c:/foo/bar`, outputting to the
+          // current directory.
+
+          outputs.add(OutputDirective { spec.asArray(), nullptr });
+          return true;
+#endif
         }
       }