[GNU] Add "#pragma once"

preprocess.c

@@ -67,6 +67,7 @@ struct Hideset {
 
 static HashMap macros;
 static CondIncl *cond_incl;
+static HashMap pragma_once;
 
 static Token *preprocess2(Token *tok);
 static Macro *find_macro(Token *tok);
@@ -781,6 +782,10 @@ static char *detect_include_guard(Token *tok) {
 }
 
 static Token *include_file(Token *tok, char *path, Token *filename_tok) {
+  // Check for "#pragma once"
+  if (hashmap_get(&pragma_once, path))
+    return tok;
+
   // If we read the same file before, and if the file was guarded
   // by the usual #ifndef ... #endif pattern, we may be able to
   // skip the file without opening it.
@@ -939,6 +944,12 @@ static Token *preprocess2(Token *tok) {
       continue;
     }
 
+    if (equal(tok, "pragma") && equal(tok->next, "once")) {
+      hashmap_put(&pragma_once, tok->file->name, (void *)1);
+      tok = skip_line(tok->next->next);
+      continue;
+    }
+
     if (equal(tok, "pragma")) {
       do {
         tok = tok->next;

test/pragma-once.c

@@ -0,0 +1,10 @@
+#include "test.h"
+
+#pragma once
+
+#include "test/pragma-once.c"
+
+int main() {
+  printf("OK\n");
+  return 0;
+}