frontend: propagate error as null instead of exit(1)

This patch propagates null when Module AST node can't be constructed correctly
instead of doing fatal(), which terminates the program. This is useful if users
use DMD frontend as a library.

Signed-off-by: Luís Ferreira <contact@lsferreira.net>

src/dmd/dmodule.d

@@ -524,7 +524,7 @@ extern (C++) final class Module : Package
             buf.printf("%s\t(%s)", ident.toChars(), m.srcfile.toChars());
             message("import    %s", buf.peekChars());
         }
-        m = m.parse();
+        if((m = m.parse()) is null) return null;
 
         // Call onImport here because if the module is going to be compiled then we
         // need to determine it early because it affects semantic analysis. This is
@@ -727,7 +727,7 @@ extern (C++) final class Module : Package
             if (buf.length & 3)
             {
                 error("odd length of UTF-32 char source %llu", cast(ulong) buf.length);
-                fatal();
+                return null;
             }
 
             const (uint)[] eBuf = cast(const(uint)[])buf;
@@ -743,7 +743,7 @@ extern (C++) final class Module : Package
                     if (u > 0x10FFFF)
                     {
                         error("UTF-32 value %08x greater than 0x10FFFF", u);
-                        fatal();
+                        return null;
                     }
                     dbuf.writeUTF8(u);
                 }
@@ -773,7 +773,7 @@ extern (C++) final class Module : Package
             if (buf.length & 1)
             {
                 error("odd length of UTF-16 char source %llu", cast(ulong) buf.length);
-                fatal();
+                return null;
             }
 
             const (ushort)[] eBuf = cast(const(ushort)[])buf;
@@ -793,26 +793,26 @@ extern (C++) final class Module : Package
                         if (i >= eBuf.length)
                         {
                             error("surrogate UTF-16 high value %04x at end of file", u);
-                            fatal();
+                            return null;
                         }
                         const u2 = readNext(&eBuf[i]);
                         if (u2 < 0xDC00 || 0xE000 <= u2)
                         {
                             error("surrogate UTF-16 low value %04x out of range", u2);
-                            fatal();
+                            return null;
                         }
                         u = (u - 0xD7C0) << 10;
                         u |= (u2 - 0xDC00);
                     }
                     else if (u >= 0xDC00 && u <= 0xDFFF)
                     {
                         error("unpaired surrogate UTF-16 value %04x", u);
-                        fatal();
+                        return null;
                     }
                     else if (u == 0xFFFE || u == 0xFFFF)
                     {
                         error("illegal UTF-16 value %04x", u);
-                        fatal();
+                        return null;
                     }
                     dbuf.writeUTF8(u);
                 }
@@ -899,7 +899,7 @@ extern (C++) final class Module : Package
                     if (buf[0] >= 0x80)
                     {
                         error("source file must start with BOM or ASCII character, not \\x%02X", buf[0]);
-                        fatal();
+                        return null;
                     }
                 }
             }
@@ -929,6 +929,8 @@ extern (C++) final class Module : Package
                       ? UTF32ToUTF8!(Endian.little)(buf)
                       : UTF32ToUTF8!(Endian.big)(buf);
             }
+            // an error happened on UTF conversion
+            if (buf is null) return null;
         }
 
         /* If it starts with the string "Ddoc", then it's a documentation

src/dmd/frontend.d

@@ -407,7 +407,7 @@ Tuple!(Module, "module_", Diagnostics, "diagnostics") parseModule(AST = ASTCodeg
         }
     }
 
-    m.parseModule!AST();
+    m = m.parseModule!AST();
 
     Diagnostics diagnostics = {
         errors: global.errors,