Generate expressive diagnostic messages

Author: wilzbach

changelog/error-pretty.dd

@@ -0,0 +1,21 @@
+dmd now supports expressive diagnostic error messages with `-verrors=pretty`
+
+With the new CLI option `-verrors=pretty` dmd will now show the offending line directly in its error messages.
+Consider this faulty program `test.d`:
+
+---
+void foo()
+{
+
+    a = 1;
+}
+---
+
+Now run it with `-verrors=pretty`:
+
+$(CONSOLE
+> dmd -verrors=pretty test.d
+ee.d(5): $(RED Error): undefined identifier a
+    a = 1;
+    ^
+)

src/dmd/cli.d

@@ -553,6 +553,9 @@ dmd -cov -unittest myprog.d
         Option("verrors=spec",
             "show errors from speculative compiles such as __traits(compiles,...)"
         ),
+        Option("verrors=pretty",
+            "Pretty-print error messages with annotation of the erroring source line"
+        ),
         Option("-version",
             "print compiler version and exit"
         ),

src/dmd/errors.d

@@ -236,6 +236,36 @@ private void verrorPrint(const ref Loc loc, Color headerColor, const(char)* head
     else
         fputs(tmp.peekString(), stderr);
     fputc('\n', stderr);
+
+    if (global.params.prettyPrintErrors)
+    {
+        auto fp = fopen(loc.filename, "r");
+        scope(exit) fclose(fp);
+
+        if (fp)
+        {
+            char* lineptr;
+            ulong n = 1024;
+            import core.sys.posix.stdio;
+            foreach (_; 0 .. loc.linnum)
+            {
+                assert(getline(&lineptr, &n, fp) > 0);
+                if (lineptr is null)
+                    goto end;
+            }
+            if (lineptr is null)
+                goto end;
+            fprintf(stderr, "%s", lineptr);
+
+            if (loc.charnum >= 1)
+            foreach (_; 1 .. loc.charnum)
+                fputc(' ', stderr);
+
+            fputc('^', stderr);
+        }
+        fputc('\n', stderr);
+    }
+end:
     fflush(stderr);     // ensure it gets written out in case of compiler aborts
 }
 

src/dmd/globals.d

@@ -158,6 +158,7 @@ struct Param
      */
 
     bool showGaggedErrors;  // print gagged errors anyway
+    bool prettyPrintErrors; // pretty print errors with the actual line
     bool manual;            // open browser on compiler manual
     bool usage;             // print usage and exit
     bool mcpuUsage;         // print help on -mcpu switch

src/dmd/mars.d

@@ -1732,6 +1732,10 @@ private bool parseCommandLine(const ref Strings arguments, const size_t argc, re
                 {
                     params.showGaggedErrors = true;
                 }
+                else if (startsWith(p + 9, "pretty"))
+                {
+                    params.prettyPrintErrors = true;
+                }
                 else
                     goto Lerror;
             }

test/fail_compilation/fail_pretty_errors.d

@@ -0,0 +1,15 @@
+/*
+REQUIRED_ARGS: -verrors=pretty
+TEST_OUTPUT:
+---
+fail_compilation/fail_pretty_errors.d(14): Error: undefined identifier `a`
+    a = 1;
+    ^
+---
+*/
+
+void foo()
+{
+
+    a = 1;
+}