Escape backticks in error messages (#20893)

compiler/src/dmd/dsymbolsem.d

@@ -1896,8 +1896,7 @@ private extern(C++) final class DsymbolSemanticVisitor : Visitor
             const sident = se.toStringz();
             if (!sident.length || !Identifier.isValidIdentifier(sident))
             {
-                error(ns.exp.loc, "expected valid identifier for C++ namespace but got `%.*s`",
-                             cast(int)sident.length, sident.ptr);
+                error(ns.exp.loc, "expected valid identifier for C++ namespace but got `%s`", se.toErrMsg());
                 return null;
             }
             else

compiler/src/dmd/errors.d

@@ -714,7 +714,10 @@ private void verrorPrint(const(char)* format, va_list ap, ref ErrorInfo info)
         writeHighlights(con, tmp);
     }
     else
+    {
+        unescapeBackticks(tmp);
         fputs(tmp.peekChars(), stderr);
+    }
     fputc('\n', stderr);
 
     __gshared SourceLoc old_loc;
@@ -828,6 +831,7 @@ extern (C++) void halt() @safe
  * is D source code, and color syntax highlight it.
  * Modify contents of `buf` with highlighted result.
  * Many parallels to ddoc.highlightText().
+ * Double backticks are replaced by a single backtick without coloring.
  * Params:
  *      buf = text containing `...` code to highlight
  */
@@ -843,6 +847,13 @@ private void colorSyntaxHighlight(ref OutBuffer buf)
         switch (c)
         {
             case '`':
+                // A double backtick means it's part of the content, don't color
+                if (i + 1 < buf.length && buf[i + 1] == '`')
+                {
+                    buf.remove(i, 1);
+                    continue;
+                }
+
                 if (inBacktick)
                 {
                     inBacktick = false;
@@ -867,6 +878,27 @@ private void colorSyntaxHighlight(ref OutBuffer buf)
     }
 }
 
+/// Replace double backticks in `buf` with a single backtick
+void unescapeBackticks(ref OutBuffer buf)
+{
+    for (size_t i = 0; i + 1 < buf.length; ++i)
+    {
+        if (buf[i] == '`' && buf[i + 1] == '`')
+            buf.remove(i, 1);
+    }
+}
+
+unittest
+{
+    OutBuffer buf;
+    buf.writestring("x````");
+    unescapeBackticks(buf);
+    assert(buf.extractSlice() == "x``");
+
+    buf.writestring("x````");
+    colorSyntaxHighlight(buf);
+    assert(buf.extractSlice() == "x``");
+}
 
 /**
  * Embed these highlighting commands in the text stream.

compiler/src/dmd/hdrgen.d

@@ -155,13 +155,15 @@ const(char)* toErrMsg(const Dsymbol d)
  */
 private void truncateForError(ref OutBuffer buf, size_t maxLength)
 {
-    // Remove newlines
+    // Remove newlines, escape backticks ` by doubling them
     for (size_t i = 0; i < buf.length; i++)
     {
         if (buf[i] == '\r')
             buf.remove(i, 1);
         if (buf[i] == '\n')
             buf.peekSlice[i] = ' ';
+        if (buf[i] == '`')
+            i = buf.insert(i, "`");
     }
 
     // Strip trailing whitespace

compiler/test/fail_compilation/cppmangle.d

@@ -1,10 +1,10 @@
 /*
 TEST_OUTPUT:
 ---
-fail_compilation/cppmangle.d(11): Error: expected valid identifier for C++ namespace but got ``
-fail_compilation/cppmangle.d(15): Error: expected valid identifier for C++ namespace but got `0num`
+fail_compilation/cppmangle.d(11): Error: expected valid identifier for C++ namespace but got `""`
+fail_compilation/cppmangle.d(15): Error: expected valid identifier for C++ namespace but got `"0num"`
 fail_compilation/cppmangle.d(19): Error: compile time string constant (or sequence) expected, not `2`
-fail_compilation/cppmangle.d(23): Error: expected valid identifier for C++ namespace but got `invalid@namespace`
+fail_compilation/cppmangle.d(23): Error: expected valid identifier for C++ namespace but got `"invalid@namespace"`
 ---
 */