traces: Do not print unknown location

Solves 1/3 of the infinite recursion at unknown location meme.
See #8879 for ensuring we always have a trace (for stack overflows)
We might want to re-add this for finding missing location info
*while hacking on that problem only*.

src/libutil/error.cc

@@ -203,8 +203,6 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
 
     std::ostringstream oss;
 
-    auto noSource = ANSI_ITALIC " (source not available)" ANSI_NORMAL "\n";
-
     /*
      * Traces
      * ------
@@ -320,7 +318,7 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
 
             oss << "\n" << "… " << trace.hint.str() << "\n";
 
-            if (trace.pos) {
+            if (trace.pos && *trace.pos) {
                 count++;
 
                 oss << "\n" << ellipsisIndent << ANSI_BLUE << "at " ANSI_WARNING << *trace.pos << ANSI_NORMAL << ":";
@@ -329,8 +327,7 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
                     oss << "\n";
                     printCodeLines(oss, "", *trace.pos, *loc);
                     oss << "\n";
-                } else
+                }
-                    oss << noSource;
             }
         }
         oss << "\n" << prefix;
@@ -338,15 +335,14 @@ std::ostream & showErrorInfo(std::ostream & out, const ErrorInfo & einfo, bool s
 
     oss << einfo.msg << "\n";
 
-    if (einfo.errPos) {
+    if (einfo.errPos && *einfo.errPos) {
         oss << "\n" << ANSI_BLUE << "at " ANSI_WARNING << *einfo.errPos << ANSI_NORMAL << ":";
 
         if (auto loc = einfo.errPos->getCodeLines()) {
             oss << "\n";
             printCodeLines(oss, "", *einfo.errPos, *loc);
             oss << "\n";
-        } else
+        }
-            oss << noSource;
     }
 
     auto suggestions = einfo.suggestions.trim();

src/libutil/error.hh

@@ -70,6 +70,13 @@ struct AbstractPos
     uint32_t line = 0;
     uint32_t column = 0;
 
+    /**
+     * An AbstractPos may be a "null object", representing an unknown position.
+     *
+     * Return true if this position is known.
+     */
+    inline operator bool() const { return line != 0; };
+
     /**
      * Return the contents of the source file.
      */

tests/lang/eval-fail-fromTOML-timestamps.err.exp

@@ -8,5 +8,3 @@ error:
             2|   key = "value"
 
        error: while parsing a TOML string: Dates and times are not supported
-
-       at «none»:0: (source not available)

tests/lang/eval-fail-hashfile-missing.err.exp

@@ -10,10 +10,6 @@ error:
 
        … while evaluating the first argument passed to builtins.toString
 
-         at «none»:0: (source not available)
-
        … while calling the 'hashFile' builtin
 
-         at «none»:0: (source not available)
-
        error: opening file '/pwd/lang/this-file-is-definitely-not-there-7392097': No such file or directory

tests/lang/eval-fail-set-override.err.exp

@@ -1,6 +1,4 @@
 error:
        … while evaluating the `__overrides` attribute
 
-         at «none»:0: (source not available)
-
        error: value is an integer while a set was expected

tests/lang/eval-fail-substring.err.exp

@@ -8,5 +8,3 @@ error:
             2|
 
        error: negative start position in 'substring'
-
-       at «none»:0: (source not available)

tests/lang/eval-fail-to-path.err.exp

@@ -9,6 +9,4 @@ error:
 
        … while evaluating the first argument passed to builtins.toPath
 
-         at «none»:0: (source not available)
-
        error: string 'foo/bar' doesn't represent an absolute path