Improve error message for including missing library symbols (#17856)

Fixes #17839

src/runtime_debug.js

@@ -45,6 +45,14 @@ function missingLibrarySymbol(sym) {
         // Can't `abort()` here because it would break code that does runtime
         // checks.  e.g. `if (typeof SDL === 'undefined')`.
         var msg = '`' + sym + '` is a library symbol and not included by default; add it to your library.js __deps or to DEFAULT_LIBRARY_FUNCS_TO_INCLUDE on the command line';
+        // DEFAULT_LIBRARY_FUNCS_TO_INCLUDE requires the name as it appears in
+        // library.js, which means $name for a JS name with no prefix, or name
+        // for a JS name like _name.
+        var librarySymbol = sym;
+        if (!librarySymbol.startsWith('_')) {
+          librarySymbol = '$' + sym;
+        }
+        msg += " (e.g. -sDEFAULT_LIBRARY_FUNCS_TO_INCLUDE=" + librarySymbol + ")";
         if (isExportedByForceFilesystem(sym)) {
           msg += '. Alternatively, forcing filesystem support (-sFORCE_FILESYSTEM) can export this for you';
         }

test/test_core.py

@@ -7145,7 +7145,7 @@ def test(expected, args=None, assert_returncode=0):
 
     # When assertions are enabled direct and indirect usage both abort with a useful error message.
     not_exported = "Aborted('ALLOC_STACK' was not exported. add it to EXPORTED_RUNTIME_METHODS (see the FAQ))"
-    not_included = "`ALLOC_STACK` is a library symbol and not included by default; add it to your library.js __deps or to DEFAULT_LIBRARY_FUNCS_TO_INCLUDE on the command line"
+    not_included = "`ALLOC_STACK` is a library symbol and not included by default; add it to your library.js __deps or to DEFAULT_LIBRARY_FUNCS_TO_INCLUDE on the command line (e.g. -sDEFAULT_LIBRARY_FUNCS_TO_INCLUDE=$ALLOC_STACK)"
     self.set_setting('ASSERTIONS')
     test(not_exported, assert_returncode=NON_ZERO)
     test(not_included, args=['-DDIRECT'])