lib.fileset: Mention trace functions in the file set evaluation error

lib/fileset/README.md

@@ -207,6 +207,5 @@ Here's a list of places in the library that need to be updated in the future:
 - > The file set library is currently somewhat limited but is being expanded to include more functions over time.
 
   in [the manual](../../doc/functions/fileset.section.md)
-- Once a tracing function exists, `__noEval` in [internal.nix](./internal.nix) should mention it
 - If/Once a function to convert `lib.sources` values into file sets exists, the `_coerce` and `toSource` functions should be updated to mention that function in the error when such a value is passed
 - If/Once a function exists that can optionally include a path depending on whether it exists, the error message for the path not existing in `_coerce` should mention the new function

lib/fileset/internal.nix

@@ -106,7 +106,9 @@ rec {
   ];
 
   _noEvalMessage = ''
-    lib.fileset: Directly evaluating a file set is not supported. Use `lib.fileset.toSource` to turn it into a usable source instead.'';
+    lib.fileset: Directly evaluating a file set is not supported.
+      To turn it into a usable source, use `lib.fileset.toSource`.
+      To pretty-print the contents, use `lib.fileset.trace` or `lib.fileset.traceVal`.'';
 
   # The empty file set without a base path
   _emptyWithoutBase = {

lib/fileset/tests.sh

@@ -327,8 +327,12 @@ expectFailure 'toSource { root = ./.; fileset = "/some/path"; }' 'lib.fileset.to
 expectFailure 'toSource { root = ./.; fileset = ./a; }' 'lib.fileset.toSource: `fileset` \('"$work"'/a\) does not exist.'
 
 # File sets cannot be evaluated directly
-expectFailure 'union ./. ./.' 'lib.fileset: Directly evaluating a file set is not supported. Use `lib.fileset.toSource` to turn it into a usable source instead.'
-expectFailure '_emptyWithoutBase' 'lib.fileset: Directly evaluating a file set is not supported. Use `lib.fileset.toSource` to turn it into a usable source instead.'
+expectFailure 'union ./. ./.' 'lib.fileset: Directly evaluating a file set is not supported.
+\s*To turn it into a usable source, use `lib.fileset.toSource`.
+\s*To pretty-print the contents, use `lib.fileset.trace` or `lib.fileset.traceVal`.'
+expectFailure '_emptyWithoutBase' 'lib.fileset: Directly evaluating a file set is not supported.
+\s*To turn it into a usable source, use `lib.fileset.toSource`.
+\s*To pretty-print the contents, use `lib.fileset.trace` or `lib.fileset.traceVal`.'
 
 # Past versions of the internal representation are supported
 expectEqual '_coerce "<tests>: value" { _type = "fileset"; _internalVersion = 0; _internalBase = ./.; }' \