lib.fileset.toSource: Mention fromSource in errors

lib/fileset/README.md

@@ -241,5 +241,4 @@ 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)
-- 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/default.nix

@@ -153,7 +153,12 @@ If a directory does not recursively contain any file, it is omitted from the sto
       sourceFilter = _toSourceFilter fileset;
     in
     if ! isPath root then
-      if isStringLike root then
+      if root ? _isLibCleanSourceWith then
+        throw ''
+          lib.fileset.toSource: `root` is a `lib.sources`-based value, but it should be a path instead.
+              To use a `lib.sources`-based value, convert it to a file set using `lib.fileset.fromSource` and pass it as `fileset`.
+              Note that this only works for sources created from paths.''
+      else if isStringLike root then
         throw ''
           lib.fileset.toSource: `root` ("${toString root}") is a string-like value, but it should be a path instead.
               Paths in strings are not supported by `lib.fileset`, use `lib.sources` or derivations instead.''

lib/fileset/internal.nix

@@ -170,7 +170,12 @@ rec {
       else
         value
     else if ! isPath value then
-      if isStringLike value then
+      if value ? _isLibCleanSourceWith then
+        throw ''
+          ${context} is a `lib.sources`-based value, but it should be a file set or a path instead.
+              To convert a `lib.sources`-based value to a file set you can use `lib.fileset.fromSource`.
+              Note that this only works for sources created from paths.''
+      else if isStringLike value then
         throw ''
           ${context} ("${toString value}") is a string-like value, but it should be a file set or a path instead.
               Paths represented as strings are not supported by `lib.fileset`, use `lib.sources` or derivations instead.''

lib/fileset/tests.sh

@@ -339,6 +339,10 @@ checkFileset() {
 expectFailure 'toSource { root = "/nix/store/foobar"; fileset = ./.; }' 'lib.fileset.toSource: `root` \("/nix/store/foobar"\) is a string-like value, but it should be a path instead.
 \s*Paths in strings are not supported by `lib.fileset`, use `lib.sources` or derivations instead.'
 
+expectFailure 'toSource { root = cleanSourceWith { src = ./.; }; fileset = ./.; }' 'lib.fileset.toSource: `root` is a `lib.sources`-based value, but it should be a path instead.
+\s*To use a `lib.sources`-based value, convert it to a file set using `lib.fileset.fromSource` and pass it as `fileset`.
+\s*Note that this only works for sources created from paths.'
+
 # Only paths are accepted as `root`
 expectFailure 'toSource { root = 10; fileset = ./.; }' 'lib.fileset.toSource: `root` is of type int, but it should be a path instead.'
 
@@ -376,6 +380,9 @@ rm -rf *
 expectFailure 'toSource { root = ./.; fileset = 10; }' 'lib.fileset.toSource: `fileset` is of type int, but it should be a file set or a path instead.'
 expectFailure 'toSource { root = ./.; fileset = "/some/path"; }' 'lib.fileset.toSource: `fileset` \("/some/path"\) is a string-like value, but it should be a file set or a path instead.
 \s*Paths represented as strings are not supported by `lib.fileset`, use `lib.sources` or derivations instead.'
+expectFailure 'toSource { root = ./.; fileset = cleanSourceWith { src = ./.; }; }' 'lib.fileset.toSource: `fileset` is a `lib.sources`-based value, but it should be a file set or a path instead.
+\s*To convert a `lib.sources`-based value to a file set you can use `lib.fileset.fromSource`.
+\s*Note that this only works for sources created from paths.'
 
 # Path coercion errors for non-existent paths
 expectFailure 'toSource { root = ./.; fileset = ./a; }' 'lib.fileset.toSource: `fileset` \('"$work"'/a\) does not exist.'