python3.tests.nix-pythonprefix-mypy: Avoid unnecessary rebuilds

[infinisil]

Description of changes

E.g. when the Nix is changed, see #301014

Things done

• Built python3.tests.nix-pythonprefix-mypy on x86_64-linux

---

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[philiptaron]

$ diff -r -u4 /nix/store/gq6b77q97cpsbxs8rx070wfl533s4lpw-typeddep /nix/store/rsbc8gc512fmsamc8n383x97ixf0sics-source
Only in /nix/store/gq6b77q97cpsbxs8rx070wfl533s4lpw-typeddep: default.nix

I had to build this specific derivation with this command, btw:

nix-build --expr 'let p = import ./. { }; in p.python3.pkgs.callPackage pkgs/development/interpreters/python/tests/test_nix_pythonprefix/typeddep/default.nix { }'

[infinisil]

I mentioned the attribute that builds this in the description :P

[philiptaron]

I mentioned the attribute that builds this in the description :P

The derivation mentioned definitely causes this package to be built, but it doesn't directly show it, so my workflow of nix derivation show to find the inputSrc and diff it vs. what's on master right now didn't work.

[infinisil]

Here's a hack to print excluded files by default :P:

diff --git a/lib/fileset/default.nix b/lib/fileset/default.nix
index bb327226731e..32bfcf21ecf6 100644
--- a/lib/fileset/default.nix
+++ b/lib/fileset/default.nix
@@ -123,6 +123,7 @@ let
     isPath
     pathExists
     seq
+    trace
     typeOf
     nixVersion
     ;
@@ -473,6 +474,8 @@ in {
             - Set `fileset` to a file set that cannot contain files outside the `root` (${toString root}). This could change the files included in the result.''
     else
       seq sourceFilter
+      trace "lib.fileset.toSource: These files are excluded:"
+      seq (_printFileset (_difference (_singleton root) fileset))
       cleanSourceWith {
         name = "source";
         src = root;