Consider this snapcraft parts recipe:
ccache:
plugin: nil
build-packages:
- ccache
override-build: 'true'
override-stage: 'true'
override-prime: 'true'
And according to the description of snapcraft help nil:
The nil plugin is useful for parts with no source.
However after running snapcraft pull ccache, non-snapcraft-generated files in the source tree are pulled into the part’s sourcedir:
parts/ccache/src
├── guvcview-brlin_source.tar.bz2
├── guvcview.strace.log
├── icons
│ ├── guvcview256.png
│ └── guvcview.png
├── LICENSE
├── nohup.out
├── README.md
└── screenshots
└── view-image-control.png
2 directories, 8 files
IMO the behavior is unexpected and should be a snapcraft bug.
Workaround
Override the entire pull step using scriptlets.