### Bug Description At the beginning of the **build** stage, a part's source files are copied into the `build` directory (see the [relevant source code](https://github.com/canonical/craft-parts/blob/139cbce69c1ef9e62c0ecf36b7567061299f3444/craft_parts/executor/part_handler.py#L463)). This happens if the build is not an update and if the plugin is not **out-of-source**. Out-of-source plugins are those which are known to not modify source files during build (e.g. `cmake`, `qmake`, `go-use`, `meson`). The issue here is that the `nil` plugin is supposed to be an empty plugin, where every instruction is explicitly set by the developer. Instead, if the `nil` plugin is used, the copy operation mentioned above is performed anyway as part of the **build** stage, and there is nothing a developer can do to prevent that. This is an issue if the project packs large files, like [inference snaps](https://documentation.ubuntu.com/inference-snaps/) do when packing larger models. This behavior is unexpected and can leave a developer wondering why their build takes more space than anticipated. The same could be said about the `dump` plugin, whose purpose is to move files from **src** to **install**, there's arguably no need for it to copy source files into **build**. ### Workaround A **workaround** for this issue is to use an **out-of-source** plugin (such as `cmake`) so the copy in **build** is skipped at the cost of confusion in the _snapcraft.yaml_ file, and unneeded build dependencies. Dropping the `source` keyword in the part description also prevents the copy from happening, but will also prevent the build system from noticing if the files on disk change to trigger an update. ### To Reproduce Setup an example project with a 1 GiB data file: ```bash mkdir -p ~/source/craft_size_playground cd ~/source/craft_size_playground mkdir data # Create large file (1 GiB) dd if=/dev/random of=data/my_file bs=1G count=1 mkdir snap touch snap/snapcraft.yaml ``` Use the following as your snapcraft.yaml: ```yaml name: craft-size-playground base: core24 version: '0.1' summary: Size test snap description: | Test snap. grade: devel confinement: strict compression: lzo parts: my-file: plugin: nil source: data override-build: | cp -rf --archive --link --no-dereference ${CRAFT_PART_SRC}/* ${CRAFT_PART_INSTALL} organize: my_file: (component/my-component) components: my-component: type: standard summary: Test component description: Just a file size test ``` Then, pack it with `snapcraft pack`. If doing multiple tests, make sure to `snapcraft clean` in between. Finally, check the build environment disk usage: ```bash sudo su cd /var/snap/lxd/common/lxd/storage-pools/default/containers/snapcraft_snapcraft-craft-size-playground-amd64-*/rootfs/root/parts du . -d 3 -h ``` The disk usage report will look like this: ``` 28K ./my-file/state 4.0K ./my-file/install 4.0K ./my-file/export 1.1G ./my-file/src 4.0K ./my-file/layer 1.1G ./my-file/build 4.0K ./my-file/run 2.1G ./my-file 2.1G . ``` The report above shows that the large file was copied two times: once in the **src** directory and another time in the **build** directory. The first copy is expected, because we are not using destructive build, and an hard-link to **src** from the project directory is not possible because it would be a cross-device link; but the second copy in **build** is unexpected and, in this example, unwanted. ### part yaml ```shell ``` ### Relevant log output ```shell Starting snapcraft, version 9.0.0.post19+dirty Logging execution to '/home/ubuntu/.local/state/snapcraft/log/snapcraft-20260605-161006.529105.log' Launching managed ubuntu 24.04 instance... Creating instance from base instance Starting instance Starting snapcraft, version 9.0.0.post19+dirty Logging execution to '/tmp/snapcraft.log' Initializing lifecycle Installing build-packages Installing build-snaps Pulling my-file Building my-file :: + cp -rf --archive --link --no-dereference /root/parts/my-file/src/my_file /root/parts/my-file/install/ Staging my-file Priming my-file Packing... Reading snap metadata... Running linters... Running linter: classic Running linter: gpu Running linter: library Running linter: metadata Creating snap package... Packed: craft-size-playground_0.1_amd64.snap, craft-size-playground+my-component.comp ```