Merge master into staging-next

3 files changed, 53 insertions, 0 deletions

diff --git a/doc/default.nix b/doc/default.nix
index 58b945c692fc..18e12c1a8aca 100644
--- a/doc/default.nix
+++ b/doc/default.nix
@@ -19,6 +19,7 @@ let
       { name = "options"; description = "NixOS / nixpkgs option handling"; }
       { name = "path"; description = "path functions"; }
       { name = "filesystem"; description = "filesystem functions"; }
+      { name = "fileset"; description = "file set functions"; }
       { name = "sources"; description = "source filtering functions"; }
       { name = "cli"; description = "command-line serialization functions"; }
       { name = "gvariant"; description = "GVariant formatted string serialization functions"; }
diff --git a/doc/functions.md b/doc/functions.md
index 09033c9e3c19..551ba522a904 100644
--- a/doc/functions.md
+++ b/doc/functions.md
@@ -8,4 +8,5 @@ functions/generators.section.md
 functions/debug.section.md
 functions/prefer-remote-fetch.section.md
 functions/nix-gitignore.section.md
+functions/fileset.section.md
 ```
diff --git a/doc/functions/fileset.section.md b/doc/functions/fileset.section.md
new file mode 100644
index 000000000000..b24ebe26cc3b
--- /dev/null
+++ b/doc/functions/fileset.section.md
@@ -0,0 +1,51 @@
+<!-- TODO: Render this document in front of function documentation in case https://github.com/nix-community/nixdoc/issues/19 is ever supported -->
+
+# File sets {#sec-fileset}
+
+The [`lib.fileset`](#sec-functions-library-fileset) library allows you to work with _file sets_.
+A file set is a mathematical set of local files that can be added to the Nix store for use in Nix derivations.
+File sets are easy and safe to use, providing obvious and composable semantics with good error messages to prevent mistakes.
+
+These sections apply to the entire library.
+See the [function reference](#sec-functions-library-fileset) for function-specific documentation.
+
+The file set library is currently very limited but is being expanded to include more functions over time.
+
+## Implicit coercion from paths to file sets {#sec-fileset-path-coercion}
+
+All functions accepting file sets as arguments can also accept [paths](https://nixos.org/manual/nix/stable/language/values.html#type-path) as arguments.
+Such path arguments are implicitly coerced to file sets containing all files under that path:
+- A path to a file turns into a file set containing that single file.
+- A path to a directory turns into a file set containing all files _recursively_ in that directory.
+
+If the path points to a non-existent location, an error is thrown.
+
+::: {.note}
+Just like in Git, file sets cannot represent empty directories.
+Because of this, a path to a directory that contains no files (recursively) will turn into a file set containing no files.
+:::
+
+:::{.note}
+File set coercion does _not_ add any of the files under the coerced paths to the store.
+Only the [`toSource`](#function-library-lib.fileset.toSource) function adds files to the Nix store, and only those files contained in the `fileset` argument.
+This is in contrast to using [paths in string interpolation](https://nixos.org/manual/nix/stable/language/values.html#type-path), which does add the entire referenced path to the store.
+:::
+
+### Example {#sec-fileset-path-coercion-example}
+
+Assume we are in a local directory with a file hierarchy like this:
+```
+├─ a/
+│  ├─ x (file)
+│  └─ b/
+│     └─ y (file)
+└─ c/
+   └─ d/
+```
+
+Here's a listing of which files get included when different path expressions get coerced to file sets:
+- `./.` as a file set contains both `a/x` and `a/b/y` (`c/` does not contain any files and is therefore omitted).
+- `./a` as a file set contains both `a/x` and `a/b/y`.
+- `./a/x` as a file set contains only `a/x`.
+- `./a/b` as a file set contains only `a/b/y`.
+- `./c` as a file set is empty, since neither `c` nor `c/d` contain any files.