From 98658ae9d25e5e715f9349fbbb79d0ba31bb810c Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Tue, 22 Mar 2022 23:31:48 +0100
Subject: Document fetchClosure

---
 doc/manual/src/release-notes/rl-next.md |  7 +++++++
 src/libexpr/primops/fetchClosure.cc     | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/doc/manual/src/release-notes/rl-next.md b/doc/manual/src/release-notes/rl-next.md
index 8fbc605e7..33eaa8a2c 100644
--- a/doc/manual/src/release-notes/rl-next.md
+++ b/doc/manual/src/release-notes/rl-next.md
@@ -4,3 +4,10 @@
 
 * `nix store make-content-addressable` has been renamed to `nix store
   make-content-addressed`.
+
+* New builtin function `builtins.fetchClosure` that copies a closure
+  from a binary cache at evaluation time and rewrites it to
+  content-addressed form (if it isn't already). Like
+  `builtins.storePath`, this allows importing pre-built store paths;
+  the difference is that it doesn't require the user to configure
+  binary caches and trusted public keys.
diff --git a/src/libexpr/primops/fetchClosure.cc b/src/libexpr/primops/fetchClosure.cc
index 47e2d2bf2..045e97dbd 100644
--- a/src/libexpr/primops/fetchClosure.cc
+++ b/src/libexpr/primops/fetchClosure.cc
@@ -110,6 +110,39 @@ static RegisterPrimOp primop_fetchClosure({
     .name = "__fetchClosure",
     .args = {"args"},
     .doc = R"(
+      Fetch a Nix store closure from a binary cache, rewriting it into
+      content-addressed form. For example,
+
+      ```nix
+      builtins.fetchClosure {
+        fromStore = "https://cache.nixos.org";
+        fromPath = /nix/store/r2jd6ygnmirm2g803mksqqjm4y39yi6i-git-2.33.1;
+        toPath = /nix/store/ldbhlwhh39wha58rm61bkiiwm6j7211j-git-2.33.1;
+      }
+      ```
+
+      fetches `/nix/store/r2jd...` from the specified binary cache,
+      and rewrites it into the content-addressed store path
+      `/nix/store/ldbh...`.
+
+      If `fromPath` is already content-addressed, or if you are
+      allowing impure evaluation (`--impure`), then `toPath` may be
+      omitted.
+
+      To find out the correct value for `toPath` given a `fromPath`,
+      you can use `nix store make-content-addressed`:
+
+      ```console
+      # nix store make-content-addressed /nix/store/r2jd6ygnmirm2g803mksqqjm4y39yi6i-git-2.33.1
+      rewrote '/nix/store/r2jd6ygnmirm2g803mksqqjm4y39yi6i-git-2.33.1' to '/nix/store/ldbhlwhh39wha58rm61bkiiwm6j7211j-git-2.33.1'
+      ```
+
+      This function is similar to `builtins.storePath` in that it
+      allows you to use a previously built store path in a Nix
+      expression. However, it is more reproducible because it requires
+      specifying a binary cache from which the path can be fetched.
+      Also, requiring a content-addressed final store path avoids the
+      need for users to configure binary cache public keys.
     )",
     .fun = prim_fetchClosure,
 });
-- 
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