Merge pull request #204626 from fricklerhandwerk/ifd-policy

document Import From Derivation (IFD) policy

2 files changed, 24 insertions, 6 deletions

diff --git a/doc/contributing/coding-conventions.chapter.md b/doc/contributing/coding-conventions.chapter.md
index d6f6cfe2358d0..275a3c7af5d2b 100644
--- a/doc/contributing/coding-conventions.chapter.md
+++ b/doc/contributing/coding-conventions.chapter.md
@@ -487,12 +487,12 @@ Preferred source hash type is sha256. There are several ways to get it.
    - `lib.fakeHash`
    - `lib.fakeSha256`
    - `lib.fakeSha512`
-   
+
    in the package expression, attempt build and extract correct hash from error messages.
 
    ::: {.warning}
    You must use one of these four fake hashes and not some arbitrarily-chosen hash.
-   
+
    See [](#sec-source-hashes-security).
    :::
 
@@ -670,3 +670,18 @@ stdenv.mkDerivation {
   ...
 }
 ```
+
+### Import From Derivation {#ssec-import-from-derivation}
+
+Import From Derivation (IFD) is disallowed in Nixpkgs for performance reasons:
+[Hydra] evaluates the entire package set, and sequential builds during evaluation would increase evaluation times to become impractical.
+
+[Hydra]: https://github.com/NixOS/hydra
+
+Import From Derivation can be worked around in some cases by committing generated intermediate files to version control and reading those instead.
+
+<!-- TODO: remove the following and link to Nix manual once https://github.com/NixOS/nix/pull/7332 is merged -->
+
+See also [NixOS Wiki: Import From Derivation].
+
+[NixOS Wiki: Import From Derivation]: https://nixos.wiki/wiki/Import_From_Derivation
diff --git a/doc/languages-frameworks/javascript.section.md b/doc/languages-frameworks/javascript.section.md
index 5a17c124a8e17..e4e6643dd66af 100644
--- a/doc/languages-frameworks/javascript.section.md
+++ b/doc/languages-frameworks/javascript.section.md
@@ -330,13 +330,16 @@ mkYarnPackage rec {
   - The `echo 9` steps comes from this answer: <https://stackoverflow.com/a/49139496>
   - Exporting the headers in `npm_config_nodedir` comes from this issue: <https://github.com/nodejs/node-gyp/issues/1191#issuecomment-301243919>
 
-## Outside of nixpkgs {#javascript-outside-nixpkgs}
+## Outside Nixpkgs {#javascript-outside-nixpkgs}
 
-There are some other options available that can't be used inside nixpkgs. Those other options are written in Nix. Importing them in nixpkgs will require moving the source code into nixpkgs. Using [Import From Derivation](https://nixos.wiki/wiki/Import_From_Derivation) is not allowed in Hydra at present. If you are packaging something outside nixpkgs, those can be considered
+There are some other tools available, which are written in the Nix language.
+These that can't be used inside Nixpkgs because they require [Import From Derivation](#ssec-import-from-derivation), which is not allowed in Nixpkgs.
+
+If you are packaging something outside Nixpkgs, consider the following:
 
 ### npmlock2nix {#javascript-npmlock2nix}
 
-[npmlock2nix](https://github.com/nix-community/npmlock2nix) aims at building node_modules without code generation. It hasn't reached v1 yet, the API might be subject to change.
+[npmlock2nix](https://github.com/nix-community/npmlock2nix) aims at building `node_modules` without code generation. It hasn't reached v1 yet, the API might be subject to change.
 
 #### Pitfalls {#javascript-npmlock2nix-pitfalls}
 
@@ -344,7 +347,7 @@ There are some [problems with npm v7](https://github.com/tweag/npmlock2nix/issue
 
 ### nix-npm-buildpackage {#javascript-nix-npm-buildpackage}
 
-[nix-npm-buildpackage](https://github.com/serokell/nix-npm-buildpackage) aims at building node_modules without code generation. It hasn't reached v1 yet, the API might change. It supports both package-lock.json and yarn.lock.
+[nix-npm-buildpackage](https://github.com/serokell/nix-npm-buildpackage) aims at building `node_modules` without code generation. It hasn't reached v1 yet, the API might change. It supports both `package-lock.json` and yarn.lock.
 
 #### Pitfalls {#javascript-nix-npm-buildpackage-pitfalls}