doc: add chapter Fixed-point arguments of build helpers

Adds "Fixed-point arguments of build helpers" chapter to the "Build helpers" section.

Co-authored-by: nicoo <nicoo@mur.at>
Co-authored-by: Silvan Mosberger <github@infinisil.com>
Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
Co-authored-by: Lin Jian <me@linj.tech>
Co-authored-by: Philip Taron <philip.taron@gmail.com>
(cherry picked from commit bbdf8601bc)

doc/build-helpers.md

@@ -17,6 +17,7 @@ There is no uniform interface for build helpers.
 [Language- or framework-specific build helpers](#chap-language-support) usually follow the style of `stdenv.mkDerivation`, which accepts an attribute set or a fixed-point function taking an attribute set.
 
 ```{=include=} chapters
+build-helpers/fixed-point-arguments.chapter.md
 build-helpers/fetchers.chapter.md
 build-helpers/trivial-build-helpers.chapter.md
 build-helpers/testers.chapter.md

doc/build-helpers/fixed-point-arguments.chapter.md

@@ -0,0 +1,74 @@
+# Fixed-point arguments of build helpers {#chap-build-helpers-finalAttrs}
+
+As mentioned in the beginning of this part, `stdenv.mkDerivation` could alternatively accept a fixed-point function. The input of such function, typically named `finalAttrs`, is expected to be the final state of the attribute set.
+A build helper like this is said to accept **fixed-point arguments**.
+
+Build helpers don't always support fixed-point arguments yet, as support in [`stdenv.mkDerivation`](#mkderivation-recursive-attributes) was first included in Nixpkgs 22.05.
+
+## Defining a build helper with `lib.extendMkDerivation` {#sec-build-helper-extendMkDerivation}
+
+Developers can use the Nixpkgs library function [`lib.customisation.extendMkDerivation`](#function-library-lib.customisation.extendMkDerivation) to define a build helper supporting fixed-point arguments from an existing one with such support, with an attribute overlay similar to the one taken by [`<pkg>.overrideAttrs`](#sec-pkg-overrideAttrs).
+
+Beside overriding, `lib.extendMkDerivation` also supports `excludeDrvArgNames` to optionally exclude some arguments in the input fixed-point argumnts from passing down the base build helper (specified as `constructDrv`).
+
+:::{.example #ex-build-helpers-extendMkDerivation}
+
+# Example definition of `mkLocalDerivation` extended from `stdenv.mkDerivation` with `lib.extendMkDerivation`
+
+We want to define a build helper named `mkLocalDerivation` that builds locally without using substitutes by default.
+
+Instead of taking a plain attribute set,
+
+```nix
+{
+  preferLocalBuild ? true,
+  allowSubstitute ? false,
+  specialArg ? (_: false),
+  ...
+}@args:
+
+stdenv.mkDerivation (
+  removeAttrs [
+    # Don't pass specialArg into mkDerivation.
+    "specialArg"
+  ] args
+  // {
+    # Arguments to pass
+    inherit preferLocalBuild allowSubstitute;
+    # Some expressions involving specialArg
+    greeting = if specialArg "hi" then "hi" else "hello";
+  }
+)
+```
+
+we could define with `lib.extendMkDerivation` an attribute overlay to make the result build helper also accepts the the attribute set's fixed point passing to the underlying `stdenv.mkDerivation`, named `finalAttrs` here:
+
+```nix
+lib.extendMkDerivation {
+  constructDrv = stdenv.mkDerivation;
+  excludeDrvArgNames = [
+    # Don't pass specialArg into mkDerivation.
+    "specialArg"
+  ];
+  extendDrvArgs =
+    finalAttrs:
+    {
+      preferLocalBuild ? true,
+      allowSubstitute ? false,
+      specialArg ? (_: false),
+      ...
+    }@args:
+    {
+      # Arguments to pass
+      inherit
+        preferLocalBuild
+        allowSubstitute
+        ;
+      # Some expressions involving specialArg
+      greeting = if specialArg "hi" then "hi" else "hello";
+    };
+}
+```
+:::
+
+If one needs to apply extra changes to the result derivation, pass the derivation transformation function to `lib.extendMkDerivation` as `lib.customisation.extendMkDerivation { transformDrv = drv: ...; }`.