libexpr/builtins: Document derivationStrict

Co-authored-by: eldritch horrors <pennae@lix.systems>
Co-authored-by: Qyriad <qyriad@qyriad.me>

Change-Id: I4cf95683da54efe830514f3a6795057cadd223c6

lix/libexpr/builtins/derivationStrict.md

@@ -0,0 +1,22 @@
+---
+name: derivationStrict
+args: [args]
+renameInGlobalScope: false
+---
+
+Constructs a [store derivation](../glossary.md#gloss-store-derivation) from the attribute set `args`
+(c.f. [derivation](#builtins-derivation). Unlike `derivation` the produced store derivation is placed
+in the store *immediately* when this builtin is called, while `derivation` may defer placing store
+derivations in the store until it is proven that they are used.
+
+It then returns a new attrset with *only* the following attributes:
+
+- `drvPath` containing the path of the store derivation;
+- For each output of the derivation (`out`, `dev`, etc): an attribute named after that output containing the output path
+
+> **Note**
+>
+> In contrast to [`builtins.derivation`](#builtins-derivation), this computes
+> the derivation set in a fully *strict* manner, i.e. the values of the attributes
+> directly computed, whereas using `builtins.derivation` will produce an attrset
+> whose values will be evaluated when they are used at a later point.

lix/libexpr/meson.build

@@ -68,6 +68,7 @@ builtin_definitions = files(
   'builtins/concatMap.md',
   'builtins/concatStringsSep.md',
   'builtins/deepSeq.md',
+  'builtins/derivationStrict.md',
   'builtins/dirOf.md',
   'builtins/div.md',
   'builtins/elem.md',

lix/libexpr/primops.cc

@@ -1227,12 +1227,6 @@ drvName, Bindings * attrs, Value & v)
     v.mkAttrs(result);
 }
 
-static RegisterPrimOp primop_derivationStrict(PrimOp{{
-    .name = "derivationStrict",
-    .arity = 1,
-    .fun = prim_derivationStrict,
-}});
-
 /* Return a placeholder string for the specified output that will be
    substituted by the corresponding output path at build time. For
    example, 'placeholder "out"' returns the string