libexpr/builtins: Document fetchMercurial

Change-Id: Ic849bd6dd10374b717d3fc257da25208f2c45525

lix/libexpr/builtins/fetchMercurial.md

@@ -0,0 +1,48 @@
+---
+name: fetchMercurial
+args: [args]
+renameInGlobalScope: false
+---
+
+Fetch a Mercurial repository. *args* can be a URL, in which case the default
+branch of the repo at that URL is fetched. Otherwise, it can be an
+attribute with the following attributes (all except `url` optional):
+
+- `url`
+
+  The URL of the repo.
+
+- `name` (default: `"source"`)
+
+  The name of the directory the repo should be exported to in the store.
+
+- `rev`
+
+  The revision to fetch.
+  This is typically a commit hash.
+
+  > **Note**
+  >
+  > Currently, `rev` can either contain a revision or a branch/tag name.
+
+The return value is an attrset containing the following keys:
+
+- `outPath` (`string`)
+
+  Resulting store path of the fetch process.
+
+- `branch` (`string`)
+
+  The branch of the fetch repository.
+
+- `rev` (`string`)
+
+  The revision that was fetched.
+
+- `revCount` (`int`)
+
+  The number of revsets for this branch.
+
+- `shortRev` (`string`)
+
+  The first *12* characters of `rev`.

lix/libexpr/extra-primops.hh

@@ -11,6 +11,7 @@ struct Value;
 void prim_addDrvOutputDependencies(EvalState & state, Value * * args, Value & v);
 void prim_fetchTree(EvalState & state, Value * * args, Value & v);
 void prim_fetchGit(EvalState & state, Value * * args, Value & v);
+void prim_fetchMercurial(EvalState & state, Value ** args, Value & v);
 void prim_fetchTarball(EvalState & state, Value * * args, Value & v);
 void prim_fetchurl(EvalState & state, Value * * args, Value & v);
 void prim_fromTOML(EvalState & state, Value * * args, Value & v);

lix/libexpr/meson.build

@@ -77,6 +77,7 @@ builtin_definitions = files(
   'builtins/elem.md',
   'builtins/elemAt.md',
   'builtins/fetchGit.md',
+  'builtins/fetchMercurial.md',
   'builtins/fetchTarball.md',
   'builtins/fetchTree.md',
   'builtins/fetchurl.md',

lix/libexpr/primops/fetchMercurial.cc

@@ -5,7 +5,7 @@
 
 namespace nix {
 
-static void prim_fetchMercurial(EvalState & state, Value * * args, Value & v)
+void prim_fetchMercurial(EvalState & state, Value ** args, Value & v)
 {
     std::string url;
     std::optional<Hash> rev;
@@ -102,11 +102,4 @@ static void prim_fetchMercurial(EvalState & state, Value * * args, Value & v)
 
     state.ctx.paths.allowPath(tree.storePath);
 }
-
-static RegisterPrimOp r_fetchMercurial({
-    .name = "fetchMercurial",
-    .arity = 1,
-    .fun = prim_fetchMercurial
-});
-
 }