dirstate-v2: Add a new experimental `exp-dirstate-v2` repository requirement

dirstate-v2: Add a new experimental `exp-dirstate-v2` repository requirement

This requirement is added to `.hg/requires` when creating a new repository
if Rust extensions are enabled and the `format.exp-dirstate-v2` config is set.
Nothing yet changes based on this requirement, but its mere presence affects
some tests (for example if they print `.hg/requires`). The next two changesets
update tests’ expected outputs accordingly.

There is no CI so far that enables this configuration.

Differential Revision: https://phab.mercurial-scm.org/D10716

--- a/mercurial/configitems.py	Fri Apr 16 00:16:43 2021 +0200
+++ b/mercurial/configitems.py	Wed May 19 13:14:59 2021 +0200
@@ -1303,6 +1303,14 @@
     experimental=True,
 )
 coreconfigitem(
+    # Enable this dirstate format *when creating a new repository*.
+    # Which format to use for existing repos is controlled by .hg/requires
+    b'format',
+    b'exp-dirstate-v2',
+    default=False,
+    experimental=True,
+)
+coreconfigitem(
     b'format',
     b'dotencode',
     default=True,

--- a/mercurial/dirstate.py	Fri Apr 16 00:16:43 2021 +0200
+++ b/mercurial/dirstate.py	Wed May 19 13:14:59 2021 +0200
@@ -39,6 +39,8 @@
 parsers = policy.importmod('parsers')
 rustmod = policy.importrust('dirstate')
 
+SUPPORTS_DIRSTATE_V2 = rustmod is not None
+
 propertycache = util.propertycache
 filecache = scmutil.filecache
 _rangemask = 0x7FFFFFFF

--- a/mercurial/localrepo.py	Fri Apr 16 00:16:43 2021 +0200
+++ b/mercurial/localrepo.py	Wed May 19 13:14:59 2021 +0200
@@ -887,6 +887,9 @@
     # Start with all requirements supported by this file.
     supported = set(localrepository._basesupported)
 
+    if dirstate.SUPPORTS_DIRSTATE_V2:
+        supported.add(requirementsmod.DIRSTATE_V2_REQUIREMENT)
+
     # Execute ``featuresetupfuncs`` entries if they belong to an extension
     # relevant to this ui instance.
     modules = {m.__name__ for n, m in extensions.extensions(ui)}
@@ -3527,6 +3530,18 @@
         if ui.configbool(b'format', b'sparse-revlog'):
             requirements.add(requirementsmod.SPARSEREVLOG_REQUIREMENT)
 
+    # experimental config: format.exp-dirstate-v2
+    if ui.configbool(b'format', b'exp-dirstate-v2'):
+        if dirstate.SUPPORTS_DIRSTATE_V2:
+            requirements.add(requirementsmod.DIRSTATE_V2_REQUIREMENT)
+        else:
+            raise error.Abort(
+                _(
+                    b"dirstate v2 format requested by config "
+                    b"but not supported (requires Rust extensions)"
+                )
+            )
+
     # experimental config: format.exp-use-copies-side-data-changeset
     if ui.configbool(b'format', b'exp-use-copies-side-data-changeset'):
         requirements.add(requirementsmod.CHANGELOGV2_REQUIREMENT)

--- a/mercurial/requirements.py	Fri Apr 16 00:16:43 2021 +0200
+++ b/mercurial/requirements.py	Wed May 19 13:14:59 2021 +0200
@@ -12,6 +12,8 @@
 STORE_REQUIREMENT = b'store'
 FNCACHE_REQUIREMENT = b'fncache'
 
+DIRSTATE_V2_REQUIREMENT = b'exp-dirstate-v2'
+
 # When narrowing is finalized and no longer subject to format changes,
 # we should move this to just "narrow" or similar.
 NARROW_REQUIREMENT = b'narrowhg-experimental'
@@ -74,9 +76,12 @@
 #   repo. Hence both of them should be stored in working copy
 # * SHARESAFE_REQUIREMENT needs to be stored in working dir to mark that rest of
 #   the requirements are stored in store's requires
+# * DIRSTATE_V2_REQUIREMENT affects .hg/dirstate, of which there is one per
+#   working directory.
 WORKING_DIR_REQUIREMENTS = {
     SPARSE_REQUIREMENT,
     SHARED_REQUIREMENT,
     RELATIVE_SHARED_REQUIREMENT,
     SHARESAFE_REQUIREMENT,
+    DIRSTATE_V2_REQUIREMENT,
 }

--- a/mercurial/upgrade_utils/actions.py	Fri Apr 16 00:16:43 2021 +0200
+++ b/mercurial/upgrade_utils/actions.py	Wed May 19 13:14:59 2021 +0200
@@ -982,6 +982,7 @@
         requirements.SHARESAFE_REQUIREMENT,
         requirements.REVLOGV2_REQUIREMENT,
         requirements.CHANGELOGV2_REQUIREMENT,
+        requirements.DIRSTATE_V2_REQUIREMENT,
     }
     for name in compression.compengines:
         engine = compression.compengines[name]