rust: module policy with importrust

We introduce two rust+c module policies and a new
`policy.importrust()` that makes use of them.

This simple approach provides runtime switching of
implementations, which is crucial for the performance
measurements such as those Octobus does with ASV.

It can also be useful for bug analysis.

It also has the advantage of making conditionals in
Rust callers more uniform, in particular
abstracting over specifics like `demandimport`

At this point, the build stays unchanged, with the rust-cpython based
`rustext` module being built if HGWITHRUSTEXT=cpython.

More transparency for the callers, i.e., just using
`policy.importmod` would be a much longer term and riskier
effort for the following reasons:

1. It would require to define common module boundaries
   for the three or four cases (pure, c, rust+ext, cffi) and that
   is premature with the Rust extension currently under heavy
   development in areas that are outside the scope of the C extensions.
2. It would imply internal API changes that are not currently wished,
   as the case of ancestors demonstrates.
3. The lack of data or property-like attributes (tp_member
   and tp_getset) in current `rust-cpython` makes it impossible to
   achieve direct transparent replacement of pure Python classes by
   Rust extension code, meaning that the caller sometimes has to be able
   to make adjustments or provide additional wrapping.

mercurial/debugcommands.py

@@ -1278,16 +1278,28 b' def debuginstall(ui, **opts):'
     fm.write('hgmodules', _("checking installed modules (%s)...\n"),
              os.path.dirname(pycompat.fsencode(__file__)))
 
-    if policy.policy in ('c', 'allow'):
+    rustandc = policy.policy in ('rust+c', 'rust+c-allow')
+    rustext = rustandc  # for now, that's the only case
+    cext = policy.policy in ('c', 'allow') or rustandc
+    nopure = cext or rustext
+    if nopure:
         err = None
         try:
-            from .cext import (
-                base85,
-                bdiff,
-                mpatch,
-                osutil,
-            )
-            dir(bdiff), dir(mpatch), dir(base85), dir(osutil) # quiet pyflakes
+            if cext:
+                from .cext import (
+                    base85,
+                    bdiff,
+                    mpatch,
+                    osutil,
+                )
+                # quiet pyflakes
+                dir(bdiff), dir(mpatch), dir(base85), dir(osutil)
+            if rustext:
+                from .rustext import (
+                    ancestor,
+                    dirstate,
+                )
+                dir(ancestor), dir(dirstate) # quiet pyflakes
         except Exception as inst:
             err = stringutil.forcebytestr(inst)
             problems += 1

mercurial/policy.py

@@ -13,6 +13,9 b' import sys'
 # Rules for how modules can be loaded. Values are:
 #
 #    c - require C extensions
+#    rust+c - require Rust and C extensions
+#    rust+c-allow - allow Rust and C extensions with fallback to pure Python
+#                   for each
 #    allow - allow pure Python implementation when C loading fails
 #    cffi - required cffi versions (implemented within pure module)
 #    cffi-allow - allow pure Python implementation if cffi version is missing
@@ -29,6 +32,9 b" policy = b'allow'"
     b'cffi': (r'cffi', None),
     b'cffi-allow': (r'cffi', r'pure'),
     b'py': (None, r'pure'),
+    # For now, rust policies impact importrust only
+    b'rust+c': (r'cext', None),
+    b'rust+c-allow': (r'cext', r'pure'),
 }
 
 try:
@@ -107,3 +113,34 b' def importmod(modname):'
                 raise
     pn, mn = _modredirects.get((purepkg, modname), (purepkg, modname))
     return _importfrom(pn, mn)
+
+def _isrustpermissive():
+    """Assuming the policy is a Rust one, tell if it's permissive."""
+    return policy.endswith(b'-allow')
+
+def importrust(modname, member=None, default=None):
+    """Import Rust module according to policy and availability.
+
+    If policy isn't a Rust one, this returns `default`.
+
+    If either the module or its member is not available, this returns `default`
+    if policy is permissive and raises `ImportError` if not.
+    """
+    if not policy.startswith(b'rust'):
+        return default
+
+    try:
+        mod = _importfrom(r'rustext', modname)
+    except ImportError:
+        if _isrustpermissive():
+            return default
+        raise
+    if member is None:
+        return mod
+
+    try:
+        return getattr(mod, member)
+    except AttributeError:
+        if _isrustpermissive():
+            return default
+        raise ImportError(r"Cannot import name %s" % member)

setup.py

@@ -556,10 +556,17 b' class hgbuildpy(build_py):'
         if self.distribution.pure:
             modulepolicy = 'py'
         elif self.build_lib == '.':
-            # in-place build should run without rebuilding C extensions
-            modulepolicy = 'allow'
+            # in-place build should run without rebuilding C
+            # and Rust extensions
+            if hgrustext == 'cpython':
+                modulepolicy = 'rust+c-allow'
+            else:
+                modulepolicy = 'allow'
         else:
-            modulepolicy = 'c'
+            if hgrustext == 'cpython':
+                modulepolicy = 'rust+c'
+            else:
+                modulepolicy = 'c'
 
         content = b''.join([
             b'# this file is autogenerated by setup.py\n',