diff mercurial/debugcommands.py @ 42451:810f66b468cd

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.

--- a/mercurial/debugcommands.py	Thu Jun 13 23:28:31 2019 +0300
+++ b/mercurial/debugcommands.py	Wed May 29 13:27:56 2019 +0200
@@ -1278,16 +1278,28 @@
     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