debuginstall: print if Rust extensions are installed

This should make it easier to use the Rust extensions.

Another patch on will be exposing a version and more useful information, but
it can only be done on top of this very patch, since it is targeting the stable
branch and the API has already changed on the default branch.

Differential Revision: https://phab.mercurial-scm.org/D8248

mercurial/debugcommands.py

@@ -1508,6 +1508,13 b' def debuginstall(ui, **opts):'
         pythonlib or _(b"unknown"),
     )
 
+    try:
+        from . import rustext
+
+        rustext.__doc__  # trigger lazy import
+    except ImportError:
+        rustext = None
+
     security = set(sslutil.supportedprotocols)
     if sslutil.hassni:
         security.add(b'sni')
@@ -1535,6 +1542,13 b' def debuginstall(ui, **opts):'
             )
         )
 
+    fm.plain(
+        _(
+            b"checking Rust extensions (%s)\n"
+            % (b'missing' if rustext is None else b'installed')
+        ),
+    )
+
     # TODO print CA cert info
 
     # hg version

tests/test-install.t

@@ -9,6 +9,7 b' hg debuginstall'
   checking Python security support (*) (glob)
     TLS 1.2 not supported by Python install; network connections lack modern security (?)
     SNI not supported by Python install; may have connectivity issues with some servers (?)
+  checking Rust extensions \((installed|missing)\) (re)
   checking Mercurial version (*) (glob)
   checking Mercurial custom build (*) (glob)
   checking module policy (*) (glob)
@@ -67,6 +68,7 b' hg debuginstall with no username'
   checking Python security support (*) (glob)
     TLS 1.2 not supported by Python install; network connections lack modern security (?)
     SNI not supported by Python install; may have connectivity issues with some servers (?)
+  checking Rust extensions \((installed|missing)\) (re)
   checking Mercurial version (*) (glob)
   checking Mercurial custom build (*) (glob)
   checking module policy (*) (glob)
@@ -113,6 +115,7 b' path variables are expanded (~ is the sa'
   checking Python security support (*) (glob)
     TLS 1.2 not supported by Python install; network connections lack modern security (?)
     SNI not supported by Python install; may have connectivity issues with some servers (?)
+  checking Rust extensions \((installed|missing)\) (re)
   checking Mercurial version (*) (glob)
   checking Mercurial custom build (*) (glob)
   checking module policy (*) (glob)
@@ -139,6 +142,7 b' not found (this is intentionally using b'
   checking Python security support (*) (glob)
     TLS 1.2 not supported by Python install; network connections lack modern security (?)
     SNI not supported by Python install; may have connectivity issues with some servers (?)
+  checking Rust extensions \((installed|missing)\) (re)
   checking Mercurial version (*) (glob)
   checking Mercurial custom build (*) (glob)
   checking module policy (*) (glob)
@@ -194,6 +198,7 b" since it's bin on most platforms but Scr"
   checking Python version (3.*) (glob)
   checking Python lib (*)... (glob)
   checking Python security support (*) (glob)
+  checking Rust extensions \((installed|missing)\) (re)
   checking Mercurial version (*) (glob)
   checking Mercurial custom build (*) (glob)
   checking module policy (*) (glob)
@@ -233,6 +238,7 b" since it's bin on most platforms but Scr"
   checking Python security support (*) (glob)
     TLS 1.2 not supported by Python install; network connections lack modern security (?)
     SNI not supported by Python install; may have connectivity issues with some servers (?)
+  checking Rust extensions \((installed|missing)\) (re)
   checking Mercurial version (*) (glob)
   checking Mercurial custom build (*) (glob)
   checking module policy (*) (glob)