debuginstall: add a line about re2 availability

Using re2 engine can massively speed up regexp. We make it simpler to check if
it is available in a given install.

mercurial/debugcommands.py

@@ -1173,6 +1173,11 b' def debuginstall(ui, **opts):'
              fm.formatlist([e.name() for e in wirecompengines
                             if e.wireprotosupport()],
                            name='compengine', fmt='%s', sep=', '))
+    re2 = 'missing'
+    if util._re2:
+        re2 = 'available'
+    fm.plain(_('checking "re2" regexp engine (%s)\n') % re2)
+    fm.data(re2=bool(util._re2))
 
     # templates
     p = templater.templatepaths()

tests/test-install.t

@@ -14,6 +14,7 b' hg debuginstall'
   checking registered compression engines (*zlib*) (glob)
   checking available compression engines (*zlib*) (glob)
   checking available compression engines for wire protocol (*zlib*) (glob)
+  checking "re2" regexp engine \((available|missing)\) (re)
   checking templates (*mercurial?templates)... (glob)
   checking default template (*mercurial?templates?map-cmdline.default) (glob)
   checking commit editor... (* -c "import sys; sys.exit(0)") (glob)
@@ -44,6 +45,7 b' hg debuginstall JSON'
     "pythonlib": "*", (glob)
     "pythonsecurity": [*], (glob)
     "pythonver": "*.*.*", (glob)
+    "re2": (true|false), (re)
     "templatedirs": "*mercurial?templates", (glob)
     "username": "test",
     "usernameerror": null,
@@ -67,6 +69,7 b' hg debuginstall with no username'
   checking registered compression engines (*zlib*) (glob)
   checking available compression engines (*zlib*) (glob)
   checking available compression engines for wire protocol (*zlib*) (glob)
+  checking "re2" regexp engine \((available|missing)\) (re)
   checking templates (*mercurial?templates)... (glob)
   checking default template (*mercurial?templates?map-cmdline.default) (glob)
   checking commit editor... (* -c "import sys; sys.exit(0)") (glob)
@@ -110,6 +113,7 b' path variables are expanded (~ is the sa'
   checking registered compression engines (*zlib*) (glob)
   checking available compression engines (*zlib*) (glob)
   checking available compression engines for wire protocol (*zlib*) (glob)
+  checking "re2" regexp engine \((available|missing)\) (re)
   checking templates (*mercurial?templates)... (glob)
   checking default template (*mercurial?templates?map-cmdline.default) (glob)
   checking commit editor... (* -c "import sys; sys.exit(0)") (glob)
@@ -217,6 +221,7 b" since it's bin on most platforms but Scr"
   checking registered compression engines (*) (glob)
   checking available compression engines (*) (glob)
   checking available compression engines for wire protocol (*) (glob)
+  checking "re2" regexp engine \((available|missing)\) (re)
   checking templates ($TESTTMP/installenv/*/site-packages/mercurial/templates)... (glob)
   checking default template ($TESTTMP/installenv/*/site-packages/mercurial/templates/map-cmdline.default) (glob)
   checking commit editor... (*) (glob)