Patchwork [V2] revset: introduce new operator "##" to concatenate strings/symbols at runtime

diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -2840,6 +2840,10 @@ 
         newtree = revset.findaliases(ui, tree)
         if newtree != tree:
             ui.note(revset.prettyformat(newtree), "\n")
+        tree = newtree
+        newtree = revset.foldconcat(tree)
+        if newtree != tree:
+            ui.note(revset.prettyformat(newtree), "\n")
         if opts["optimize"]:
             weight, optimizedtree = revset.optimize(newtree, True)
             ui.note("* optimized:\n", revset.prettyformat(optimizedtree), "\n")
diff --git a/mercurial/help/revsets.txt b/mercurial/help/revsets.txt
--- a/mercurial/help/revsets.txt
+++ b/mercurial/help/revsets.txt
@@ -81,6 +81,19 @@ 
 defines three aliases, ``h``, ``d``, and ``rs``. ``rs(0:tip, author)`` is
 exactly equivalent to ``reverse(sort(0:tip, author))``.
 
+An infix operator ``##`` can concatenate strings and identifiers into
+one string. For example::
+
+  [revsetalias]
+  issue($1) = grep(r'\bissue[ :]?' ## $1 ## r'\b|\bbug\(' ## $1 ## r'\)')
+
+``issue(1234)`` is equivalent to ``grep(r'\bissue[ :]?1234\b|\bbug\(1234\)')``
+in this case. This matches against all of "issue 1234", "issue:1234",
+"issue1234" and "bug(1234)".
+
+All other prefix, infix and postfix operators have lower priority than
+``##``. For example, ``$1 ## $2~2`` is equivalent to ``($1 ## $2)~2``.
+
 Command line equivalents for :hg:`log`::
 
   -f    ->  ::.
diff --git a/mercurial/revset.py b/mercurial/revset.py
--- a/mercurial/revset.py
+++ b/mercurial/revset.py
@@ -102,7 +102,8 @@ 
     return baseset(sorted(reachable))
 
 elements = {
-    "(": (20, ("group", 1, ")"), ("func", 1, ")")),
+    "(": (21, ("group", 1, ")"), ("func", 1, ")")),
+    "##": (20, None, ("_concat", 20)),
     "~": (18, None, ("ancestor", 18)),
     "^": (18, None, ("parent", 18), ("parentpost", 18)),
     "-": (5, ("negate", 19), ("minus", 5)),
@@ -148,6 +149,9 @@ 
         elif c == '.' and program[pos:pos + 2] == '..': # look ahead carefully
             yield ('..', None, pos)
             pos += 1 # skip ahead
+        elif c == '#' and program[pos:pos + 2] == '##': # look ahead carefully
+            yield ('##', None, pos)
+            pos += 1 # skip ahead
         elif c in "():,-|&+!~^": # handle simple operators
             yield (c, None, pos)
         elif (c in '"\'' or c == 'r' and
@@ -2155,6 +2159,27 @@ 
                 alias.warned = True
     return tree
 
+def foldconcat(tree):
+    """Fold elements to be concatenated by `##`
+    """
+    if not isinstance(tree, tuple) or tree[0] in ('string', 'symbol'):
+        return tree
+    if tree[0] == '_concat':
+        pending = [tree]
+        l = []
+        while pending:
+            e = pending.pop()
+            if e[0] == '_concat':
+                pending.extend(reversed(e[1:]))
+            elif e[0] in ('string', 'symbol'):
+                l.append(e[1])
+            else:
+                msg = _("\"##\" can't concatenate \"%s\" element") % (e[0])
+                raise error.ParseError(msg)
+        return ('string', ''.join(l))
+    else:
+        return tuple(foldconcat(t) for t in tree)
+
 def parse(spec, lookup=None):
     p = parser.parser(tokenize, elements)
     return p.parse(spec, lookup=lookup)
@@ -2170,6 +2195,7 @@ 
         raise error.ParseError(_("invalid token"), pos)
     if ui:
         tree = findaliases(ui, tree, showwarning=ui.warn)
+    tree = foldconcat(tree)
     weight, tree = optimize(tree, True)
     def mfunc(repo, subset):
         if util.safehasattr(subset, 'isascending'):
diff --git a/tests/test-revset.t b/tests/test-revset.t
--- a/tests/test-revset.t
+++ b/tests/test-revset.t
@@ -1123,6 +1123,54 @@ 
   $ cd ../repo
   $ log 'remote(".a.b.c.", "../remote3")'
 
+tests for concatenation of strings/symbols by "##"
+
+  $ try "278 ## '5f5' ## 1ee ## 'ce5'"
+  (_concat
+    (_concat
+      (_concat
+        ('symbol', '278')
+        ('string', '5f5'))
+      ('symbol', '1ee'))
+    ('string', 'ce5'))
+  ('string', '2785f51eece5')
+  0
+
+  $ echo 'cat4($1, $2, $3, $4) = $1 ## $2 ## $3 ## $4' >> .hg/hgrc
+  $ try "cat4(278, '5f5', 1ee, 'ce5')"
+  (func
+    ('symbol', 'cat4')
+    (list
+      (list
+        (list
+          ('symbol', '278')
+          ('string', '5f5'))
+        ('symbol', '1ee'))
+      ('string', 'ce5')))
+  (_concat
+    (_concat
+      (_concat
+        ('symbol', '278')
+        ('string', '5f5'))
+      ('symbol', '1ee'))
+    ('string', 'ce5'))
+  ('string', '2785f51eece5')
+  0
+
+(check concatenation in alias nesting)
+
+  $ echo 'cat2($1, $2) = $1 ## $2' >> .hg/hgrc
+  $ echo 'cat2x2($1, $2, $3, $4) = cat2($1 ## $2, $3 ## $4)' >> .hg/hgrc
+  $ log "cat2x2(278, '5f5', 1ee, 'ce5')"
+  0
+
+(check operator priority)
+
+  $ echo 'cat2n2($1, $2, $3, $4) = $1 ## $2 or $3 ## $4~2' >> .hg/hgrc
+  $ log "cat2n2(2785f5, 1eece5, 24286f, 4ae135)"
+  0
+  4
+
   $ cd ..
 
 test author/desc/keyword in problematic encoding