revset: move subscript relation functions to its own dict

This will help adding more relation functions in extensions.

We skip short names (that consist of one letter) while raising
UnknownIdentifier because such names cannot be suggested anyway: the similarity
cutoff in dispatch._getsimilar() is currently 0.6.

mercurial/revset.py

@@ -218,15 +218,7 def notset(repo, subset, x, order):
 def relationset(repo, subset, x, y, order):
     raise error.ParseError(_("can't use a relation in this context"))
 
-def relsubscriptset(repo, subset, x, y, z, order):
-    # this is pretty basic implementation of 'x#y[z]' operator, still
-    # experimental so undocumented. see the wiki for further ideas.
-    # https://www.mercurial-scm.org/wiki/RevsetOperatorPlan
-    rel = getsymbol(y)
-    n = getinteger(z, _("relation subscript must be an integer"))
-
-    # TODO: perhaps this should be a table of relation functions
-    if rel in ('g', 'generations'):
+def generationsrel(repo, subset, x, rel, n, order):
         # TODO: support range, rewrite tests, and drop startdepth argument
         # from ancestors() and descendants() predicates
         if n <= 0:
@@ -235,7 +227,18 def relsubscriptset(repo, subset, x, y,
         else:
             return _descendants(repo, subset, x, startdepth=n, stopdepth=n + 1)
 
-    raise error.UnknownIdentifier(rel, ['generations'])
+def relsubscriptset(repo, subset, x, y, z, order):
+    # this is pretty basic implementation of 'x#y[z]' operator, still
+    # experimental so undocumented. see the wiki for further ideas.
+    # https://www.mercurial-scm.org/wiki/RevsetOperatorPlan
+    rel = getsymbol(y)
+    n = getinteger(z, _("relation subscript must be an integer"))
+
+    if rel in subscriptrelations:
+        return subscriptrelations[rel](repo, subset, x, rel, n, order)
+
+    relnames = [r for r in subscriptrelations.keys() if len(r) > 1]
+    raise error.UnknownIdentifier(rel, relnames)
 
 def subscriptset(repo, subset, x, y, order):
     raise error.ParseError(_("can't use a subscript in this context"))
@@ -2215,6 +2218,11 methods = {
     "parentpost": parentpost,
 }
 
+subscriptrelations = {
+    "g": generationsrel,
+    "generations": generationsrel,
+}
+
 def lookupfn(repo):
     return lambda symbol: scmutil.isrevsymbol(repo, symbol)
 

tests/test-revset.t

@@ -649,6 +649,17 parse errors of relation, subscript and
   hg: parse error: relation subscript must be an integer
   [255]
 
+suggested relations
+
+  $ hg debugrevspec '.#generafions[0]'
+  hg: parse error: unknown identifier: generafions
+  (did you mean generations?)
+  [255]
+
+  $ hg debugrevspec '.#f[0]'
+  hg: parse error: unknown identifier: f
+  [255]
+
 parsed tree at stages:
 
   $ hg debugrevspec -p all '()'