feat: lib: Allow special string %account in auto-posting rules (#1975)

This allows using the special string `%account` in auto posting rules.
When run, this will be substituted with the account name of the matched
posting.

hledger-lib/Hledger/Data/TransactionModifier.hs

@@ -109,6 +109,7 @@ tmPostingRuleToFunction verbosetags styles query querytxt tmpr =
   \p -> styleAmounts styles . renderPostingCommentDates $ pr
       { pdate    = pdate  pr <|> pdate  p
       , pdate2   = pdate2 pr <|> pdate2 p
+      , paccount = account' p
       , pamount  = amount' p
       , pcomment = pcomment pr & (if verbosetags then (`commentAddTag` ("generated-posting",qry)) else id)
       , ptags    = ptags pr
@@ -119,6 +120,10 @@ tmPostingRuleToFunction verbosetags styles query querytxt tmpr =
     pr = tmprPosting tmpr
     qry = "= " <> querytxt
     symq = filterQuery (liftA2 (||) queryIsSym queryIsAmt) query
+    account' = if accountTemplate `T.isInfixOf` paccount pr
+                 then \p -> T.replace accountTemplate (paccount p) $ paccount pr
+                 else const $ paccount pr
+      where accountTemplate = "%account"
     amount' = case postingRuleMultiplier tmpr of
         Nothing -> const $ pamount pr
         Just n  -> \p ->

hledger/hledger.m4.md

@@ -2706,6 +2706,9 @@ the rule's postings are added to that transaction, immediately below the matched
 Note these generated postings are temporary, existing only for the duration of the report,
 and only when `--auto` is used; they are not saved in the journal file by hledger.
 
+The postings can contain the special string `%account` which will be expanded to
+the account name of the matched account.
+
 Generated postings' amounts can depend on the matched posting's amount.
 So auto postings can be useful for, eg, adding tax postings with a standard percentage.
 AMOUNT can be:

hledger/test/journal/auto-postings.test

@@ -426,3 +426,22 @@ $ hledger -f- print --forecast --auto --explicit
     a   1
 
 $ hledger -f- --auto check
+
+# 22. any instance of %account in the account name is substituted for the original account
+<
+2023-01-06 * A client
+    Income:Revenue  -1190 €  ; tax: 19%
+    Assets:Bank
+
+= acct:Income tag:tax=19%
+    %account              *-0.15966387 ; 1-100/119
+    Liabilities:VAT        *0.15966387 ; 1-100/119
+
+$ hledger -f- --auto print
+2023-01-06 * A client
+    Income:Revenue            -1190 €  ; tax: 19%
+    Income:Revenue      190.0000053 €  ; 1-100/119
+    Liabilities:VAT    -190.0000053 €  ; 1-100/119
+    Assets:Bank
+
+>=0