--- a/mercurial/cmdutil.py	Tue Feb 11 11:18:52 2020 +0100
+++ b/mercurial/cmdutil.py	Fri Dec 20 15:50:13 2019 -0800
@@ -1410,12 +1410,15 @@
 
 
 def copy(ui, repo, pats, opts, rename=False):
+    check_incompatible_arguments(opts, b'forget', [b'dry_run'])
+
     # called with the repo lock held
     #
     # hgsep => pathname that uses "/" to separate directories
     # ossep => pathname that uses os.sep to separate directories
     cwd = repo.getcwd()
     targets = {}
+    forget = opts.get(b"forget")
     after = opts.get(b"after")
     dryrun = opts.get(b"dry_run")
     ctx = repo[None]
@@ -1423,6 +1426,24 @@
 
     uipathfn = scmutil.getuipathfn(repo, legacyrelativevalue=True)
 
+    if forget:
+        match = scmutil.match(wctx, pats, opts)
+
+        current_copies = wctx.p1copies()
+        current_copies.update(wctx.p2copies())
+
+        for f in wctx.walk(match):
+            if f in current_copies:
+                wctx[f].markcopied(None)
+            elif match.exact(f):
+                ui.warn(
+                    _(
+                        b'%s: not unmarking as copy - file is not marked as copied\n'
+                    )
+                    % uipathfn(f)
+                )
+        return
+
     def walkpat(pat):
         srcs = []
         m = scmutil.match(ctx, [pat], opts, globbed=True)