Show set-cmd-text command bindings in completion.

Fixes #5942.

For bindings like `o -> :set-cmd-text -s :open`, the user conceptually
expects that `o` maps to `open`, yet they do not show in the command
completion UI.

With this patch, a binding to a command of form
`set-cmd-text [flags...] :<cmd> [cmdargs]` will be treated as if it were
bound to `<cmd> [cmdargs]` instead for the purpose of completion.
Bindings to `set-cmd-text --append` are ignored.

qutebrowser/config/config.py

@@ -162,13 +162,40 @@ class KeyConfig:
                 bindings[key] = binding
         return bindings
 
+    def _strip_leading_flags(self, cmdline: str) -> List[str]:
+        """Split cmdline at whitespace until the first non-flag."""
+        first, _, rest = cmdline.partition(" ")
+        if first.startswith("-"):
+            return [first] + self._strip_leading_flags(rest)
+        return [cmdline]
+
+    def _implied_cmd(self, cmdline: str) -> str:
+        """Return cmdline, or the implied cmd if cmdline is a set-cmd-text."""
+        if not cmdline.startswith("set-cmd-text "):
+            return cmdline
+        cmdline = cmdline[len("set-cmd-text "):]
+        *flags, cmd = self._strip_leading_flags(cmdline)
+        if "-a" in flags or "--append" in flags or not cmd.startswith(":"):
+            return ""  # doesn't look like this sets a command
+        return cmd.lstrip(":")
+
     def get_reverse_bindings_for(self, mode: str) -> '_ReverseBindings':
-        """Get a dict of commands to a list of bindings for the mode."""
+        """Get a dict of commands to a list of bindings for the mode.
+
+        This is intented for user-facing display of keybindings.
+        As such, bindings for 'set-cmd-text [flags] :<cmd> ...' are translated
+        to '<cmd> ...', as from the user's perspective these keys behave like
+        bindings for '<cmd>' (that allow for further input before running).
+
+        See #5942.
+        """
         cmd_to_keys: KeyConfig._ReverseBindings = {}
         bindings = self.get_bindings_for(mode)
         for seq, full_cmd in sorted(bindings.items()):
             for cmd in full_cmd.split(';;'):
-                cmd = cmd.strip()
+                cmd = self._implied_cmd(cmd.strip())
+                if not cmd:
+                    continue
                 cmd_to_keys.setdefault(cmd, [])
                 # Put bindings involving modifiers last
                 if any(info.modifiers for info in seq):

tests/unit/config/test_config.py

@@ -198,6 +198,26 @@ class TestKeyConfig:
         # Chained command
         ({'a': 'message-info foo ;; message-info bar'},
          {'message-info foo': ['a'], 'message-info bar': ['a']}),
+        # Command using set-cmd-text (#5942)
+        (
+            {
+                "o": "set-cmd-text -s :open",
+                "O": "set-cmd-text -s :open -t",
+                "go": "set-cmd-text :open {url:pretty}",
+                # all of these should be ignored
+                "/": "set-cmd-text /",
+                "?": "set-cmd-text ?",
+                ":": "set-cmd-text :",
+                "a": "set-cmd-text no_leading_colon",
+                "b": "set-cmd-text -s -a :skip_cuz_append",
+                "c": "set-cmd-text --append :skip_cuz_append",
+            },
+            {
+                "open": ["o"],
+                "open -t": ["O"],
+                "open {url:pretty}": ["go"],
+            }
+        ),
     ])
     def test_get_reverse_bindings_for(self, key_config_stub, config_stub,
                                       no_bindings, bindings, expected):