Improve keymap errors from command typos (#3931)

* Improve keymap errors from command typos

Currently, opening helix with a config containing a bad command mapping
fails with a cryptic error. For example, say we have a config (bad.toml)
with a command name that doesn't exist:

    [keys.normal]
    b = "buffer_close" # should be ":buffer-close"

When we `hx -c bad.toml`, we get...

> Bad config: data did not match any variant of untagged enum KeyTrie for key `keys.normal` at line 1 column 1
> Press <ENTER> to continue with default config

This is because of the way that Serde tries to deserialize untagged
enums such as `helix_term::keymap::KeyTrie`. From the Serde docs[^1]:

> Serde will try to match the data against each variant in order and the
> first one that deserializes successfully is the one returned.

`MappableCommand::deserialize` fails (returns an Err variant) when a
command does not exist. Serde interprets this as the `KeyTrie::Leaf`
variant failing to match and declares that the input data doesn't
"match any variant of untagged enum KeyTrie."

Luckily the variants of KeyTrie are orthogonal in structure: we can tell
them apart by the type hints from a `serde:🇩🇪:Visitor`. This change
uses a custom Deserialize implementation along with a Visitor that
discerns which variant of the KeyTrie applies. With this change, the
above failure becomes:

> Bad config: No command named 'buffer_close' for key `keys.normal.b` at line 2 column 5
> Press <ENTER> to continue with default config

We also provide more explicit information about the expectations on
the field. A config with an unexpected type produces a message with
that information and the expectation:

    [keys.normal]
    b = 1

> Bad config: invalid type: integer `1`, expected a command, list of commands, or sub-keymap for key `keys.normal.b` at line 2 column 5
> Press <ENTER> to continue with default config

[^1]: https://serde.rs/enum-representations.html#untagged

* Update helix-term/src/keymap.rs

Co-authored-by: Ivan Tham <pickfire@riseup.net>

Co-authored-by: Ivan Tham <pickfire@riseup.net>

helix-term/src/keymap.rs

@@ -144,14 +144,70 @@ impl DerefMut for KeyTrieNode {
     }
 }
 
-#[derive(Debug, Clone, PartialEq, Deserialize)]
-#[serde(untagged)]
+#[derive(Debug, Clone, PartialEq)]
 pub enum KeyTrie {
     Leaf(MappableCommand),
     Sequence(Vec<MappableCommand>),
     Node(KeyTrieNode),
 }
 
+impl<'de> Deserialize<'de> for KeyTrie {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        deserializer.deserialize_any(KeyTrieVisitor)
+    }
+}
+
+struct KeyTrieVisitor;
+
+impl<'de> serde::de::Visitor<'de> for KeyTrieVisitor {
+    type Value = KeyTrie;
+
+    fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
+        write!(formatter, "a command, list of commands, or sub-keymap")
+    }
+
+    fn visit_str<E>(self, command: &str) -> Result<Self::Value, E>
+    where
+        E: serde::de::Error,
+    {
+        command
+            .parse::<MappableCommand>()
+            .map(KeyTrie::Leaf)
+            .map_err(E::custom)
+    }
+
+    fn visit_seq<S>(self, mut seq: S) -> Result<Self::Value, S::Error>
+    where
+        S: serde::de::SeqAccess<'de>,
+    {
+        let mut commands = Vec::new();
+        while let Some(command) = seq.next_element::<&str>()? {
+            commands.push(
+                command
+                    .parse::<MappableCommand>()
+                    .map_err(serde::de::Error::custom)?,
+            )
+        }
+        Ok(KeyTrie::Sequence(commands))
+    }
+
+    fn visit_map<M>(self, mut map: M) -> Result<Self::Value, M::Error>
+    where
+        M: serde::de::MapAccess<'de>,
+    {
+        let mut mapping = HashMap::new();
+        let mut order = Vec::new();
+        while let Some((key, value)) = map.next_entry::<KeyEvent, KeyTrie>()? {
+            mapping.insert(key, value);
+            order.push(key);
+        }
+        Ok(KeyTrie::Node(KeyTrieNode::new("", mapping, order)))
+    }
+}
+
 impl KeyTrie {
     pub fn node(&self) -> Option<&KeyTrieNode> {
         match *self {