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::de::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>

0 files changed, 0 insertions, 0 deletions