mirror of
https://github.com/nushell/nushell.git
synced 2025-05-05 15:32:56 +00:00
55de232a1c
# Description
While working on something else, I noticed that
`Value::follow_cell_path` receives `self`.
While it would be ideal for the signature to be `(&'a self, cell_path)
-> &'a Value`, that's not possible because:
1. Selecting a row from a list and field from a record can be done with
a reference but selecting a column from a table requires creating a new
list.
2. `Value::Custom` returns new `Value`s when indexed.
So the signature becomes `(&'a self, cell_path) -> Cow<'a, Value>`.
Another complication that arises is, once a new `Value` is created, and
it is further indexed, the `current` variable
1. can't be `&'a Value`, as the lifetime requirement means it can't
refer to local variables
2. _shouldn't_ be `Cow<'a, Value>`, as once it becomes an owned value,
it can't be borrowed ever again, as `current` is derived from its
previous value in further iterations. So once it's owned, it can't be
indexed by reference, leading to more clones
We need `current` to have _two_ possible lifetimes
1. `'out`: references derived from `&self`
2. `'local`: references derived from an owned value stored in a local
variable
```rust
enum MultiLife<'out, 'local, T>
where
'out: 'local,
T: ?Sized,
{
Out(&'out T),
Local(&'local T),
}
```
With `current: MultiLife<'out, '_, Value>`, we can traverse values with
minimal clones, and we can transform it to `Cow<'out, Value>` easily
(`MultiLife::Out -> Cow::Borrowed, MultiLife::Local -> Cow::Owned`) to
return it
# User-Facing Changes
# Tests + Formatting
# After Submitting
---------
Co-authored-by: Bahex <17417311+Bahex@users.noreply.github.com>
Implementation of the Nushell language server.
See the Language Server Protocol specification
Internal Nushell crate
This crate implements components of Nushell and is not designed to support plugin authors or other users directly.