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# 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>
nu-protocol
The nu-protocol crate holds the definitions of structs/traits that are used throughout Nushell. This gives us one way to expose them to many other crates, as well as make these definitions available to each other, without causing mutually recursive dependencies.