Variable completions. (#3666)

In Nu we have variables (E.g. $var-name) and these contain `Value` types.
This means we can bind to variables any structured data and column path syntax
(E.g. `$variable.path.to`) allows flexibility for "querying" said structures.

Here we offer completions for these. For example, in a Nushell session the
variable `$nu` contains environment values among other things. If we wanted to
see in the screen some environment variable (say the var `SHELL`) we do:

```
> echo $nu.env.SHELL
```

with completions we can now do: `echo $nu.env.S[\TAB]` and we get suggestions
that start at the column path `$nu.env` with vars starting with the letter `S`
in this case `SHELL` appears in the suggestions.

crates/nu-parser/src/lib.rs

@@ -9,11 +9,9 @@ mod lex;
 mod parse;
 mod scope;
 mod shapes;
-mod signature;
 
 pub use lex::lexer::{lex, parse_block};
 pub use lex::tokens::{LiteBlock, LiteCommand, LiteGroup, LitePipeline};
 pub use parse::{classify_block, garbage, parse, parse_full_column_path, parse_math_expression};
 pub use scope::ParserScope;
 pub use shapes::shapes;
-pub use signature::{Signature, SignatureRegistry};

crates/nu-parser/src/scope.rs

@@ -3,8 +3,6 @@ use nu_source::Spanned;
 use std::{fmt::Debug, sync::Arc};
 
 pub trait ParserScope: Debug {
-    fn get_names(&self) -> Vec<String>;
-
     fn get_signature(&self, name: &str) -> Option<nu_protocol::Signature>;
 
     fn has_signature(&self, name: &str) -> bool;

crates/nu-parser/src/signature.rs

@@ -1,48 +0,0 @@
-use std::fmt::Debug;
-
-use nu_source::{DebugDocBuilder, HasSpan, PrettyDebugWithSource, Span};
-
-pub trait SignatureRegistry: Debug {
-    fn has(&self, name: &str) -> bool;
-    fn get(&self, name: &str) -> Option<nu_protocol::Signature>;
-    fn clone_box(&self) -> Box<dyn SignatureRegistry>;
-}
-
-impl SignatureRegistry for Box<dyn SignatureRegistry> {
-    fn has(&self, name: &str) -> bool {
-        (&**self).has(name)
-    }
-    fn get(&self, name: &str) -> Option<nu_protocol::Signature> {
-        (&**self).get(name)
-    }
-    fn clone_box(&self) -> Box<dyn SignatureRegistry> {
-        (&**self).clone_box()
-    }
-}
-
-#[derive(Debug, Clone)]
-pub struct Signature {
-    pub(crate) unspanned: nu_protocol::Signature,
-    span: Span,
-}
-
-impl Signature {
-    pub fn new(unspanned: nu_protocol::Signature, span: impl Into<Span>) -> Signature {
-        Signature {
-            unspanned,
-            span: span.into(),
-        }
-    }
-}
-
-impl HasSpan for Signature {
-    fn span(&self) -> Span {
-        self.span
-    }
-}
-
-impl PrettyDebugWithSource for Signature {
-    fn pretty_debug(&self, source: &str) -> DebugDocBuilder {
-        self.unspanned.pretty_debug(source)
-    }
-}