Limited mutable variables (#7089)

This adds support for (limited) mutable variables. Mutable variables are created with mut much the same way immutable variables are made with let.

Mutable variables allow mutation via the assignment operator (=).

❯ mut x = 100
❯ $x = 200
❯ print $x
200

Mutable variables are limited in that they're only tended to be used in the local code block. Trying to capture a local variable will result in an error:

❯ mut x = 123; {|| $x }
Error: nu::parser::expected_keyword (link)

  × Capture of mutable variable.

The intent of this limitation is to reduce some of the issues with mutable variables in general: namely they make code that's harder to reason about. By reducing the scope that a mutable variable can be used it, we can help create local reasoning about them.

Mutation can occur with fields as well, as in this case:

❯ mut y = {abc: 123}
❯ $y.abc = 456
❯ $y

On a historical note: mutable variables are something that we resisted for quite a long time, leaning as much as we could on the functional style of pipelines and dataflow. That said, we've watched folks struggle to work with reduce as an approximation for patterns that would be trivial to express with local mutation. With that in mind, we're leaning towards the happy path.

crates/nu-parser/src/parse_keywords.rs

@@ -2781,8 +2781,12 @@ pub fn parse_let(
                         }
 
                         let mut idx = 0;
-                        let (lvalue, err) =
-                            parse_var_with_opt_type(working_set, &spans[1..(span.0)], &mut idx);
+                        let (lvalue, err) = parse_var_with_opt_type(
+                            working_set,
+                            &spans[1..(span.0)],
+                            &mut idx,
+                            false,
+                        );
                         error = error.or(err);
 
                         let var_name =
@@ -2858,6 +2862,129 @@ pub fn parse_let(
     )
 }
 
+pub fn parse_mut(
+    working_set: &mut StateWorkingSet,
+    spans: &[Span],
+    expand_aliases_denylist: &[usize],
+) -> (Pipeline, Option<ParseError>) {
+    let name = working_set.get_span_contents(spans[0]);
+
+    if name == b"mut" {
+        if let Some((span, err)) = check_name(working_set, spans) {
+            return (Pipeline::from_vec(vec![garbage(*span)]), Some(err));
+        }
+
+        if let Some(decl_id) = working_set.find_decl(b"mut", &Type::Any) {
+            let cmd = working_set.get_decl(decl_id);
+            let call_signature = cmd.signature().call_signature();
+
+            if spans.len() >= 4 {
+                // This is a bit of by-hand parsing to get around the issue where we want to parse in the reverse order
+                // so that the var-id created by the variable isn't visible in the expression that init it
+                for span in spans.iter().enumerate() {
+                    let item = working_set.get_span_contents(*span.1);
+                    if item == b"=" && spans.len() > (span.0 + 1) {
+                        let mut error = None;
+
+                        let mut idx = span.0;
+                        let (rvalue, err) = parse_multispan_value(
+                            working_set,
+                            spans,
+                            &mut idx,
+                            &SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::Expression)),
+                            expand_aliases_denylist,
+                        );
+                        error = error.or(err);
+
+                        if idx < (spans.len() - 1) {
+                            error = error.or(Some(ParseError::ExtraPositional(
+                                call_signature,
+                                spans[idx + 1],
+                            )));
+                        }
+
+                        let mut idx = 0;
+                        let (lvalue, err) = parse_var_with_opt_type(
+                            working_set,
+                            &spans[1..(span.0)],
+                            &mut idx,
+                            true,
+                        );
+                        error = error.or(err);
+
+                        let var_name =
+                            String::from_utf8_lossy(working_set.get_span_contents(lvalue.span))
+                                .to_string();
+
+                        if ["in", "nu", "env", "nothing"].contains(&var_name.as_str()) {
+                            error =
+                                error.or(Some(ParseError::MutBuiltinVar(var_name, lvalue.span)));
+                        }
+
+                        let var_id = lvalue.as_var();
+                        let rhs_type = rvalue.ty.clone();
+
+                        if let Some(var_id) = var_id {
+                            working_set.set_variable_type(var_id, rhs_type);
+                        }
+
+                        let call = Box::new(Call {
+                            decl_id,
+                            head: spans[0],
+                            arguments: vec![
+                                Argument::Positional(lvalue),
+                                Argument::Positional(rvalue),
+                            ],
+                            redirect_stdout: true,
+                            redirect_stderr: false,
+                        });
+
+                        return (
+                            Pipeline::from_vec(vec![Expression {
+                                expr: Expr::Call(call),
+                                span: nu_protocol::span(spans),
+                                ty: Type::Any,
+                                custom_completion: None,
+                            }]),
+                            error,
+                        );
+                    }
+                }
+            }
+            let ParsedInternalCall {
+                call,
+                error: err,
+                output,
+            } = parse_internal_call(
+                working_set,
+                spans[0],
+                &spans[1..],
+                decl_id,
+                expand_aliases_denylist,
+            );
+
+            return (
+                Pipeline {
+                    expressions: vec![Expression {
+                        expr: Expr::Call(call),
+                        span: nu_protocol::span(spans),
+                        ty: output,
+                        custom_completion: None,
+                    }],
+                },
+                err,
+            );
+        }
+    }
+    (
+        garbage_pipeline(spans),
+        Some(ParseError::UnknownState(
+            "internal error: mut statement unparseable".into(),
+            span(spans),
+        )),
+    )
+}
+
 pub fn parse_source(
     working_set: &mut StateWorkingSet,
     spans: &[Span],