Switch let/let-env family to init with math expressions (#8545)

# Description

This is an experiment to see what switching the `let/let-env` family to
math expressions for initialisers would be like.

# User-Facing Changes

This would require any commands you call from `let x = <command here>`
(and similar family) to call the command in parentheses. `let x = (foo)`
to call `foo`.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-parser/src/parse_keywords.rs

@@ -3102,7 +3102,10 @@ pub fn parse_let_or_const(
                             working_set,
                             spans,
                             &mut idx,
-                            &SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::Expression)),
+                            &SyntaxShape::Keyword(
+                                b"=".to_vec(),
+                                Box::new(SyntaxShape::MathExpression),
+                            ),
                             expand_aliases_denylist,
                         );
                         error = error.or(err);
@@ -3236,7 +3239,10 @@ pub fn parse_mut(
                             working_set,
                             spans,
                             &mut idx,
-                            &SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::Expression)),
+                            &SyntaxShape::Keyword(
+                                b"=".to_vec(),
+                                Box::new(SyntaxShape::MathExpression),
+                            ),
                             expand_aliases_denylist,
                         );
                         error = error.or(err);

crates/nu-parser/src/parser.rs

@@ -71,7 +71,12 @@ pub fn is_math_expression_like(
         return false;
     }
 
-    if bytes == b"true" || bytes == b"false" || bytes == b"null" || bytes == b"not" {
+    if bytes == b"true"
+        || bytes == b"false"
+        || bytes == b"null"
+        || bytes == b"not"
+        || bytes == b"if"
+    {
         return true;
     }
 
@@ -1112,12 +1117,12 @@ pub fn parse_call(
     for word_span in spans[cmd_start..].iter() {
         // Find the longest group of words that could form a command
 
-        if is_math_expression_like(working_set, *word_span, expand_aliases_denylist) {
-            let bytes = working_set.get_span_contents(*word_span);
-            if bytes != b"true" && bytes != b"false" && bytes != b"null" && bytes != b"not" {
-                break;
-            }
-        }
+        // if is_math_expression_like(working_set, *word_span, expand_aliases_denylist) {
+        //     let bytes = working_set.get_span_contents(*word_span);
+        //     if bytes != b"true" && bytes != b"false" && bytes != b"null" && bytes != b"not" {
+        //         break;
+        //     }
+        // }
 
         name_spans.push(*word_span);
 
@@ -4993,7 +4998,20 @@ pub fn parse_math_expression(
 
     let first_span = working_set.get_span_contents(spans[0]);
 
-    if first_span == b"not" {
+    if first_span == b"if" {
+        // If expression
+        if spans.len() > 1 {
+            return parse_call(working_set, spans, spans[0], expand_aliases_denylist, false);
+        } else {
+            return (
+                garbage(spans[0]),
+                Some(ParseError::Expected(
+                    "expression".into(),
+                    Span::new(spans[0].end, spans[0].end),
+                )),
+            );
+        }
+    } else if first_span == b"not" {
         if spans.len() > 1 {
             let (remainder, err) = parse_math_expression(
                 working_set,

crates/nu-parser/tests/test_parser.rs

@@ -27,7 +27,7 @@ impl Command for Let {
             .required("var_name", SyntaxShape::VarWithOptType, "variable name")
             .required(
                 "initial_value",
-                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::Expression)),
+                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::MathExpression)),
                 "equals sign followed by value",
             )
     }
@@ -1555,7 +1555,7 @@ mod input_types {
 
         fn signature(&self) -> nu_protocol::Signature {
             Signature::build("if")
-                .required("cond", SyntaxShape::Expression, "condition to check")
+                .required("cond", SyntaxShape::MathExpression, "condition to check")
                 .required(
                     "then_block",
                     SyntaxShape::Block,