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-cmd-lang/src/core_commands/const_.rs

@@ -21,7 +21,7 @@ impl Command for Const {
             .required("const_name", SyntaxShape::VarWithOptType, "constant 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 constant value",
             )
             .category(Category::Core)

crates/nu-cmd-lang/src/core_commands/if_.rs

@@ -20,7 +20,7 @@ impl Command for If {
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("if")
             .input_output_types(vec![(Type::Any, Type::Any)])
-            .required("cond", SyntaxShape::Expression, "condition to check")
+            .required("cond", SyntaxShape::MathExpression, "condition to check")
             .required(
                 "then_block",
                 SyntaxShape::Block,

crates/nu-cmd-lang/src/core_commands/let_.rs

@@ -22,7 +22,7 @@ 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",
             )
             .category(Category::Core)

crates/nu-cmd-lang/src/core_commands/mut_.rs

@@ -22,7 +22,7 @@ impl Command for Mut {
             .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",
             )
             .category(Category::Core)

crates/nu-cmd-lang/src/core_commands/while_.rs

@@ -21,7 +21,7 @@ impl Command for While {
         Signature::build("while")
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .allow_variants_without_examples(true)
-            .required("cond", SyntaxShape::Expression, "condition to check")
+            .required("cond", SyntaxShape::MathExpression, "condition to check")
             .required(
                 "block",
                 SyntaxShape::Block,

crates/nu-command/src/env/let_env.rs

@@ -24,7 +24,7 @@ impl Command for LetEnv {
             .required("var_name", SyntaxShape::String, "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",
             )
             .category(Category::Env)

crates/nu-command/tests/commands/use_.rs

@@ -172,7 +172,7 @@ fn use_export_env_combined() {
             "spam.nu",
             r#"
                 alias bar = foo
-                export-env { let-env FOO = bar }
+                export-env { let-env FOO = (bar) }
                 def foo [] { 'foo' }
             "#,
         )]);

crates/nu-engine/src/eval.rs

@@ -698,6 +698,29 @@ pub fn eval_expression_with_input(
             input = eval_subexpression(engine_state, stack, block, input)?;
         }
 
+        elem @ Expression {
+            expr: Expr::FullCellPath(full_cell_path),
+            ..
+        } => match &full_cell_path.head {
+            Expression {
+                expr: Expr::Subexpression(block_id),
+                span,
+                ..
+            } => {
+                let block = engine_state.get_block(*block_id);
+
+                // FIXME: protect this collect with ctrl-c
+                input = eval_subexpression(engine_state, stack, block, input)?;
+                let value = input.into_value(*span);
+                input = value
+                    .follow_cell_path(&full_cell_path.tail, false)?
+                    .into_pipeline_data()
+            }
+            _ => {
+                input = eval_expression(engine_state, stack, elem)?.into_pipeline_data();
+            }
+        },
+
         elem => {
             input = eval_expression(engine_state, stack, elem)?.into_pipeline_data();
         }

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,

src/tests/test_engine.rs

@@ -146,7 +146,7 @@ fn date_comparison() -> TestResult {
 #[test]
 fn let_sees_input() -> TestResult {
     run_test(
-        r#"def c [] { let x = str length; $x }; "hello world" | c"#,
+        r#"def c [] { let x = (str length); $x }; "hello world" | c"#,
         "11",
     )
 }

src/tests/test_modules.rs

@@ -91,7 +91,7 @@ fn module_def_import_uses_internal_command() -> TestResult {
 #[test]
 fn module_env_import_uses_internal_command() -> TestResult {
     run_test(
-        r#"module foo { def b [] { "2" }; export-env { let-env a = b }  }; use foo; $env.a"#,
+        r#"module foo { def b [] { "2" }; export-env { let-env a = (b) }  }; use foo; $env.a"#,
         "2",
     )
 }