Errors when let in, let env and similar commands are passed. (#5866)

* throw `let nu/env/nothing/in` error in parsing

* add tests and fmt

* fix clippy

* suggestions

* fmt

* `lvalue.span` instead of `spans[1]`

* clippy

* fmt

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

@@ -0,0 +1,15 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn let_parse_error() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        let in = 3
+        "#
+    ));
+
+    assert!(actual
+        .err
+        .contains("'in' is the name of a builtin Nushell variable"));
+}

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

@@ -30,6 +30,7 @@ mod into_filesize;
 mod into_int;
 mod last;
 mod length;
+mod let_;
 mod lines;
 mod ls;
 mod math;

crates/nu-parser/src/errors.rs

@@ -88,6 +88,14 @@ pub enum ParseError {
     )]
     LetInPipeline(String, String, #[label("let in pipeline")] Span),
 
+    #[error("Let used with builtin variable name.")]
+    #[diagnostic(
+        code(nu::parser::let_builtin_var),
+        url(docsrs),
+        help("'{0}' is the name of a builtin Nushell variable. `let` cannot assign to it.")
+    )]
+    LetBuiltinVar(String, #[label("already a builtin variable")] Span),
+
     #[error("Incorrect value")]
     #[diagnostic(code(nu::parser::incorrect_value), url(docsrs), help("{2}"))]
     IncorrectValue(String, #[label("unexpected {0}")] Span, String),
@@ -311,6 +319,7 @@ impl ParseError {
             ParseError::UnexpectedKeyword(_, s) => *s,
             ParseError::BuiltinCommandInPipeline(_, s) => *s,
             ParseError::LetInPipeline(_, _, s) => *s,
+            ParseError::LetBuiltinVar(_, s) => *s,
             ParseError::IncorrectValue(_, s, _) => *s,
             ParseError::MultipleRestParams(s) => *s,
             ParseError::VariableNotFound(s) => *s,

crates/nu-parser/src/parse_keywords.rs

@@ -2286,6 +2286,15 @@ pub fn parse_let(
                             parse_var_with_opt_type(working_set, &spans[1..(span.0)], &mut idx);
                         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::LetBuiltinVar(var_name, lvalue.span)));
+                        }
+
                         let var_id = lvalue.as_var();
                         let rhs_type = rvalue.ty.clone();