Require let to be a statement (#594)

crates/nu-parser/src/errors.rs

@@ -195,6 +195,14 @@ pub enum ParseError {
     #[diagnostic(code(nu::parser::file_not_found), url(docsrs))]
     FileNotFound(String, #[label("File not found: {0}")] Span),
 
+    #[error("'let' statements can't be part of a pipeline")]
+    #[diagnostic(
+        code(nu::parser::let_not_statement),
+        url(docsrs),
+        help("use parens to assign to a variable\neg) let x = ('hello' | str length)")
+    )]
+    LetNotStatement(#[label = "let statement part of a pipeline"] Span),
+
     #[error("{0}")]
     #[diagnostic()]
     LabeledError(String, String, #[label("{1}")] Span),

crates/nu-parser/src/parse_keywords.rs

@@ -973,6 +973,10 @@ pub fn parse_let(
                         );
                         error = error.or(err);
 
+                        if idx < (spans.len() - 1) {
+                            error = error.or(Some(ParseError::ExtraPositional(spans[idx + 1])));
+                        }
+
                         let mut idx = 0;
                         let (lvalue, err) =
                             parse_var_with_opt_type(working_set, &spans[1..(span.0)], &mut idx);

crates/nu-parser/src/parser.rs

@@ -3352,6 +3352,16 @@ pub fn parse_block(
                     })
                     .collect::<Vec<Expression>>();
 
+                if let Some(let_call_id) = working_set.find_decl(b"let") {
+                    for expr in output.iter() {
+                        if let Expr::Call(x) = &expr.expr {
+                            if let_call_id == x.decl_id && output.len() != 1 && error.is_none() {
+                                error = Some(ParseError::LetNotStatement(expr.span));
+                            }
+                        }
+                    }
+                }
+
                 for expr in output.iter_mut().skip(1) {
                     if expr.has_in_variable(working_set) {
                         *expr = wrap_expr_with_collect(working_set, expr);