disallow blocks as first-class values (#9587)

# Description

This PR disallows blocks as first-class values by removing the ability
to create them using the `block` syntax shape or type. Now, the parser
will only ever be able to create closures as first-class values.

This means that `{ 3 }` will always be treated as a closure, unless used
in the specifically supported use case of the literal being given as an
arg to `for`, `if` and other built-in block users.

Note: first-class value here means "value that can be passed into
commands and held in variables"

# User-Facing Changes

This may break some user scripts as `: block` is no longer allows as a
type annotation. Note: these cases were not actually supported before,
as, for example, passing a block that accessed a variable would have
errored when the block was later evaluated.

Closures do not have the restriction mentioned above and are the much
safer value to pass as first-class, so now they are the only block-like
value to be allowed to be passed.

# Tests + Formatting
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crates/nu-parser/src/parser.rs

@@ -2687,7 +2687,15 @@ pub fn parse_shape_name(
     let result = match bytes {
         b"any" => SyntaxShape::Any,
         b"binary" => SyntaxShape::Binary,
-        b"block" => SyntaxShape::Block, //FIXME: Blocks should have known output types
+        b"block" => {
+            working_set.error(ParseError::LabeledErrorWithHelp {
+                error: "Blocks are not support as first-class values".into(),
+                label: "blocks are not supported as values".into(),
+                help: "Use 'closure' instead of 'block'".into(),
+                span,
+            });
+            SyntaxShape::Any
+        }
         b"bool" => SyntaxShape::Boolean,
         b"cell-path" => SyntaxShape::CellPath,
         b"closure" => SyntaxShape::Closure(None), //FIXME: Blocks should have known output types
@@ -2919,10 +2927,19 @@ fn prepare_inner_span(
     }
 }
 
-pub fn parse_type(_working_set: &StateWorkingSet, bytes: &[u8]) -> Type {
+pub fn parse_type(working_set: &mut StateWorkingSet, bytes: &[u8], span: Span) -> Type {
     match bytes {
         b"binary" => Type::Binary,
-        b"block" => Type::Block,
+        b"block" => {
+            working_set.error(ParseError::LabeledErrorWithHelp {
+                error: "Blocks are not support as first-class values".into(),
+                label: "blocks are not supported as values".into(),
+                help: "Use 'closure' instead of 'block'".into(),
+                span,
+            });
+
+            Type::Any
+        }
         b"bool" => Type::Bool,
         b"cellpath" => Type::CellPath,
         b"closure" => Type::Closure,
@@ -3080,18 +3097,18 @@ pub fn parse_var_with_opt_type(
         // We end with colon, so the next span should be the type
         if *spans_idx + 1 < spans.len() {
             *spans_idx += 1;
-            let type_bytes = working_set.get_span_contents(spans[*spans_idx]);
+            let type_bytes = working_set.get_span_contents(spans[*spans_idx]).to_vec();
 
-            let ty = parse_type(working_set, type_bytes);
+            let ty = parse_type(working_set, &type_bytes, spans[*spans_idx]);
 
             let var_name = bytes[0..(bytes.len() - 1)].to_vec();
 
             if !is_variable(&var_name) {
                 working_set.error(ParseError::Expected(
                     "valid variable name",
-                    spans[*spans_idx],
+                    spans[*spans_idx - 1],
                 ));
-                return (garbage(spans[*spans_idx]), None);
+                return (garbage(spans[*spans_idx - 1]), None);
             }
 
             let id = working_set.add_variable(var_name, spans[*spans_idx - 1], ty.clone(), mutable);

src/tests/test_type_check.rs

@@ -43,3 +43,19 @@ fn date_plus_duration() -> TestResult {
     let expected = "2023-04-20";
     run_test(input, expected)
 }
+
+#[test]
+fn block_not_first_class_def() -> TestResult {
+    fail_test(
+        "def foo [x: block] { do $x }",
+        "Blocks are not support as first-class values",
+    )
+}
+
+#[test]
+fn block_not_first_class_let() -> TestResult {
+    fail_test(
+        "let x: block = { 3 }",
+        "Blocks are not support as first-class values",
+    )
+}