Handle closures that are invalid for Aleo.

In particular,

1. Insert a dummy instruction for empty closures.
SnarkVM won't parse closures with no instructions.

2. Give a parse error for Leo functions with
no parameters (rather than just generating
an invalid Aleo closure).

Fixes #28401

compiler/parser/src/parser/file.rs

@@ -349,7 +349,11 @@ impl<N: Network> ParserContext<'_, N> {
         let name = self.expect_identifier()?;
 
         // Parse parameters.
-        let (inputs, ..) = self.parse_paren_comma_list(|p| p.parse_input().map(Some))?;
+        let (inputs, _, span) = self.parse_paren_comma_list(|p| p.parse_input().map(Some))?;
+
+        if inputs.is_empty() && matches!(variant, Variant::Function) {
+            return Err(ParserError::empty_function_arglist(span).into());
+        }
 
         // Parse return type.
         let output = match self.eat(&Token::Arrow) {

compiler/passes/src/code_generation/visit_program.rs

@@ -247,6 +247,12 @@ impl<'a> CodeGenerator<'a> {
 
         //  Construct and append the function body.
         let block_string = self.visit_block(&function.block);
+        if block_string.lines().all(|line| line.starts_with("    output ")) {
+            // There are no real instructions, which is invalid in Aleo, so
+            // add a dummy instruction.
+            function_string.push_str("    assert.eq true true;\n");
+        }
+
         function_string.push_str(&block_string);
 
         function_string

errors/src/errors/parser/parser_errors.rs

@@ -365,4 +365,11 @@ create_messages!(
         msg: format!("Digit {digit} invalid in radix {radix} (token {token})."),
         help: None,
     }
+
+    @formatted
+    empty_function_arglist {
+        args: (),
+        msg: format!("Cannot define a function with no parameters."),
+        help: None,
+    }
 );

tests/expectations/parser/functions/empty2.out

@@ -1,22 +1,8 @@
 namespace = "Parse"
-expectation = "Pass"
-
-[[outputs]]
-
-[outputs.imports]
-
-[outputs.stubs]
-
-[outputs.program_scopes.test]
-program_id = '{"name":"test","network":"\"{\\\"id\\\":\\\"1\\\",\\\"name\\\":\\\"aleo\\\",\\\"span\\\":\\\"{\\\\\\\"lo\\\\\\\":0,\\\\\\\"hi\\\\\\\":0}\\\"}\""}'
-consts = []
-structs = []
-mappings = []
-functions = [[
-    "x",
-    { annotations = [], variant = "Function", identifier = '{"id":"2","name":"x","span":"{\"lo\":39,\"hi\":40}"}', input = [], id = 5, output = [{ mode = "None", id = 3, type_ = { Integer = "U8" }, span = { lo = 46, hi = 48 } }], output_type = { Integer = "U8" }, block = { statements = [], id = 4, span = { lo = 49, hi = 51 } }, span = { lo = 30, hi = 51 } },
-]]
-
-[outputs.program_scopes.test.span]
-lo = 2
-hi = 52
+expectation = "Fail"
+outputs = ["""
+Error [EPAR0370044]: Cannot define a function with no parameters.
+    --> test:4:15
+     |
+   4 |     function x() -> u8 {}
+     |               ^^"""]

tests/tests/compiler/records/owner_wrong_ty.leo

@@ -9,7 +9,7 @@ program test.aleo {
         owner: bool,
     }
 
-    function main() -> bool {
+    function main(x: field) -> bool {
         return true;
     }
 }

tests/tests/parser/functions/empty2.leo

@@ -1,7 +1,8 @@
 /*
 namespace = "Parse"
-expectation = "Pass"
+expectation = "Fail"
 */
 
 program test.aleo {    
-    function x() -> u8 {}}
+    function x() -> u8 {}
+}