Some cleanup

compiler/parser/src/parser/statement.rs

@@ -117,7 +117,9 @@ impl ParserContext<'_> {
     fn parse_return_statement(&mut self) -> Result<ReturnStatement> {
         let start = self.expect(&Token::Return)?;
         let expression = match self.token.token {
+            // If the next token is a semicolon, implicitly return a unit expression, `()`.
             Token::Semicolon => Expression::Unit(UnitExpression { span: self.token.span }),
+            // Otherwise, attempt to parse an expression.
             _ => self.parse_expression()?,
         };
         self.expect(&Token::Semicolon)?;

compiler/passes/src/code_generation/visit_expressions.rs

@@ -318,6 +318,6 @@ impl<'a> CodeGenerator<'a> {
     }
 
     fn visit_unit(&mut self, _input: &'a UnitExpression) -> (String, String) {
-        unreachable!("`UnitExpression`s should not exist in the AST at this phase of compilation.")
+        unreachable!("`UnitExpression`s should not be visited during code generation.")
     }
 }

compiler/passes/src/type_checking/check_expressions.rs

@@ -712,4 +712,12 @@ impl<'a> ExpressionVisitor<'a> for TypeChecker<'a> {
             }
         }
     }
+
+    fn visit_unit(&mut self, input: &'a UnitExpression, _additional: &Self::AdditionalInput) -> Self::Output {
+        // Unit expression are only allowed inside a return statement.
+        if !self.is_return {
+            self.emit_err(TypeCheckerError::unit_tuple(input.span()));
+        }
+        Some(Type::Unit)
+    }
 }

compiler/passes/src/type_checking/check_program.rs

@@ -73,11 +73,10 @@ impl<'a> ProgramVisitor<'a> for TypeChecker<'a> {
         for Member { identifier, type_ } in input.members.iter() {
             // Check that the member type is not a tuple.
             if matches!(type_, Type::Tuple(_)) {
-                self.emit_err(if input.is_record {
-                    TypeCheckerError::record_cannot_contain_tuple(identifier.span)
-                } else {
-                    TypeCheckerError::struct_cannot_contain_tuple(identifier.span)
-                });
+                self.emit_err(TypeCheckerError::composite_data_type_cannot_contain_tuple(
+                    if input.is_record { "record" } else { "struct" },
+                    identifier.span,
+                ));
             }
             // Ensure that there are no record members.
             self.assert_member_is_not_record(identifier.span, input.identifier.name, type_);

compiler/passes/src/type_checking/check_statements.rs

@@ -378,6 +378,7 @@ impl<'a> StatementVisitor<'a> for TypeChecker<'a> {
                 false => f.output_type.clone(),
             });
 
+        // Set the `has_return` flag.
         self.has_return = true;
 
         // Check that the return expression is not a nested tuple.
@@ -389,6 +390,11 @@ impl<'a> StatementVisitor<'a> for TypeChecker<'a> {
             }
         }
 
+        // Set the `is_return` flag.
+        self.is_return = true;
+        // Type check the associated expression.
         self.visit_expression(&input.expression, return_type);
+        // Unset the `is_return` flag.
+        self.is_return = false;
     }
 }

compiler/passes/src/type_checking/checker.rs

@@ -41,6 +41,8 @@ pub struct TypeChecker<'a> {
     pub(crate) is_finalize: bool,
     /// Whether or not we are currently traversing an imported program.
     pub(crate) is_imported: bool,
+    /// Whether or not we are currently traversing a return statement.
+    pub(crate) is_return: bool,
 }
 
 const BOOLEAN_TYPE: Type = Type::Boolean;
@@ -98,6 +100,7 @@ impl<'a> TypeChecker<'a> {
             has_finalize: false,
             is_finalize: false,
             is_imported: false,
+            is_return: false,
         }
     }
 

compiler/span/src/source_map.rs

@@ -412,8 +412,7 @@ fn analyze_source_file(src: &str, source_file_start_pos: BytePos) -> (Vec<BytePo
     let src_bytes = src.as_bytes();
 
     while i < src.len() {
-        let i_usize = i;
-        let byte = src_bytes[i_usize];
+        let byte = src_bytes[i];
 
         // How much to advance to get to the next UTF-8 char in the string.
         let mut char_len = 1;

errors/src/errors/type_checker/type_checker_error.rs

@@ -479,16 +479,9 @@ create_messages!(
     }
 
     @formatted
-    struct_cannot_contain_tuple {
-        args: (),
-        msg: format!("A struct cannot contain a tuple."),
-        help: None,
-    }
-
-    @formatted
-    record_cannot_contain_tuple {
-        args: (),
-        msg: format!("A record cannot contain a tuple."),
+    composite_data_type_cannot_contain_tuple {
+        args: (data_type: impl Display),
+        msg: format!("A {data_type} cannot contain a tuple."),
         help: None,
     }