Default switch case must be last.

CHANGELOG.md

@@ -18,6 +18,7 @@ Breaking changes
 ----------------
 
 * Negative index to an array or string yields the appropriate element/character counting from the _end_.
+* The default `_` case of a `switch` statement now must be the last case, together with a new error variant `EvalAltResult::WrongSwitchDefaultCase`.
 * `ModuleResolver` trait methods take an additional parameter `source_path` that contains the path of the current environment. This is to facilitate loading other script files always from the current directory.
 * `FileModuleResolver` now resolves relative paths under the source path if there is no base path set.
 * `FileModuleResolver::base_path` now returns `Option<&str>` which is `None` if there is no base path set.

src/parse_error.rs

@@ -113,8 +113,10 @@ pub enum ParseErrorType {
     ///
     /// Never appears under the `no_object` feature.
     DuplicatedProperty(String),
-    /// A switch case is duplicated.
+    /// A `switch` case is duplicated.
     DuplicatedSwitchCase,
+    /// The default case of a `switch` statement is not the last.
+    WrongSwitchDefaultCase,
     /// Missing a property name for custom types and maps.
     ///
     /// Never appears under the `no_object` feature.
@@ -195,6 +197,7 @@ impl ParseErrorType {
             Self::MalformedCapture(_) => "Invalid capturing",
             Self::DuplicatedProperty(_) => "Duplicated property in object map literal",
             Self::DuplicatedSwitchCase => "Duplicated switch case",
+            Self::WrongSwitchDefaultCase => "Default switch case is not the last",
             Self::PropertyExpected => "Expecting name of a property",
             Self::VariableExpected => "Expecting name of a variable",
             Self::Reserved(_) => "Invalid use of reserved keyword",

src/parser.rs

@@ -809,6 +809,7 @@ fn parse_switch(
     }
 
     let mut table = BTreeMap::<u64, Box<StmtBlock>>::new();
+    let mut def_pos = Position::NONE;
     let mut def_stmt = None;
 
     loop {
@@ -825,11 +826,13 @@ fn parse_switch(
                         .into_err(*pos),
                 )
             }
-            (Token::Underscore, _) if def_stmt.is_none() => {
+            (Token::Underscore, pos) if def_stmt.is_none() => {
+                def_pos = *pos;
                 eat_token(input, Token::Underscore);
                 None
             }
             (Token::Underscore, pos) => return Err(PERR::DuplicatedSwitchCase.into_err(*pos)),
+            _ if def_stmt.is_some() => return Err(PERR::WrongSwitchDefaultCase.into_err(def_pos)),
             _ => Some(parse_expr(input, state, lib, settings.level_up())?),
         };