Add unreachable comments.

codegen/src/rhai_module.rs

@@ -200,7 +200,7 @@ pub fn generate_body(
 
             let ns_str = syn::Ident::new(
                 match namespace {
-                    FnNamespaceAccess::Unset => unreachable!(),
+                    FnNamespaceAccess::Unset => unreachable!("`namespace` should be set"),
                     FnNamespaceAccess::Global => "Global",
                     FnNamespaceAccess::Internal => "Internal",
                 },

src/optimizer.rs

@@ -895,7 +895,7 @@ fn optimize_stmt(stmt: &mut Stmt, state: &mut OptimizerState, preserve_result: b
                     *stmt = stmt_block.into();
                 }
                 Expr::Stmt(..) => *stmt = Stmt::Noop(expr.position()),
-                _ => unreachable!(),
+                _ => unreachable!("`Expr::Stmt`"),
             }
         }
 
@@ -950,7 +950,7 @@ fn optimize_expr(expr: &mut Expr, state: &mut OptimizerState, _chaining: bool) {
                     optimize_expr(&mut e, state, false);
                     *expr = *e;
                 }
-                _ => unreachable!()
+                _ => unreachable!("`Expr::Stmt`")
             }
         }
         // { stmt; ... } - do not count promotion as dirty because it gets turned back into an array

src/packages/iter_basic.rs

@@ -105,7 +105,7 @@ impl<T: Debug + Copy + PartialOrd> Iterator for StepRange<T> {
                 self.dir = 0;
             }
         } else {
-            unreachable!();
+            unreachable!("`dir` != 0");
         }
 
         Some(v)

src/parser.rs

@@ -798,7 +798,7 @@ impl Engine {
                             match token {
                                 Token::LeftBracket => ASTFlags::NONE,
                                 Token::QuestionBracket => ASTFlags::NEGATED,
-                                _ => unreachable!(),
+                                _ => unreachable!("`[` or `?[`"),
                             },
                             false,
                             settings.level_up(),
@@ -1693,7 +1693,7 @@ impl Engine {
                     let opt = match token {
                         Token::LeftBracket => ASTFlags::NONE,
                         Token::QuestionBracket => ASTFlags::NEGATED,
-                        _ => unreachable!(),
+                        _ => unreachable!("`[` or `?[`"),
                     };
                     self.parse_index_chain(input, state, lib, expr, opt, true, settings.level_up())?
                 }
@@ -1717,7 +1717,7 @@ impl Engine {
                     let op_flags = match op {
                         Token::Period => ASTFlags::NONE,
                         Token::Elvis => ASTFlags::NEGATED,
-                        _ => unreachable!(),
+                        _ => unreachable!("`.` or `?.`"),
                     };
                     Self::make_dot_expr(state, expr, rhs, ASTFlags::NONE, op_flags, tail_pos)?
                 }

src/tokenizer.rs

@@ -248,7 +248,7 @@ impl fmt::Debug for Position {
             }
 
             #[cfg(feature = "no_position")]
-            unreachable!();
+            unreachable!("no position");
         }
     }
 }