conlang: Add Quote expression as a hack for testing

Possibly removed later, or replaced with something that turns Conlang AST nodes into Conlang data structures.

compiler/cl-ast/src/ast.rs

@@ -347,6 +347,8 @@ pub enum ExprKind {
     /// An empty expression: `(` `)`
     #[default]
     Empty,
+    /// A backtick-quoted expression
+    Quote(Quote),
     /// A local bind instruction, `let` [`Sym`] `=` [`Expr`]
     Let(Let),
     /// An [Assign]ment expression: [`Expr`] (`=` [`Expr`])\+
@@ -396,6 +398,12 @@ pub enum ExprKind {
     Continue,
 }
 
+/// A backtick-quoted subexpression-literal
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct Quote {
+    pub quote: Box<ExprKind>,
+}
+
 /// A local variable declaration [Stmt]
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Let {

compiler/cl-ast/src/ast_impl.rs

@@ -415,6 +415,7 @@ mod display {
         fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
             match self {
                 ExprKind::Empty => "()".fmt(f),
+                ExprKind::Quote(v) => v.fmt(f),
                 ExprKind::Let(v) => v.fmt(f),
                 ExprKind::Assign(v) => v.fmt(f),
                 ExprKind::Modify(v) => v.fmt(f),
@@ -442,6 +443,13 @@ mod display {
         }
     }
 
+    impl Display for Quote {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            let Self { quote } = self;
+            write!(f, "`{quote}`")
+        }
+    }
+
     impl Display for Let {
         fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
             let Self { mutable, name, ty, init } = self;
@@ -764,6 +772,7 @@ mod convert {
         }
         impl From for ExprKind {
             Let => ExprKind::Let,
+            Quote => ExprKind::Quote,
             Assign => ExprKind::Assign,
             Modify => ExprKind::Modify,
             Binary => ExprKind::Binary,

compiler/cl-ast/src/ast_visitor/fold.rs

@@ -529,6 +529,7 @@ pub fn or_fold_stmt_kind<F: Fold + ?Sized>(folder: &mut F, kind: StmtKind) -> St
 pub fn or_fold_expr_kind<F: Fold + ?Sized>(folder: &mut F, kind: ExprKind) -> ExprKind {
     match kind {
         ExprKind::Empty => ExprKind::Empty,
+        ExprKind::Quote(q) => ExprKind::Quote(q), // quoted expressions are left unmodified
         ExprKind::Let(l) => ExprKind::Let(folder.fold_let(l)),
         ExprKind::Assign(a) => ExprKind::Assign(folder.fold_assign(a)),
         ExprKind::Modify(m) => ExprKind::Modify(folder.fold_modify(m)),

compiler/cl-ast/src/ast_visitor/visit.rs

@@ -452,6 +452,7 @@ pub fn or_visit_stmt_kind<'a, V: Visit<'a>>(visitor: &mut V, kind: &'a StmtKind)
 pub fn or_visit_expr_kind<'a, V: Visit<'a>>(visitor: &mut V, e: &'a ExprKind) {
     match e {
         ExprKind::Empty => {}
+        ExprKind::Quote(_q) => {} // Quoted expressions are left unvisited
         ExprKind::Let(l) => visitor.visit_let(l),
         ExprKind::Assign(a) => visitor.visit_assign(a),
         ExprKind::Modify(m) => visitor.visit_modify(m),