Add let-else to AST

compiler/rustc_ast/src/ast.rs

@@ -1005,13 +1005,42 @@ pub struct Local {
     pub id: NodeId,
     pub pat: P<Pat>,
     pub ty: Option<P<Ty>>,
-    /// Initializer expression to set the value, if any.
-    pub init: Option<P<Expr>>,
+    pub kind: LocalKind,
     pub span: Span,
     pub attrs: AttrVec,
     pub tokens: Option<LazyTokenStream>,
 }
 
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub enum LocalKind {
+    /// Local declaration.
+    /// Example: `let x;`
+    Decl,
+    /// Local declaration with an initializer.
+    /// Example: `let x = y;`
+    Init(P<Expr>),
+    /// Local declaration with an initializer and an `else` clause.
+    /// Example: `let Some(x) = y else { return };`
+    InitElse(P<Expr>, P<Block>),
+}
+
+impl LocalKind {
+    pub fn init(&self) -> Option<&Expr> {
+        match self {
+            Self::Decl => None,
+            Self::Init(i) | Self::InitElse(i, _) => Some(i),
+        }
+    }
+
+    pub fn init_else_opt(&self) -> Option<(&Expr, Option<&Block>)> {
+        match self {
+            Self::Decl => None,
+            Self::Init(init) => Some((init, None)),
+            Self::InitElse(init, els) => Some((init, Some(els))),
+        }
+    }
+}
+
 /// An arm of a 'match'.
 ///
 /// E.g., `0..=10 => { println!("match!") }` as in

compiler/rustc_ast/src/mut_visit.rs

@@ -571,11 +571,20 @@ pub fn noop_visit_parenthesized_parameter_data<T: MutVisitor>(
 }
 
 pub fn noop_visit_local<T: MutVisitor>(local: &mut P<Local>, vis: &mut T) {
-    let Local { id, pat, ty, init, span, attrs, tokens } = local.deref_mut();
+    let Local { id, pat, ty, kind, span, attrs, tokens } = local.deref_mut();
     vis.visit_id(id);
     vis.visit_pat(pat);
     visit_opt(ty, |ty| vis.visit_ty(ty));
-    visit_opt(init, |init| vis.visit_expr(init));
+    match kind {
+        LocalKind::Decl => {}
+        LocalKind::Init(init) => {
+            vis.visit_expr(init);
+        }
+        LocalKind::InitElse(init, els) => {
+            vis.visit_expr(init);
+            vis.visit_block(els);
+        }
+    }
     vis.visit_span(span);
     visit_thin_attrs(attrs, vis);
     visit_lazy_tts(tokens, vis);

compiler/rustc_ast/src/visit.rs

@@ -242,7 +242,10 @@ pub fn walk_local<'a, V: Visitor<'a>>(visitor: &mut V, local: &'a Local) {
     }
     visitor.visit_pat(&local.pat);
     walk_list!(visitor, visit_ty, &local.ty);
-    walk_list!(visitor, visit_expr, &local.init);
+    if let Some((init, els)) = local.kind.init_else_opt() {
+        visitor.visit_expr(init);
+        walk_list!(visitor, visit_block, els);
+    }
 }
 
 pub fn walk_label<'a, V: Visitor<'a>>(visitor: &mut V, label: &'a Label) {