Experimentally add *heterogeneous* try blocks

148725 moved the default to being homogeneous; this adds heterogeneous ones back under an obvious-bikeshed syntax so people can experiment with that as well.

Essentially resolves 149025 by letting them move to this syntax instead.

compiler/rustc_ast/src/ast.rs

@@ -1806,8 +1806,14 @@ pub enum ExprKind {
     /// A use expression (`x.use`). Span is of use keyword.
     Use(Box<Expr>, Span),
 
-    /// A try block (`try { ... }`).
-    TryBlock(Box<Block>),
+    /// A try block (`try { ... }`), if the type is `None`, or
+    /// A try block (`try bikeshed Ty { ... }`) if the type is `Some`.
+    ///
+    /// Note that `try bikeshed` is a *deliberately ridiculous* placeholder
+    /// syntax to avoid deciding what keyword or symbol should go there.
+    /// It's that way for experimentation only; an RFC to decide the final
+    /// semantics and syntax would be needed to put it on stabilization-track.
+    TryBlock(Box<Block>, Option<Box<Ty>>),
 
     /// An assignment (`a = foo()`).
     /// The `Span` argument is the span of the `=` token.

compiler/rustc_ast/src/visit.rs

@@ -1048,8 +1048,8 @@ macro_rules! common_visitor_and_walkers {
                     visit_visitable!($($mut)? vis, kind),
                 ExprKind::Try(subexpression) =>
                     visit_visitable!($($mut)? vis, subexpression),
-                ExprKind::TryBlock(body) =>
-                    visit_visitable!($($mut)? vis, body),
+                ExprKind::TryBlock(body, optional_type) =>
+                    visit_visitable!($($mut)? vis, body, optional_type),
                 ExprKind::Lit(token) =>
                     visit_visitable!($($mut)? vis, token),
                 ExprKind::IncludedBytes(bytes) =>