Generate a method for static retrieval of the SyntaxKind of a node, where possible

This will help for the quote macro for `ast::make`.

crates/syntax/src/ast.rs

@@ -42,6 +42,14 @@ pub use self::{
 /// the same representation: a pointer to the tree root and a pointer to the
 /// node itself.
 pub trait AstNode {
+    /// This panics if the `SyntaxKind` is not statically known.
+    fn kind() -> SyntaxKind
+    where
+        Self: Sized,
+    {
+        panic!("dynamic `SyntaxKind` for `AstNode::kind()`")
+    }
+
     fn can_cast(kind: SyntaxKind) -> bool
     where
         Self: Sized;

xtask/src/codegen/grammar.rs

@@ -162,6 +162,13 @@ fn generate_nodes(kinds: KindsSrc, grammar: &AstSrc) -> String {
                 },
                 quote! {
                     impl AstNode for #name {
+                        #[inline]
+                        fn kind() -> SyntaxKind
+                        where
+                            Self: Sized
+                        {
+                            #kind
+                        }
                         #[inline]
                         fn can_cast(kind: SyntaxKind) -> bool {
                             kind == #kind