peepmatic: Introduce the peepmatic-macro crate

This crate provides the derive macros used by `peepmatic`, notable AST-related
derives that enumerate child AST nodes, and operator-related derives that
provide helpers for type checking.

cranelift/peepmatic/crates/macro/src/into_dyn_ast_ref.rs

@@ -0,0 +1,23 @@
+use quote::quote;
+use syn::DeriveInput;
+use syn::Result;
+
+pub fn derive_into_dyn_ast_ref(input: &DeriveInput) -> Result<impl quote::ToTokens> {
+    let ty = &input.ident;
+
+    let opts = crate::PeepmaticOpts::from_attrs(&mut input.attrs.clone())?;
+    if opts.no_into_dyn_node {
+        return Ok(quote! {});
+    }
+
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    Ok(quote! {
+        impl #impl_generics From<&'a #ty #ty_generics> for DynAstRef<'a> #where_clause {
+            #[inline]
+            fn from(x: &'a #ty #ty_generics) -> Self {
+                Self::#ty(x)
+            }
+        }
+    })
+}