Playing with proc macros

peg_macro/.gitignore

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+/target

peg_macro/Cargo.toml

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+[package]
+name = "peg_macro"
+version = "0.1.0"
+edition = "2024"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+quote = "1.0.40"

peg_macro/src/lib.rs

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+use proc_macro::TokenStream;
+use quote::quote;
+
+#[proc_macro]
+pub fn grammar(ts: TokenStream) -> TokenStream {
+    let token = ts.into_iter().next().expect("must have at least one token");
+    let result = match token {
+        proc_macro::TokenTree::Literal(literal) => {
+            let s = literal.to_string();
+            let c = s.chars().next().unwrap_or('\0');
+            match c {
+                '0'..'9' => {
+                    let val: usize = s.parse().unwrap();
+                    quote! { Some(crate::peg::Foo::Bar(#val)) }
+                }
+                '"' => {
+                    let inner = &s[1..s.len() - 1];
+                    quote! { Some(crate::peg::Foo::Baz(#inner.into())) }
+                }
+                _ => quote! { None },
+            }
+        }
+        _ => quote! { None },
+    };
+
+    quote! { println!("Hello, world! {:?}", #result); }.into()
+}