Only eval args once

mono-fmt-macro/src/to_tokens.rs

@@ -1,6 +1,6 @@
 use std::cell::Cell;
 
-use proc_macro2::{Ident, TokenStream};
+use proc_macro2::{Ident, Span, TokenStream};
 use quote::{quote, ToTokens};
 
 use crate::{
@@ -17,6 +17,14 @@ pub(crate) struct Scoped<'a, T> {
     inner: &'a T,
 }
 
+fn pos_arg_ident(idx: usize) -> Ident {
+    Ident::new(&format!("__pos_arg_{idx}"), Span::mixed_site())
+}
+
+fn named_arg_ident(name: impl std::fmt::Display) -> Ident {
+    Ident::new(&format!("__named_arg_{name}"), Span::mixed_site())
+}
+
 impl<'a, T> Scoped<'a, T> {
     pub fn new(input: &'a Input, inner: &'a T, current_position: &'a Cell<usize>) -> Self {
         Self {
@@ -46,10 +54,33 @@ impl ToTokens for Scoped<'_, Format<'_>> {
     fn to_tokens(&self, tokens: &mut TokenStream) {
         let parts = self.inner.pieces.iter().map(|piece| self.scope(piece));
 
+        let input = &self.input;
+
+        let pos_args = input.positional_args.iter();
+        let named_args = input.named_args.iter().map(|(_, expr)| expr);
+
+        let args = pos_args.chain(named_args);
+
+        let pos_idents = input
+            .positional_args
+            .iter()
+            .enumerate()
+            .map(|(idx, _)| pos_arg_ident(idx));
+
+        let named_idents = input
+            .named_args
+            .iter()
+            .map(|(name, _)| named_arg_ident(name));
+
+        let idents = pos_idents.chain(named_idents);
+
         tokens.extend(quote! {
-            (
+            #[allow(unused_parens)]
+            match (#(#args),*) {
+                (#(#idents),*) => (
                     #(#parts),*
                 )
+            }
         })
     }
 }
@@ -80,20 +111,16 @@ impl ToTokens for Scoped<'_, FormatArg<'_>> {
                 let current_position = self.current_position.get();
                 self.current_position.set(current_position + 1);
 
-                self.input.positional_args[current_position].to_token_stream()
-            }
-            Some(FormatArgRef::Positional(idx)) => {
-                self.input.positional_args[idx].to_token_stream()
+                pos_arg_ident(current_position)
             }
+            Some(FormatArgRef::Positional(idx)) => pos_arg_ident(idx),
             Some(FormatArgRef::Named(name)) => self
                 .input
                 .named_args
                 .iter()
                 .find(|(arg, _)| arg == name)
-                .map(|(_, expr)| expr.to_token_stream())
-                .unwrap_or_else(|| {
-                    Ident::new(name, self.input.format_str.span()).to_token_stream()
-                }),
+                .map(|(name, _)| named_arg_ident(name))
+                .unwrap_or_else(|| Ident::new(name, self.input.format_str.span())),
         };
 
         let opt_ty = opt_ty_tokens(self.scope(&self.inner.format_spec.formatter_args));

src/lib.rs

@@ -116,8 +116,19 @@ mod tests {
 }
 
 pub mod uwu {
-    fn test_format_debug_hex() {
-        assert_eq!(format!("{:02x?}", b"Foo\0"), "[46, 6f, 6f, 00]");
-        assert_eq!(format!("{:02X?}", b"Foo\0"), "[46, 6F, 6F, 00]");
+    use std::cell::Cell;
+
+    fn test_expansion() {
+        let evil = Cell::new(0);
+        format!(
+            "{0}{0}{1}{owo}",
+            {
+                evil.set(evil.get() + 1);
+                0
+            },
+            5,
+            owo = "owo",
+        );
+        // assert_eq!(evil.get(), 1);
     }
 }

tests/base.rs

@@ -1,3 +1,5 @@
+use std::cell::Cell;
+
 #[macro_use]
 extern crate mono_fmt;
 
@@ -17,3 +19,13 @@ fn test_pointer_formats_data_pointer() {
     assert_eq!(format!("{s:p}"), format!("{:p}", s.as_ptr()));
     assert_eq!(format!("{b:p}"), format!("{:p}", b.as_ptr()));
 }
+
+#[test]
+fn only_eval_once() {
+    let evil = Cell::new(0);
+    let _ = format!("{0} {0}", {
+        evil.set(evil.get() + 1);
+        0
+    });
+    assert_eq!(evil.get(), 1);
+}