Add experimental cfg for using the proc-macro-expand API

rust-lang/rust#90765

.github/workflows/nightly.yml

@@ -18,6 +18,21 @@ jobs:
         command: test
         args: --workspace --locked --all-features
 
+    name: cargo +nightly test --cfg stylish_proc_macro_expand
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: hecrj/setup-rust-action@v1
+      with:
+        rust-version: nightly
+    - uses: actions-rs/cargo@v1
+      env:
+        RUSTFLAGS: --cfg stylish_proc_macro_expand
+      with:
+        toolchain: nightly
+        command: test
+        args: --workspace --locked --all-features
+
   fmt:
     name: cargo +nightly fmt --check
     runs-on: ubuntu-latest

core/src/lib.rs

@@ -96,6 +96,19 @@ pub mod 𓀄 {
 /// ```
 #[macro_export]
 macro_rules! format_args {
+    ($fmt:literal $(, $($arg:tt)*)?) => {
+        $crate::_format_args!($fmt $(, $($arg)*)?)
+    };
+    ($fmt:expr $(, $($arg:tt)*)?) => {
+        $crate::_format_args!($fmt $(, $($arg)*)?)
+    };
+}
+
+#[cfg(feature = "macros")]
+#[cfg(not(stylish_proc_macro_expand))]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! _format_args {
     ($fmt:literal $(, $($arg:tt)*)?) => {
         $crate::𓀄::format_args!(crate=$crate, $fmt $(, $($arg)*)?)
     };
@@ -105,3 +118,13 @@ macro_rules! format_args {
         })
     };
 }
+
+#[cfg(feature = "macros")]
+#[cfg(stylish_proc_macro_expand)]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! _format_args {
+    ($fmt:expr $(, $($arg:tt)*)?) => {
+        $crate::𓀄::format_args!(crate=$crate, $fmt $(, $($arg)*)?)
+    };
+}

macros/src/lib.rs

@@ -11,6 +11,7 @@
 #![warn(unused_extern_crates)]
 #![warn(unused_import_braces)]
 #![warn(variant_size_differences)]
+#![cfg_attr(stylish_proc_macro_expand, feature(proc_macro_expand))]
 
 use std::collections::HashMap;
 
@@ -48,7 +49,23 @@ impl syn::parse::Parse for ArgsInput {
         } else {
             None
         };
+        #[cfg(not(stylish_proc_macro_expand))]
         let format = input.parse()?;
+        #[cfg(stylish_proc_macro_expand)]
+        let format = {
+            use syn::spanned::Spanned;
+            let expr = input.parse::<Expr>()?;
+            let span = expr.span();
+            let tokens = proc_macro::TokenStream::from(expr.into_token_stream());
+            let expanded = tokens.expand_expr().map_err(|e| {
+                syn::parse::Error::new(span, format!("failed to expand format string: {e}"))
+            })?;
+            #[cfg(stylish_proc_macro_expand_debug)]
+            if expanded.to_string() != tokens.to_string() {
+                eprintln!("{tokens} => {expanded}");
+            }
+            syn::parse(expanded)?
+        };
         let mut positional_args = Vec::new();
         let mut named_args = Vec::new();
         let mut onto_named = false;

tests/tests.rs

@@ -182,4 +182,12 @@ mod tests {
             r#""a"b"#
         );
     }
+
+    #[cfg(stylish_proc_macro_expand)]
+    #[test]
+    fn extended_builtin_macros() {
+        // `file!()` is not supported by `with_builtin_macros`, but it works with
+        // `proc_macro_expand`
+        assert_eq!(stylish::plain::format!(file!()), file!());
+    }
 }