Pass through format args directly to support named arguments etc

Author: andrewhickman

Cargo.toml

@@ -12,6 +12,7 @@ autotests = false
 
 [dependencies]
 quote = "1.0.3"
+proc-macro2 = "1.0.9"
 
 [dependencies.syn]
 features = ["full", "parsing"]

src/lib.rs

@@ -2,38 +2,17 @@ extern crate proc_macro;
 
 use proc_macro::TokenStream;
 use quote::quote;
-use syn::parse::{Parse, ParseStream};
-use syn::punctuated::Punctuated;
-use syn::token::Comma;
 
 #[proc_macro_attribute]
 pub fn error_context(args: TokenStream, input: TokenStream) -> TokenStream {
-    let Args { fmt, args } = syn::parse_macro_input!(args as Args);
+    let args: proc_macro2::TokenStream = args.into();
     let mut input = syn::parse_macro_input!(input as syn::ItemFn);
 
     let body = &input.block;
     let return_ty = &input.sig.output;
     input.block = syn::parse_quote!({
-        (|| #return_ty #body)().map_err(|err| err.context(format!(#fmt, #args)).into())
+        (|| #return_ty #body)().map_err(|err| err.context(format!(#args)).into())
     });
 
     quote!(#input).into()
 }
-
-struct Args {
-    fmt: syn::LitStr,
-    args: Punctuated<syn::Expr, Comma>,
-}
-
-impl Parse for Args {
-    fn parse(input: ParseStream) -> syn::Result<Self> {
-        let fmt = input.parse()?;
-        let args = if let Some(_) = input.parse::<Option<Comma>>()? {
-            Punctuated::parse_separated_nonempty(input)?
-        } else {
-            Punctuated::new()
-        };
-
-        Ok(Args { fmt, args })
-    }
-}

tests/fmt_named_arg.rs

@@ -0,0 +1,13 @@
+use fn_error_context::error_context;
+
+#[error_context("context {arg}", arg = arg)]
+fn do_stuff(arg: u32) -> anyhow::Result<()> {
+    anyhow::bail!("error {}", arg)
+}
+
+fn main() {
+    assert_eq!(
+        format!("{:#}", do_stuff(1).unwrap_err()),
+        "context 1: error 1"
+    );
+}

tests/missing_fmt.stderr

@@ -1,5 +1,7 @@
-error: unexpected end of input, expected literal
+error: requires at least a format string argument
  --> $DIR/missing_fmt.rs:3:1
   |
 3 | #[error_context]
   | ^^^^^^^^^^^^^^^^
+  |
+  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

tests/tests.rs

@@ -14,4 +14,5 @@ fn tests() {
     tests.pass("tests/as_ref.rs");
     tests.compile_fail("tests/fmt_missing_arg.rs");
     tests.compile_fail("tests/fmt_unused_arg.rs");
+    tests.pass("tests/fmt_named_arg.rs");
 }