Accept shorthand for the format string

impl/src/attr.rs

@@ -36,7 +36,9 @@ impl Parse for Display {
             args.extend(once(token));
         }
 
-        Ok(Display { fmt, args })
+        let mut display = Display { fmt, args };
+        display.expand_shorthand();
+        Ok(display)
     }
 }
 

impl/src/fmt.rs

@@ -0,0 +1,96 @@
+use crate::attr::Display;
+use proc_macro2::TokenStream;
+use quote::quote_spanned;
+use syn::{Ident, LitStr};
+
+impl Display {
+    // Transform `"error {var}"` to `"error {}", var`.
+    pub fn expand_shorthand(&mut self) {
+        if !self.args.is_empty() {
+            return;
+        }
+
+        let span = self.fmt.span();
+        let fmt = self.fmt.value();
+        let mut read = fmt.as_str();
+        let mut out = String::new();
+        let mut args = TokenStream::new();
+
+        while let Some(brace) = read.find('{') {
+            out += &read[..brace + 1];
+            read = &read[brace + 1..];
+            if read.starts_with('{') {
+                out.push('{');
+                read = &read[1..];
+                continue;
+            }
+            let next = match read.chars().next() {
+                Some(next) => next,
+                None => return,
+            };
+            let var = match next {
+                '0'..='9' => take_int(&mut read),
+                'a'..='z' | 'A'..='Z' | '_' => take_ident(&mut read),
+                _ => return,
+            };
+            let ident = Ident::new(&var, span);
+            args.extend(quote_spanned!(span=> , #ident));
+        }
+
+        out += read;
+        self.fmt = LitStr::new(&out, self.fmt.span());
+        self.args = args;
+    }
+}
+
+fn take_int(read: &mut &str) -> String {
+    let mut int = String::new();
+    int.push('_');
+    for (i, ch) in read.char_indices() {
+        match ch {
+            '0'..='9' => int.push(ch),
+            _ => {
+                *read = &read[i..];
+                break;
+            }
+        }
+    }
+    int
+}
+
+fn take_ident(read: &mut &str) -> String {
+    let mut ident = String::new();
+    for (i, ch) in read.char_indices() {
+        match ch {
+            'a'..='z' | 'A'..='Z' | '_' => ident.push(ch),
+            _ => {
+                *read = &read[i..];
+                break;
+            }
+        }
+    }
+    ident
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use proc_macro2::Span;
+
+    fn assert(input: &str, fmt: &str, args: &str) {
+        let mut display = Display {
+            fmt: LitStr::new(input, Span::call_site()),
+            args: TokenStream::new(),
+        };
+        display.expand_shorthand();
+        assert_eq!(fmt, display.fmt.value());
+        assert_eq!(args, display.args.to_string());
+    }
+
+    #[test]
+    fn test_expand() {
+        assert("error {var}", "error {}", ", var");
+        assert("fn main() {{ }}", "fn main() {{ }}", "");
+        assert("{v} {v:?} {0} {0:?}", "{} {:?} {} {:?}", ", v , v , _0 , _0");
+    }
+}

impl/src/lib.rs

@@ -2,6 +2,7 @@ extern crate proc_macro;
 
 mod attr;
 mod expand;
+mod fmt;
 
 use proc_macro::TokenStream;
 use syn::{parse_macro_input, DeriveInput};