support optional fields

You can do `$?` to try and parse something.
This is convenient.

Author: nikomatsakis

crates/formality-macros/src/debug.rs

@@ -149,7 +149,7 @@ fn debug_variant_with_attr(
         stream.extend(match op {
             spec::FormalitySpecOp::Field {
                 name,
-                mode: FieldMode::Single,
+                mode: FieldMode::Single | FieldMode::Optional,
             } => {
                 quote_spanned! {
                     name.span() =>

crates/formality-macros/src/parse.rs

@@ -157,6 +157,17 @@ fn parse_variant_with_attr(
                 }
             }
 
+            spec::FormalitySpecOp::Field {
+                name,
+                mode: FieldMode::Optional,
+            } => {
+                quote_spanned! {
+                    name.span() => 
+                    let (#name, text) = parse::CoreParse::parse_opt(scope, text)?;
+                    let #name = #name.unwrap_or_default();
+                }
+            }
+
             spec::FormalitySpecOp::Field {
                 name,
                 mode: FieldMode::Many,

crates/formality-macros/src/spec.rs

@@ -45,6 +45,9 @@ pub enum FieldMode {
     /// If the next op is a fixed character, stop parsing when we see that.
     /// Otherwise parse as many we can greedily.
     Comma,
+
+    /// $?x -- parse `x` if we can, but otherwise use `Default`
+    Optional,
 }
 
 impl syn::parse::Parse for FormalitySpec {
@@ -124,6 +127,7 @@ fn parse_variable_binding(
             let mode = match punct.as_char() {
                 ',' => FieldMode::Comma,
                 '*' => FieldMode::Many,
+                '?' => FieldMode::Optional,
                 '$' => return Ok(FormalitySpecOp::Char { punct }),
                 _ => return error(),
             };