Don't generate match expressions for unit structs

While the code generated for unit structs would *technically* work, it
accidentally created a variable binding the value to the struct name.
That struct name is naturally CamelCase, which would then create
compiler warnings inside the macro about `snake_case`.

I don't think this was directly caused by my PR, but empty structs are
now treated as unit structs, and unit structs were broken, so this
surfaced the issue.

crates/turbo-tasks-macros-shared/src/expand.rs

@@ -22,14 +22,15 @@ use syn::{
 pub fn match_expansion<
     EN: Fn(&Ident, &FieldsNamed) -> (TokenStream, TokenStream),
     EU: Fn(&Ident, &FieldsUnnamed) -> (TokenStream, TokenStream),
-    U: Fn(&Ident) -> (TokenStream, TokenStream),
+    U: Fn(&Ident) -> TokenStream,
 >(
     derive_input: &DeriveInput,
     expand_named: &EN,
     expand_unnamed: &EU,
     expand_unit: &U,
 ) -> TokenStream {
     let ident = &derive_input.ident;
+    let expand_unit = move |ident| (TokenStream::new(), expand_unit(ident));
     match &derive_input.data {
         Data::Enum(DataEnum { variants, .. }) => {
             let (variants_idents, (variants_fields_capture, expansion)): (
@@ -63,9 +64,19 @@ pub fn match_expansion<
             let (captures, expansion) =
                 expand_fields(ident, fields, expand_named, expand_unnamed, expand_unit);
 
-            quote! {
-                match self {
-                    #ident #captures => #expansion
+            if fields.is_empty() {
+                assert!(captures.is_empty());
+                // a match expression here doesn't make sense as there's no fields to capture,
+                // just pass through the inner expression.
+                expansion
+            } else {
+                match fields {
+                    Fields::Named(_) | Fields::Unnamed(_) => quote! {
+                        match self {
+                            #ident #captures => #expansion
+                        }
+                    },
+                    Fields::Unit => unreachable!(),
                 }
             }
         }
@@ -82,6 +93,10 @@ pub fn match_expansion<
 }
 
 /// Formats the fields of any structure or enum variant.
+///
+/// Empty lists of named or unnamed fields are treated as unit structs, as they
+/// are semantically identical, and the `expand_unit` codepath can usually
+/// generate better code.
 pub fn expand_fields<
     'ident,
     'fields,

crates/turbo-tasks-macros/src/derive/deterministic_hash_macro.rs

@@ -62,6 +62,6 @@ fn hash_unnamed(_ident: &Ident, fields: &FieldsUnnamed) -> (TokenStream2, TokenS
 }
 
 /// Hashes a unit struct or enum variant (e.g. `struct Foo;`, `Foo::Bar`).
-fn hash_unit(_ident: &Ident) -> (TokenStream2, TokenStream2) {
-    (quote! {}, quote! { { } })
+fn hash_unit(_ident: &Ident) -> TokenStream2 {
+    quote! { { } }
 }

crates/turbo-tasks-macros/src/derive/trace_raw_vcs_macro.rs

@@ -56,6 +56,6 @@ fn trace_unnamed(_ident: &Ident, fields: &FieldsUnnamed) -> (TokenStream2, Token
     )
 }
 
-fn trace_unit(_ident: &Ident) -> (TokenStream2, TokenStream2) {
-    (quote! {}, quote! { { } })
+fn trace_unit(_ident: &Ident) -> TokenStream2 {
+    quote! { { } }
 }

crates/turbo-tasks-macros/src/derive/value_debug_format_macro.rs

@@ -116,14 +116,11 @@ fn format_unnamed(ident: &Ident, fields: &FieldsUnnamed) -> (TokenStream2, Token
 }
 
 /// Formats a unit struct or enum variant (e.g. `struct Foo;`, `Foo::Bar`).
-fn format_unit(ident: &Ident) -> (TokenStream2, TokenStream2) {
-    (
-        quote! {},
-        quote! {
-            FormattingStruct::new_unnamed(
-                stringify!(#ident),
-                vec![],
-            )
-        },
-    )
+fn format_unit(ident: &Ident) -> TokenStream2 {
+    quote! {
+        FormattingStruct::new_unnamed(
+            stringify!(#ident),
+            vec![],
+        )
+    }
 }