hide unexpected_cfgs warnings in proc-macro generated code (#287)

* replace #[derive(Debug)] with manual impls

The new impls should be identical, but they have the advantage that we
can move them around. See the next patch for why this is useful.

* allow unexpected_cfgs lint

AFAICT it's not possible to set `#[allow(unexpected_cfgs)]` on a
`#[cfg(...)]` or `#[cfg_attr(...)]` directly. Instead, we have to crate
a new scope and use `#[allow(...)]` on that. This patch wraps all uses
of `target_arch = "spirv"` in an unnamed constant scope and uses
`#[allow(unexpected_cfgs)]` on it.

derive/src/traits.rs

@@ -607,19 +607,27 @@ fn generate_checked_bit_pattern_struct(
   let field_name = &field_names[..];
   let field_ty = &field_tys[..];
 
-  let derive_dbg =
-    quote!(#[cfg_attr(not(target_arch = "spirv"), derive(Debug))]);
-
   Ok((
     quote! {
         #[doc = #GENERATED_TYPE_DOCUMENTATION]
         #repr
         #[derive(Clone, Copy, #crate_name::AnyBitPattern)]
-        #derive_dbg
         #[allow(missing_docs)]
         pub struct #bits_ty {
             #(#field_name: <#field_ty as #crate_name::CheckedBitPattern>::Bits,)*
         }
+
+        #[allow(unexpected_cfgs)]
+        const _: () = {
+          #[cfg(not(target_arch = "spirv"))]
+          impl ::core::fmt::Debug for #bits_ty {
+            fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
+              let mut debug_struct = ::core::fmt::Formatter::debug_struct(f, ::core::stringify!(#bits_ty));
+              #(::core::fmt::DebugStruct::field(&mut debug_struct, ::core::stringify!(#field_name), &self.#field_name);)*
+              ::core::fmt::DebugStruct::finish(&mut debug_struct)
+            }
+          }
+        };
     },
     quote! {
         type Bits = #bits_ty;
@@ -711,9 +719,6 @@ fn generate_checked_bit_pattern_enum_with_fields(
   let representation = get_repr(&input.attrs)?;
   let vis = &input.vis;
 
-  let derive_dbg =
-    quote!(#[cfg_attr(not(target_arch = "spirv"), derive(Debug))]);
-
   match representation.repr {
     Repr::Rust => unreachable!(),
     repr @ (Repr::C | Repr::CWithDiscriminant(_)) => {
@@ -793,27 +798,42 @@ fn generate_checked_bit_pattern_enum_with_fields(
         quote! {
           #[doc = #GENERATED_TYPE_DOCUMENTATION]
           #[derive(::core::clone::Clone, ::core::marker::Copy, #crate_name::AnyBitPattern)]
-          #derive_dbg
           #bits_repr
           #vis struct #bits_ty_ident {
             tag: #integer,
             payload: #variants_union_ident,
           }
 
+          #[allow(unexpected_cfgs)]
+          const _: () = {
+            #[cfg(not(target_arch = "spirv"))]
+            impl ::core::fmt::Debug for #bits_ty_ident {
+              fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
+                let mut debug_struct = ::core::fmt::Formatter::debug_struct(f, ::core::stringify!(#bits_ty_ident));
+                ::core::fmt::DebugStruct::field(&mut debug_struct, "tag", &self.tag);
+                ::core::fmt::DebugStruct::field(&mut debug_struct, "payload", &self.payload);
+                ::core::fmt::DebugStruct::finish(&mut debug_struct)
+              }
+            }
+          };
+
           #[derive(::core::clone::Clone, ::core::marker::Copy, #crate_name::AnyBitPattern)]
           #[repr(C)]
           #[allow(non_snake_case)]
           #vis union #variants_union_ident {
             #(#union_fields,)*
           }
 
-          #[cfg(not(target_arch = "spirv"))]
-          impl ::core::fmt::Debug for #variants_union_ident {
-            fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
-              let mut debug_struct = ::core::fmt::Formatter::debug_struct(f, ::core::stringify!(#variants_union_ident));
-              ::core::fmt::DebugStruct::finish_non_exhaustive(&mut debug_struct)
+          #[allow(unexpected_cfgs)]
+          const _: () = {
+            #[cfg(not(target_arch = "spirv"))]
+            impl ::core::fmt::Debug for #variants_union_ident {
+              fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
+                let mut debug_struct = ::core::fmt::Formatter::debug_struct(f, ::core::stringify!(#variants_union_ident));
+                ::core::fmt::DebugStruct::finish_non_exhaustive(&mut debug_struct)
+              }
             }
-          }
+          };
 
           #(#variant_struct_definitions)*
         },
@@ -930,14 +950,17 @@ fn generate_checked_bit_pattern_enum_with_fields(
             #(#union_fields,)*
           }
 
-          #[cfg(not(target_arch = "spirv"))]
-          impl ::core::fmt::Debug for #bits_ty_ident {
-            fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
-              let mut debug_struct = ::core::fmt::Formatter::debug_struct(f, ::core::stringify!(#bits_ty_ident));
-              ::core::fmt::DebugStruct::field(&mut debug_struct, "tag", unsafe { &self.__tag });
-              ::core::fmt::DebugStruct::finish_non_exhaustive(&mut debug_struct)
+          #[allow(unexpected_cfgs)]
+          const _: () = {
+            #[cfg(not(target_arch = "spirv"))]
+            impl ::core::fmt::Debug for #bits_ty_ident {
+              fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
+                let mut debug_struct = ::core::fmt::Formatter::debug_struct(f, ::core::stringify!(#bits_ty_ident));
+                ::core::fmt::DebugStruct::field(&mut debug_struct, "tag", unsafe { &self.__tag });
+                ::core::fmt::DebugStruct::finish_non_exhaustive(&mut debug_struct)
+              }
             }
-          }
+          };
 
           #(#variant_struct_definitions)*
         },