hide helper traits from calling code

Makes a couple of changes to avoid helper traits from appearing anywhere in the
caller code.

It changes the name of the `Display` trait method to reduce the chance that an
inherent method conflicts with the type in question, causing undesirable
results. It also wraps the helper trait and declarations into an anonymous const
context, meaning the helper traits aren't visible to the calling context.

src/expand.rs

@@ -4,38 +4,46 @@ use quote::{format_ident, quote};
 use syn::{Data, DataEnum, DataStruct, DeriveInput, Error, Fields, Result};
 
 pub fn derive(input: &DeriveInput) -> Result<TokenStream> {
-    match &input.data {
+    let impls = match &input.data {
         Data::Struct(data) => impl_struct(input, data),
         Data::Enum(data) => impl_enum(input, data),
         Data::Union(_) => Err(Error::new_spanned(input, "Unions are not supported")),
-    }
+    }?;
+
+    let helpers = specialization();
+    Ok(quote! {
+        const _: () = {
+            #helpers
+            #impls
+        };
+    })
 }
 
 #[cfg(feature = "std")]
 fn specialization() -> TokenStream {
     quote! {
         trait DisplayToDisplayDoc {
-            fn get_display(&self) -> Self;
+            fn __displaydoc_display(&self) -> &Self;
         }
 
-        impl<T: core::fmt::Display> DisplayToDisplayDoc for &T {
-            fn get_display(&self) -> Self {
+        impl<T: core::fmt::Display> DisplayToDisplayDoc for T {
+            fn __displaydoc_display(&self) -> &Self {
                 self
             }
         }
 
         trait PathToDisplayDoc {
-            fn get_display(&self) -> std::path::Display<'_>;
+            fn __displaydoc_display(&self) -> std::path::Display<'_>;
         }
 
         impl PathToDisplayDoc for std::path::Path {
-            fn get_display(&self) -> std::path::Display<'_> {
+            fn __displaydoc_display(&self) -> std::path::Display<'_> {
                 self.display()
             }
         }
 
         impl PathToDisplayDoc for std::path::PathBuf {
-            fn get_display(&self) -> std::path::Display<'_> {
+            fn __displaydoc_display(&self) -> std::path::Display<'_> {
                 self.display()
             }
         }
@@ -74,13 +82,7 @@ fn impl_struct(input: &DeriveInput, data: &DataStruct) -> Result<TokenStream> {
         }
     });
 
-    let needed_traits = specialization();
-
-    Ok(quote! {
-        #needed_traits
-
-        #display
-    })
+    Ok(quote! { #display })
 }
 
 fn impl_enum(input: &DeriveInput, data: &DataEnum) -> Result<TokenStream> {
@@ -93,7 +95,7 @@ fn impl_enum(input: &DeriveInput, data: &DataEnum) -> Result<TokenStream> {
         .map(|variant| attr::display(&variant.attrs))
         .collect::<Result<Vec<_>>>()?;
 
-    let display = if displays.iter().any(Option::is_some) {
+    if displays.iter().any(Option::is_some) {
         let arms = data
             .variants
             .iter()
@@ -115,7 +117,7 @@ fn impl_enum(input: &DeriveInput, data: &DataEnum) -> Result<TokenStream> {
                 })
             })
             .collect::<Result<Vec<_>>>()?;
-        Some(quote! {
+        Ok(quote! {
             impl #impl_generics core::fmt::Display for #ty #ty_generics #where_clause {
                 fn fmt(&self, formatter: &mut core::fmt::Formatter) -> core::fmt::Result {
                     #[allow(unused_variables)]
@@ -126,14 +128,6 @@ fn impl_enum(input: &DeriveInput, data: &DataEnum) -> Result<TokenStream> {
             }
         })
     } else {
-        return Err(Error::new_spanned(input, "Missing doc comments"));
-    };
-
-    let needed_traits = specialization();
-
-    Ok(quote! {
-        #needed_traits
-
-        #display
-    })
+        Err(Error::new_spanned(input, "Missing doc comments"))
+    }
 }

src/fmt.rs

@@ -3,11 +3,6 @@ use proc_macro2::TokenStream;
 use quote::quote_spanned;
 use syn::{Ident, LitStr};
 
-#[cfg(feature = "std")]
-const IS_STD: bool = true;
-#[cfg(not(feature = "std"))]
-const IS_STD: bool = false;
-
 macro_rules! peek_next {
     ($read:ident) => {
         match $read.chars().next() {
@@ -49,8 +44,8 @@ impl Display {
 
             let next = peek_next!(read);
 
-            let arg = if IS_STD && next == '}' {
-                quote_spanned!(span=> , (&#ident).get_display())
+            let arg = if cfg!(feature = "std") && next == '}' {
+                quote_spanned!(span=> , #ident.__displaydoc_display())
             } else {
                 quote_spanned!(span=> , #ident)
             };
@@ -120,11 +115,33 @@ mod tests {
         assert(
             "{v} {v:?} {0} {0:?}",
             "{} {:?} {} {:?}",
-            ", ( & v ) . get_display ( ) , v , ( & _0 ) . get_display ( ) , _0",
+            ", v . __displaydoc_display ( ) , v , _0 . __displaydoc_display ( ) , _0",
+        );
+        assert(
+            "error {var}",
+            "error {}",
+            ", var . __displaydoc_display ( )",
+        );
+
+        assert(
+            "The path {0}",
+            "The path {}",
+            ", _0 . __displaydoc_display ( )",
+        );
+        assert("The path {0:?}", "The path {:?}", ", _0");
+    }
+
+    #[test]
+    #[cfg_attr(feature = "std", ignore)]
+    fn test_nostd_expand() {
+        assert(
+            "{v} {v:?} {0} {0:?}",
+            "{} {:?} {} {:?}",
+            ", v , v , _0 , _0",
         );
-        assert("error {var}", "error {}", ", ( & var ) . get_display ( )");
+        assert("error {var}", "error {}", ", var");
 
-        // assert("The path {0.display()}", "The path {}", "0.display()");
-        // assert("The path {0.display():?}", "The path {:?}", "0.display()");
+        assert("The path {0}", "The path {}", ", _0");
+        assert("The path {0:?}", "The path {:?}", ", _0");
     }
 }