Implement transparent translation for newtypes and enums

aws_macros/src/tag.rs

@@ -1,10 +1,107 @@
 use proc_macro::TokenStream;
 use quote::quote;
 
+#[derive(Debug)]
+enum TransparentKind {
+    NewtypeStruct {
+        ty: syn::Type,
+    },
+    SimpleEnum {
+        variants: Vec<(syn::Ident, Option<syn::LitStr>)>,
+    },
+}
+
 #[derive(Debug)]
 enum Translator {
     Serde,
     Manual,
+    Transparent(TransparentKind),
+}
+
+fn parse_enum_attributes(attrs: &mut Vec<syn::Attribute>) -> Option<syn::LitStr> {
+    let index_of_tag_attribute = attrs
+        .iter()
+        .enumerate()
+        .filter(|&(_i, attr)| attr.style == syn::AttrStyle::Outer)
+        .find_map(|(i, attr)| match attr.meta {
+            syn::Meta::List(ref meta_list) => {
+                if let (Some(name), 1) = (
+                    meta_list.path.segments.first(),
+                    meta_list.path.segments.len(),
+                ) {
+                    if name.ident == "tag" {
+                        Some((i, meta_list.clone()))
+                    } else {
+                        None
+                    }
+                } else {
+                    None
+                }
+            }
+            _ => None,
+        });
+
+    match index_of_tag_attribute {
+        Some((i, meta_list)) => {
+            // `i` came from `enumerate()` and is guaranteed to be in bounds
+            let removed_attribute = attrs.remove(i);
+            drop(removed_attribute);
+
+            let expr: syn::Expr =
+                syn::parse(meta_list.tokens.into()).expect("expected expr in macro attribute");
+
+            match expr {
+                syn::Expr::Assign(ref assign) => {
+                    match *assign.left {
+                        syn::Expr::Path(ref exprpath) => {
+                            let segments = &exprpath.path.segments;
+                            let (Some(segment), 1) = (segments.first(), segments.len()) else {
+                                panic!("invalid attribute key")
+                            };
+
+                            assert!(segment.ident == "rename", "invalid attribute key");
+                        }
+                        _ => panic!("invalid expression in enum variant attribute, left side"),
+                    }
+
+                    match *assign.right {
+                        syn::Expr::Lit(ref expr_lit) => match expr_lit.lit {
+                            syn::Lit::Str(ref lit_str) => Some(lit_str.clone()),
+                            _ => panic!("invalid literal in enum variant attribute"),
+                        },
+                        _ => panic!("invalid expression in enum variant attribute, right side"),
+                    }
+                }
+                _ => panic!("invalid expression in enum variant attribute"),
+            }
+        }
+        None => None,
+    }
+}
+
+fn parse_transparent_enum(mut e: syn::ItemEnum) -> (Translator, syn::Item) {
+    let variants = e
+        .variants
+        .iter_mut()
+        .map(|variant| {
+            assert!(
+                variant.discriminant.is_none(),
+                "variant cannot have an explicit discriminant"
+            );
+            match variant.fields {
+                syn::Fields::Unit => (),
+                _ => panic!("enum cannot have fields in variants"),
+            }
+            let rename = parse_enum_attributes(&mut variant.attrs);
+
+            (variant.ident.clone(), rename)
+        })
+        .collect::<Vec<(syn::Ident, Option<syn::LitStr>)>>();
+
+    (
+        Translator::Transparent(TransparentKind::SimpleEnum { variants }),
+        e.into(),
+    )
 }
 
 pub(crate) fn transform(attr: TokenStream, item: TokenStream) -> TokenStream {
@@ -12,6 +109,8 @@ pub(crate) fn transform(attr: TokenStream, item: TokenStream) -> TokenStream {
 
     let expr: syn::Expr = syn::parse(attr).expect("expected expr in macro attribute");
 
+    let elem = syn::parse_macro_input!(item as syn::Item);
+
     let syn::Expr::Assign(assign) = expr else {
         panic!("invalid expression in macro attribute")
     };
@@ -28,7 +127,7 @@ pub(crate) fn transform(attr: TokenStream, item: TokenStream) -> TokenStream {
         _ => panic!("invalid expression in tag field attribute, left side"),
     }
 
-    let translator = match *assign.right {
+    let (translator, elem) = match *assign.right {
         syn::Expr::Path(ref exprpath) => {
             let segments = &exprpath.path.segments;
             let (Some(segment), 1) = (segments.first(), segments.len()) else {
@@ -36,16 +135,40 @@ pub(crate) fn transform(attr: TokenStream, item: TokenStream) -> TokenStream {
             };
 
             match segment.ident.to_string().as_str() {
-                "serde" => Translator::Serde,
-                "manual" => Translator::Manual,
+                "serde" => (Translator::Serde, elem),
+                "manual" => (Translator::Manual, elem),
+                "transparent" => match elem {
+                    syn::Item::Struct(ref s) => match s.fields {
+                        syn::Fields::Unnamed(ref fields) => {
+                            let (Some(field), 1) = (fields.unnamed.first(), fields.unnamed.len())
+                            else {
+                                panic!(
+                                        "transparent translation is only available for newtype-style macros"
+                                    )
+                            };
+                            (
+                                Translator::Transparent(TransparentKind::NewtypeStruct {
+                                    ty: field.ty.clone(),
+                                }),
+                                elem,
+                            )
+                        }
+                        _ => panic!(
+                            "transparent translation is only available for newtype-style macros"
+                        ),
+                    },
+
+                    syn::Item::Enum(e) => parse_transparent_enum(e),
+                    _ => {
+                        panic!("transparent translation is only available for newtype-style macros")
+                    }
+                },
                 t => panic!("invalid translator {t}"),
             }
         }
         _ => panic!("invalid expression in tag field attribute, left side"),
     };
 
-    let elem = syn::parse_macro_input!(item as syn::Item);
-
     let name = match elem {
         syn::Item::Struct(ref s) => &s.ident,
         syn::Item::Enum(ref e) => &e.ident,
@@ -69,6 +192,82 @@ pub(crate) fn transform(attr: TokenStream, item: TokenStream) -> TokenStream {
                 type Translator = #root::tags::TranslateManual;
             }
         },
+        Translator::Transparent(kind) => match kind {
+            TransparentKind::NewtypeStruct { ty } => quote! {
+                impl #root::tags::TranslatableManual for #name {}
+
+                impl #root::tags::TagValue<#name> for #name {
+                    type Error = #root::tags::ParseTagValueError;
+                    type Translator = #root::tags::TranslateManual;
+                }
+
+                impl TryFrom<#root::tags::RawTagValue> for #name {
+                    type Error = #root::tags::ParseTagValueError;
+
+                    fn try_from(value: #root::tags::RawTagValue) -> Result<Self, Self::Error> {
+                        Ok(Self(<#ty as #root::tags::TagValue<#ty>>::from_raw_tag(value)?))
+                    }
+                }
+
+                impl From<#name> for RawTagValue {
+                    fn from(value: #name) -> Self {
+                        <#ty as #root::tags::TagValue<#ty>>::into_raw_tag(value.0)
+                    }
+                }
+            },
+
+            TransparentKind::SimpleEnum { variants } => {
+                let (into_raw_tag_mapping, from_raw_tag_mapping): (Vec<_>, Vec<_>) = variants
+                    .into_iter()
+                    .map(|(variant, rename)| {
+                        let lit = rename
+                            .map(|r| r.value())
+                            .unwrap_or_else(|| variant.to_string());
+                        (
+                            quote! {
+                                #name::#variant => #root::tags::RawTagValue::new(#lit.to_owned()),
+                            },
+                            quote! {
+                                #lit => Self::#variant,
+                            },
+                        )
+                    })
+                    .unzip();
+
+                quote! {
+                    impl #root::tags::TranslatableManual for #name {}
+
+                    impl #root::tags::TagValue<#name> for #name {
+                        type Error = #root::tags::ParseTagValueError;
+                        type Translator = #root::tags::TranslateManual;
+                    }
+
+                    impl From<#name> for #root::tags::RawTagValue {
+                        fn from(value: #name) -> Self {
+                            match value {
+                                #(#into_raw_tag_mapping)
+                                *
+                            }
+                        }
+                    }
+
+                    impl TryFrom<#root::tags::RawTagValue> for #name {
+                        type Error = #root::tags::ParseTagValueError;
+
+                        fn try_from(value: #root::tags::RawTagValue) -> Result<Self, Self::Error> {
+                            Ok(match value.as_str() {
+                                #(#from_raw_tag_mapping)
+                                *
+                                _ => return Err(#root::tags::ParseTagValueError::InvalidValue {
+                                    value,
+                                    message: "invalid enum value".to_owned(),
+                                }),
+                            })
+                        }
+                    }
+                }
+            }
+        },
     };
 
     quote! {