Add support for generics

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## Unreleased
 
+- Fixed: Structs with generics are now supported (https://github.com/heroku-buildpacks/cache_diff/pull/12)
 - Fixed: Use fully qulified path to `::std::vec::Vec` (https://github.com/heroku-buildpacks/cache_diff/pull/8)
 
 ## 1.1.0

cache_diff_derive/src/cache_diff_container.rs

@@ -35,6 +35,8 @@ use syn::{DataStruct, FieldsNamed, Ident};
 pub(crate) struct CacheDiffContainer {
     /// The identifier of a struct e.g. `struct Metadata {version: String}` would be `Metadata`
     pub(crate) identifier: Ident,
+    /// Info about generics, lifetimes and where clauses i.e. `struct Metadata<T> { name: T }`
+    pub(crate) generics: syn::Generics,
     /// An optional path to a custom diff function
     pub(crate) custom: Option<syn::Path>, // #[cache_diff(custom = <function>)]
     /// One or more named fields
@@ -44,6 +46,7 @@ pub(crate) struct CacheDiffContainer {
 impl CacheDiffContainer {
     pub(crate) fn from_ast(input: &syn::DeriveInput) -> syn::Result<Self> {
         let identifier = input.ident.clone();
+        let generics = input.generics.clone();
         let mut container_custom = None;
 
         for attribute in input
@@ -93,6 +96,7 @@ impl CacheDiffContainer {
         } else {
             Ok(CacheDiffContainer {
                 identifier,
+                generics,
                 custom: container_custom,
                 fields,
             })

cache_diff_derive/src/lib.rs

@@ -16,7 +16,7 @@ pub fn cache_diff(item: TokenStream) -> TokenStream {
 fn create_cache_diff(item: proc_macro2::TokenStream) -> syn::Result<proc_macro2::TokenStream> {
     let ast: DeriveInput = syn::parse2(item).unwrap();
     let container = CacheDiffContainer::from_ast(&ast)?;
-    let struct_identifier = &container.identifier;
+    let ident = &container.identifier;
 
     let custom_diff = if let Some(ref custom_fn) = container.custom {
         quote::quote! {
@@ -48,9 +48,9 @@ fn create_cache_diff(item: proc_macro2::TokenStream) -> syn::Result<proc_macro2:
             }
         });
     }
-
+    let (impl_generics, type_generics, where_clause) = container.generics.split_for_impl();
     Ok(quote::quote! {
-        impl cache_diff::CacheDiff for #struct_identifier {
+        impl #impl_generics ::cache_diff::CacheDiff for #ident #type_generics #where_clause {
             fn diff(&self, old: &Self) -> ::std::vec::Vec<String> {
                 let mut differences = ::std::vec::Vec::new();
                 #custom_diff

usage/tests/fails/generic_missing_bounds.rs

@@ -0,0 +1,19 @@
+use cache_diff::CacheDiff;
+
+#[derive(CacheDiff)]
+struct Example<T> {
+    name: String,
+    other: T,
+}
+
+fn main() {
+    let now = Example::<String> {
+        name: "Richard".to_string(),
+        other: "John Jacob Jingleheimer Schmidt (his name is my name too)".to_string(),
+    };
+
+    let _ = now.diff(&Example::<String> {
+        name: "Richard".to_string(),
+        other: "schneems".to_string(),
+    });
+}

usage/tests/fails/generic_missing_bounds.stderr

@@ -0,0 +1,30 @@
+error[E0369]: binary operation `!=` cannot be applied to type `T`
+ --> tests/fails/generic_missing_bounds.rs:3:10
+  |
+3 | #[derive(CacheDiff)]
+  |          ^^^^^^^^^
+  |
+  = note: this error originates in the derive macro `CacheDiff` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider restricting type parameter `T` with trait `PartialEq`
+  |
+4 | struct Example<T: std::cmp::PartialEq> {
+  |                 +++++++++++++++++++++
+
+error[E0277]: `T` doesn't implement `std::fmt::Display`
+ --> tests/fails/generic_missing_bounds.rs:3:10
+  |
+3 | #[derive(CacheDiff)]
+  |          ^^^^^^^^^ `T` cannot be formatted with the default formatter
+  |
+  = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
+  = note: required for `&T` to implement `std::fmt::Display`
+note: required by a bound in `fmt_value`
+ --> $WORKSPACE/cache_diff/src/lib.rs
+  |
+  |     fn fmt_value<T: std::fmt::Display>(&self, value: &T) -> String {
+  |                     ^^^^^^^^^^^^^^^^^ required by this bound in `CacheDiff::fmt_value`
+  = note: this error originates in the derive macro `CacheDiff` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: consider restricting type parameter `T` with trait `Display`
+  |
+4 | struct Example<T: std::fmt::Display> {
+  |                 +++++++++++++++++++

usage/tests/pass/struct_with_generics.rs

@@ -0,0 +1,42 @@
+use cache_diff::CacheDiff;
+
+#[derive(CacheDiff)]
+struct Example<T>
+where
+    T: std::fmt::Display + Eq,
+{
+    name: String,
+    other: T,
+}
+
+#[derive(CacheDiff)]
+struct ExampleToo<T, V>
+where
+    T: std::fmt::Display + Eq,
+    V: std::fmt::Display + PartialEq,
+{
+    one: T,
+    two: V,
+}
+
+fn main() {
+    let now = Example::<String> {
+        name: "Richard".to_string(),
+        other: "John Jacob Jingleheimer Schmidt (his name is my name too)".to_string(),
+    };
+
+    let _ = now.diff(&Example::<String> {
+        name: "Richard".to_string(),
+        other: "schneems".to_string(),
+    });
+
+    let new = ExampleToo::<String, String> {
+        one: "One".to_string(),
+        two: "Two".to_string(),
+    };
+
+    let _ = new.diff(&ExampleToo::<String, String> {
+        one: "Won".to_string(),
+        two: "Too".to_string(),
+    });
+}