feature: support EnumFrom macro

Cargo.toml

@@ -10,5 +10,10 @@ license = "MIT"
 proc-macro = true
 
 [dependencies]
+proc-macro2 = "1.0.86"
+quote = "1.0.37"
+syn = { version = "2.0.77", features = ["extra-traits"] }
+
+[dev-dependencies]
 anyhow = "1.0.87"
 tokio = { version = "1.40.0", features = ["rt", "rt-multi-thread", "macros"] }

examples/enum_macro.rs

@@ -0,0 +1,28 @@
+use template::EnumFrom;
+
+#[allow(unused)]
+#[derive(Debug, EnumFrom)]
+enum Direction {
+    Up(DirectionUp),
+    Down,
+    Left(u32),
+    Right(u32, u32),
+}
+
+#[allow(unused)]
+#[derive(Debug)]
+struct DirectionUp {
+    speed: u32,
+}
+
+impl DirectionUp {
+    fn new(speed: u32) -> Self {
+        Self { speed }
+    }
+}
+
+fn main() {
+    let up: Direction = DirectionUp::new(42).into();
+    let left: Direction = 10.into();
+    println!("Up: {:?}, Left: {:?}", up, left);
+}

src/lib.rs

@@ -1 +1,49 @@
-// empty
+// proc macro crate
+
+use proc_macro::TokenStream;
+use quote::quote;
+
+// for enum, we'd like to generate From impls for each variant
+#[proc_macro_derive(EnumFrom)]
+pub fn derive_enum_from(input: TokenStream) -> TokenStream {
+    let input = syn::parse_macro_input!(input as syn::DeriveInput);
+    // println!("{:#?}", input);
+
+    // get the ident
+    let ident = input.ident;
+    //get enum variants
+    let variants = match input.data {
+        syn::Data::Enum(data) => data.variants,
+        _ => panic!("EnumFrom can only be applied to enums"),
+    };
+    //for each variant, get the ident and fields
+    let from_impls = variants.iter().map(|v| {
+        let var = &v.ident;
+        match &v.fields {
+            syn::Fields::Unnamed(fields) => {
+                // only support one field
+                if fields.unnamed.len() != 1 {
+                    quote! {}
+                } else {
+                    let field = fields.unnamed.first().expect("Should have 1 field");
+                    let ty = &field.ty;
+                    quote! {
+                        impl From<#ty> for #ident {
+                            fn from(v: #ty) -> Self {
+                                #ident::#var(v)
+                            }
+                        }
+                    }
+                }
+            }
+            syn::Fields::Unit => quote! {},
+            syn::Fields::Named(_fields) => quote! {},
+        }
+    });
+
+    // quote return proc-macro2 TokenStream so we need to convert it to TokenStream
+    quote! {
+        #(#from_impls)*
+    }
+    .into()
+}