From 63163d57d9f6a58b89948089360b497940ac467c Mon Sep 17 00:00:00 2001
From: yuk1ty <yuk1ty@users.noreply.github.com>
Date: Fri, 8 Jul 2022 10:35:28 +0900
Subject: [PATCH] Implement rename helper attribute

---
 README.md               | 12 ++++++++++++
 econf-derive/src/lib.rs | 41 ++++++++++++++++++++++++++++++++++++-----
 econf/tests/basics.rs   | 34 ++++++++++++++++++++++++++++++++++
 3 files changed, 82 insertions(+), 5 deletions(-)

diff --git a/README.md b/README.md
index 0129af9..96533b4 100644
--- a/README.md
+++ b/README.md
@@ -150,5 +150,17 @@ struct A {
 }
 ```
 
+## Renaming fields
+
+Load a field with the given name instead of its Rust's field name. This is helpful if the environment variable name and Rust's field name don't match:
+
+```
+#[derive(LoadEnv)]
+struct A {
+    x: bool,
+    #[econf(rename = "ANOTHER_Y")]
+    y: u64, // will be loaded from an environment variable `ANOTHER_Y`
+}
+```
 
 License: MIT
diff --git a/econf-derive/src/lib.rs b/econf-derive/src/lib.rs
index 2ed071f..d9c5839 100644
--- a/econf-derive/src/lib.rs
+++ b/econf-derive/src/lib.rs
@@ -4,7 +4,7 @@ use proc_macro::TokenStream;
 
 use proc_macro2::{Ident, TokenStream as TokenStream2};
 use quote::quote;
-use syn::{Data, DeriveInput, Field, Fields, Meta, NestedMeta, parse_macro_input};
+use syn::{parse_macro_input, Data, DeriveInput, Field, Fields, Lit, Meta, NestedMeta};
 
 #[proc_macro_derive(LoadEnv, attributes(econf))]
 pub fn load_env(input: TokenStream) -> TokenStream {
@@ -34,6 +34,27 @@ fn is_skip(f: &Field) -> bool {
     })
 }
 
+fn find_renaming(f: &Field) -> Option<String> {
+    f.attrs
+        .iter()
+        .filter(|attr| attr.path.is_ident("econf"))
+        .filter_map(|attr| match attr.parse_meta().unwrap() {
+            Meta::List(meta) => Some(meta.nested),
+            _ => None,
+        })
+        .flat_map(|nested| nested.into_iter())
+        .filter_map(|nested| match nested {
+            NestedMeta::Meta(Meta::NameValue(value)) if value.ident.to_string() == "rename" => {
+                Some(value)
+            }
+            _ => None,
+        })
+        .find_map(|value| match value.lit {
+            Lit::Str(token) => Some(token.value()),
+            _ => None,
+        })
+}
+
 fn content(name: &Ident, data: &Data) -> TokenStream2 {
     match data {
         Data::Struct(data) => match &data.fields {
@@ -45,8 +66,13 @@ fn content(name: &Ident, data: &Data) -> TokenStream2 {
                             #ident: self.#ident,
                         };
                     }
-                    quote! {
-                        #ident: self.#ident.load(&(path.to_owned() + "_" + stringify!(#ident)), loader),
+                    match find_renaming(f) {
+                        Some(overwritten_name) => quote! {
+                            #ident: self.#ident.load(&(path.to_owned() + "_" + #overwritten_name), loader),
+                        },
+                        None => quote! {
+                            #ident: self.#ident.load(&(path.to_owned() + "_" + stringify!(#ident)), loader),
+                        }
                     }
                 });
                 quote! {
@@ -62,8 +88,13 @@ fn content(name: &Ident, data: &Data) -> TokenStream2 {
                     if is_skip(f) {
                         return quote! { self.#i, };
                     }
-                    quote! {
-                        self.#i.load(&(path.to_owned() + "_" + &#i.to_string()), loader),
+                    match find_renaming(f) {
+                        Some(overwritten_name) => quote! {
+                            self.#i.load(&(path.to_owned() + "_" + #overwritten_name), loader),
+                        },
+                        None => quote! {
+                            self.#i.load(&(path.to_owned() + "_" + &#i.to_string()), loader),
+                        },
                     }
                 });
                 quote! {
diff --git a/econf/tests/basics.rs b/econf/tests/basics.rs
index 35a8d84..c5d2e59 100644
--- a/econf/tests/basics.rs
+++ b/econf/tests/basics.rs
@@ -356,6 +356,40 @@ fn skipped() {
     assert_eq!(a.v3.s, "initial".to_string());
 }
 
+#[derive(LoadEnv)]
+struct NestedRenamed {
+    s: String,
+}
+
+#[derive(LoadEnv)]
+struct Renamed {
+    v1: bool,
+    #[econf(rename = "example_1")]
+    v2: u32,
+    #[econf(rename = "example_2")]
+    v3: NestedRenamed,
+}
+
+#[test]
+fn renamed() {
+    std::env::set_var("RENAMED_V1", "true");
+    std::env::set_var("RENAMED_EXAMPLE_1", "42");
+    std::env::set_var("RENAMED_EXAMPLE_2_S", "renamed text");
+
+    let a = Renamed {
+        v1: false,
+        v2: 0,
+        v3: NestedRenamed {
+            s: "initial".to_string(),
+        },
+    };
+
+    let a = econf::load(a, "renamed");
+    assert_eq!(a.v1, true);
+    assert_eq!(a.v2, 42);
+    assert_eq!(a.v3.s, "renamed text".to_string());
+}
+
 #[derive(LoadEnv)]
 struct Net {
     n1: IpAddr,