Add #[derive(FromRef)] (#1430)

* add `#[derive(FromRef)]`

* tests

* don't support skipping fields

probably wouldn't work at all since the whole state likely needs `Clone`

* UI tests

* changelog

* changelog link

* revert hello-world example, used for testing

* Re-export `#[derive(FromRef)]`

* Don't need to return `Result`

* use `collect` instead of quoting the iterator

* Mention it in axum's changelog

axum-macros/src/from_ref.rs

@@ -0,0 +1,39 @@
+use proc_macro2::{Ident, TokenStream};
+use quote::quote_spanned;
+use syn::{spanned::Spanned, Field, ItemStruct};
+
+pub(crate) fn expand(item: ItemStruct) -> TokenStream {
+    item.fields
+        .iter()
+        .enumerate()
+        .map(|(idx, field)| expand_field(&item.ident, idx, field))
+        .collect()
+}
+
+fn expand_field(state: &Ident, idx: usize, field: &Field) -> TokenStream {
+    let field_ty = &field.ty;
+    let span = field.ty.span();
+
+    let body = if let Some(field_ident) = &field.ident {
+        quote_spanned! {span=> state.#field_ident.clone() }
+    } else {
+        let idx = syn::Index {
+            index: idx as _,
+            span: field.span(),
+        };
+        quote_spanned! {span=> state.#idx.clone() }
+    };
+
+    quote_spanned! {span=>
+        impl ::axum::extract::FromRef<#state> for #field_ty {
+            fn from_ref(state: &#state) -> Self {
+                #body
+            }
+        }
+    }
+}
+
+#[test]
+fn ui() {
+    crate::run_ui_tests("from_ref");
+}