1 files changed, 175 insertions, 0 deletions
diff --git a/third_party/rust/darling/examples/consume_fields.rs b/third_party/rust/darling/examples/consume_fields.rs
new file mode 100644
index 0000000000..f5cd435b90
--- /dev/null
+++ b/third_party/rust/darling/examples/consume_fields.rs
@@ -0,0 +1,175 @@
+// The use of fields in debug print commands does not count as "used",
+// which causes the fields to trigger an unwanted dead code warning.
+#![allow(dead_code)]
+
+//! This example shows how to do struct and field parsing using darling.
+
+use darling::{ast, FromDeriveInput, FromField, FromMeta};
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+use syn::parse_str;
+
+/// A speaking volume. Deriving `FromMeta` will cause this to be usable
+/// as a string value for a meta-item key.
+#[derive(Debug, Clone, Copy, FromMeta)]
+#[darling(default)]
+enum Volume {
+    Normal,
+    Whisper,
+    Shout,
+}
+
+impl Default for Volume {
+    fn default() -> Self {
+        Volume::Normal
+    }
+}
+
+/// Support parsing from a full derive input. Unlike FromMeta, this isn't
+/// composable; each darling-dependent crate should have its own struct to handle
+/// when its trait is derived.
+#[derive(Debug, FromDeriveInput)]
+// This line says that we want to process all attributes declared with `my_trait`,
+// and that darling should panic if this receiver is given an enum.
+#[darling(attributes(my_trait), supports(struct_any))]
+struct MyInputReceiver {
+    /// The struct ident.
+    ident: syn::Ident,
+
+    /// The type's generics. You'll need these any time your trait is expected
+    /// to work with types that declare generics.
+    generics: syn::Generics,
+
+    /// Receives the body of the struct or enum. We don't care about
+    /// struct fields because we previously told darling we only accept structs.
+    data: ast::Data<(), MyFieldReceiver>,
+
+    /// The Input Receiver demands a volume, so use `Volume::Normal` if the
+    /// caller doesn't provide one.
+    #[darling(default)]
+    volume: Volume,
+}
+
+impl ToTokens for MyInputReceiver {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let MyInputReceiver {
+            ref ident,
+            ref generics,
+            ref data,
+            volume,
+        } = *self;
+
+        let (imp, ty, wher) = generics.split_for_impl();
+        let fields = data
+            .as_ref()
+            .take_struct()
+            .expect("Should never be enum")
+            .fields;
+
+        // Generate the format string which shows each field and its name
+        let fmt_string = fields
+            .iter()
+            .enumerate()
+            .map(|(i, f)| {
+                // We have to preformat the ident in this case so we can fall back
+                // to the field index for unnamed fields. It's not easy to read,
+                // unfortunately.
+                format!(
+                    "{} = {{}}",
+                    f.ident
+                        .as_ref()
+                        .map(|v| format!("{}", v))
+                        .unwrap_or_else(|| format!("{}", i))
+                )
+            })
+            .collect::<Vec<_>>()
+            .join(", ");
+
+        // Generate the actual values to fill the format string.
+        let field_list = fields
+            .into_iter()
+            .enumerate()
+            .map(|(i, f)| {
+                let field_volume = f.volume.unwrap_or(volume);
+
+                // This works with named or indexed fields, so we'll fall back to the index so we can
+                // write the output as a key-value pair.
+                let field_ident = f.ident
+                    .as_ref()
+                    .map(|v| quote!(#v))
+                    .unwrap_or_else(|| {
+                        let i = syn::Index::from(i);
+                        quote!(#i)
+                    });
+
+                match field_volume {
+                    Volume::Normal => quote!(self.#field_ident),
+                    Volume::Shout => {
+                        quote!(::std::string::ToString::to_string(&self.#field_ident).to_uppercase())
+                    }
+                    Volume::Whisper => {
+                        quote!(::std::string::ToString::to_string(&self.#field_ident).to_lowercase())
+                    }
+                }
+            })
+            .collect::<Vec<_>>();
+
+        tokens.extend(quote! {
+            impl #imp Speak for #ident #ty #wher {
+                fn speak(&self, writer: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
+                    write!(writer, #fmt_string, #(#field_list),*)
+                }
+            }
+        });
+    }
+}
+
+#[derive(Debug, FromField)]
+#[darling(attributes(my_trait))]
+struct MyFieldReceiver {
+    /// Get the ident of the field. For fields in tuple or newtype structs or
+    /// enum bodies, this can be `None`.
+    ident: Option<syn::Ident>,
+
+    /// This magic field name pulls the type from the input.
+    ty: syn::Type,
+
+    /// We declare this as an `Option` so that during tokenization we can write
+    /// `field.volume.unwrap_or(derive_input.volume)` to facilitate field-level
+    /// overrides of struct-level settings.
+    ///
+    /// Because this field is an `Option`, we don't need to include `#[darling(default)]`
+    volume: Option<Volume>,
+}
+
+fn main() {
+    let input = r#"#[derive(MyTrait)]
+#[my_trait(volume = "shout")]
+pub struct Foo {
+    #[my_trait(volume = "whisper")]
+    bar: bool,
+
+    baz: i64,
+}"#;
+
+    let parsed = parse_str(input).unwrap();
+    let receiver = MyInputReceiver::from_derive_input(&parsed).unwrap();
+    let tokens = quote!(#receiver);
+
+    println!(
+        r#"
+INPUT:
+
+{}
+
+PARSED AS:
+
+{:?}
+
+EMITS:
+
+{}
+    "#,
+        input, receiver, tokens
+    );
+}