[5/n] drop default field values

daft-derive/src/internals/imp.rs

@@ -627,6 +627,12 @@ impl DiffFields {
         // future.
         f.attrs = vec![];
 
+        // Drop default field values (Rust RFC 3681). The default is an
+        // expression of the original field type, so retaining it while
+        // rewriting `f.ty` would emit something like `a: <u32 as
+        // Diffable>::Diff<'__daft> = 5`, which is definitely wrong.
+        f.default = None;
+
         Some((f, config))
     }
 

daft-derive/tests/integration/default_field_values.rs

@@ -0,0 +1,106 @@
+//! Test that default field values don't make their way into generated diff
+//! structs.
+//!
+//! Default field values ([Rust RFC 3681]) let a struct field carry an
+//! initializer, as in `struct Foo { a: u32 = 5 }`. Daft drops these in its own
+//! struct definitions.
+//!
+//! These tests live here rather than as snapshot tests for two reasons:
+//! because trybuild runs against stable where (as of Rust 1.97) default values
+//! aren't supported, and because prettyplease 0.3.0 seems to eat default field
+//! values. Worth revisiting in the future.
+//!
+//! [Rust RFC 3681]: https://rust-lang.github.io/rfcs/3681-default-field-values.html
+
+use quote::ToTokens;
+
+#[path = "../../src/internals/mod.rs"]
+mod internals;
+
+/// Parse `input`, run the derive macro over it, and return the generated diff
+/// struct.
+fn derive_diff_struct(input: &str) -> syn::ItemStruct {
+    let parsed: syn::DeriveInput =
+        syn::parse_str(input).expect("input parsed as a DeriveInput");
+    let diff_name = format!("{}Diff", parsed.ident);
+
+    let generated = internals::derive_diffable(parsed).into_token_stream();
+    let file: syn::File =
+        syn::parse2(generated.clone()).unwrap_or_else(|err| {
+            panic!("generated code parsed as a file: {err}\n{generated}");
+        });
+
+    file.items
+        .into_iter()
+        .find_map(|item| match item {
+            syn::Item::Struct(s) if s.ident == diff_name => Some(s),
+            _ => None,
+        })
+        .unwrap_or_else(|| {
+            panic!("generated code defines {diff_name}:\n{generated}");
+        })
+}
+
+/// Assert that no field of `s` carries a default value.
+fn assert_no_field_defaults(s: &syn::ItemStruct) {
+    let kept: Vec<_> = s
+        .fields
+        .iter()
+        .enumerate()
+        .filter(|(_, f)| f.default.is_some())
+        .map(|(i, f)| match &f.ident {
+            Some(ident) => ident.to_string(),
+            None => i.to_string(),
+        })
+        .collect();
+
+    assert!(
+        kept.is_empty(),
+        "fields of {} kept a default value from the original struct: {:?}\n{}",
+        s.ident,
+        kept,
+        s.to_token_stream(),
+    );
+}
+
+/// Return the names of `s`'s fields, in declaration order.
+fn field_names(s: &syn::ItemStruct) -> Vec<String> {
+    s.fields
+        .iter()
+        .map(|f| f.ident.as_ref().expect("named field").to_string())
+        .collect()
+}
+
+/// Test that default on a named field is dropped from the diff struct.
+///
+/// The rest of the struct still diffs normally when a default is present.
+#[test]
+fn named_field_default_is_dropped() {
+    let diff =
+        derive_diff_struct("struct HasDefault { a: u32 = 5, b: String }");
+    assert_no_field_defaults(&diff);
+    assert_eq!(field_names(&diff), ["a", "b"]);
+}
+
+/// Test that a default on a `#[daft(leaf)]` field is dropped too.
+#[test]
+fn leaf_field_default_is_dropped() {
+    let diff = derive_diff_struct(
+        "struct HasLeafDefault { #[daft(leaf)] a: u32 = 5 }",
+    );
+    assert_no_field_defaults(&diff);
+    assert_eq!(field_names(&diff), ["a"]);
+}
+
+/// Test that a default on an ignored field doesn't reappear.
+///
+/// `#[daft(ignore)]` drops the field entirely, so the diff struct is empty and
+/// there is nothing for a default to attach to.
+#[test]
+fn ignored_field_default_is_dropped() {
+    let diff = derive_diff_struct(
+        "struct HasIgnoredDefault { #[daft(ignore)] a: u32 = 5, b: String }",
+    );
+    assert_no_field_defaults(&diff);
+    assert_eq!(field_names(&diff), ["b"]);
+}

daft-derive/tests/integration/main.rs

@@ -4,4 +4,5 @@
 //! test modules don't have to build a separate binary.
 
 mod basic;
+mod default_field_values;
 mod generics;