Implement `default_overrides_default_fields` lint

Detect when a manual `Default` implementation isn't using the existing default field values and suggest using `..` instead:

```
error: `Default` impl doesn't use the declared default field values
  --> $DIR/manual-default-impl-could-be-derived.rs:14:1
   |
LL | / impl Default for A {
LL | |     fn default() -> Self {
LL | |         A {
LL | |             y: 0,
   | |                - this field has a default value
...  |
LL | | }
   | |_^
   |
   = help: use the default values in the `impl` with `Struct { mandatory_field, .. }` to avoid them diverging over time
note: the lint level is defined here
  --> $DIR/manual-default-impl-could-be-derived.rs:5:9
   |
LL | #![deny(default_overrides_default_fields)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

2 files changed, 188 insertions, 0 deletions

diff --git a/compiler/rustc_lint/src/default_could_be_derived.rs b/compiler/rustc_lint/src/default_could_be_derived.rs
new file mode 100644
index 00000000000..d95cbb05158
--- /dev/null
+++ b/compiler/rustc_lint/src/default_could_be_derived.rs
@@ -0,0 +1,185 @@
+use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::Diag;
+use rustc_hir as hir;
+use rustc_middle::ty;
+use rustc_session::{declare_lint, impl_lint_pass};
+use rustc_span::Symbol;
+use rustc_span::symbol::sym;
+
+use crate::{LateContext, LateLintPass};
+
+declare_lint! {
+    /// The `default_overrides_default_fields` lint checks for manual `impl` blocks of the
+    /// `Default` trait of types with default field values.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![feature(default_field_values)]
+    /// struct Foo {
+    ///     x: i32 = 101,
+    ///     y: NonDefault,
+    /// }
+    ///
+    /// struct NonDefault;
+    ///
+    /// #[deny(default_overrides_default_fields)]
+    /// impl Default for Foo {
+    ///     fn default() -> Foo {
+    ///         Foo { x: 100, y: NonDefault }
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Manually writing a `Default` implementation for a type that has
+    /// default field values runs the risk of diverging behavior between
+    /// `Type { .. }` and `<Type as Default>::default()`, which would be a
+    /// foot-gun for users of that type that would expect these to be
+    /// equivalent. If `Default` can't be derived due to some fields not
+    /// having a `Default` implementation, we encourage the use of `..` for
+    /// the fields that do have a default field value.
+    pub DEFAULT_OVERRIDES_DEFAULT_FIELDS,
+    Deny,
+    "detect `Default` impl that should use the type's default field values",
+    @feature_gate = default_field_values;
+}
+
+#[derive(Default)]
+pub(crate) struct DefaultCouldBeDerived;
+
+impl_lint_pass!(DefaultCouldBeDerived => [DEFAULT_OVERRIDES_DEFAULT_FIELDS]);
+
+impl<'tcx> LateLintPass<'tcx> for DefaultCouldBeDerived {
+    fn check_impl_item(&mut self, cx: &LateContext<'_>, impl_item: &hir::ImplItem<'_>) {
+        // Look for manual implementations of `Default`.
+        let Some(default_def_id) = cx.tcx.get_diagnostic_item(sym::Default) else { return };
+        let hir::ImplItemKind::Fn(_sig, body_id) = impl_item.kind else { return };
+        let assoc = cx.tcx.associated_item(impl_item.owner_id);
+        let parent = assoc.container_id(cx.tcx);
+        if cx.tcx.has_attr(parent, sym::automatically_derived) {
+            // We don't care about what `#[derive(Default)]` produces in this lint.
+            return;
+        }
+        let Some(trait_ref) = cx.tcx.impl_trait_ref(parent) else { return };
+        let trait_ref = trait_ref.instantiate_identity();
+        if trait_ref.def_id != default_def_id {
+            return;
+        }
+        let ty = trait_ref.self_ty();
+        let ty::Adt(def, _) = ty.kind() else { return };
+
+        // We now know we have a manually written definition of a `<Type as Default>::default()`.
+
+        let hir = cx.tcx.hir();
+
+        let type_def_id = def.did();
+        let body = hir.body(body_id);
+
+        // FIXME: evaluate bodies with statements and evaluate bindings to see if they would be
+        // derivable.
+        let hir::ExprKind::Block(hir::Block { stmts: _, expr: Some(expr), .. }, None) =
+            body.value.kind
+        else {
+            return;
+        };
+
+        // Keep a mapping of field name to `hir::FieldDef` for every field in the type. We'll use
+        // these to check for things like checking whether it has a default or using its span for
+        // suggestions.
+        let orig_fields = match hir.get_if_local(type_def_id) {
+            Some(hir::Node::Item(hir::Item {
+                kind:
+                    hir::ItemKind::Struct(hir::VariantData::Struct { fields, recovered: _ }, _generics),
+                ..
+            })) => fields.iter().map(|f| (f.ident.name, f)).collect::<FxHashMap<_, _>>(),
+            _ => return,
+        };
+
+        // We check `fn default()` body is a single ADT literal and get all the fields that are
+        // being set.
+        let hir::ExprKind::Struct(_qpath, fields, tail) = expr.kind else { return };
+
+        // We have a struct literal
+        //
+        // struct Foo {
+        //     field: Type,
+        // }
+        //
+        // impl Default for Foo {
+        //     fn default() -> Foo {
+        //         Foo {
+        //             field: val,
+        //         }
+        //     }
+        // }
+        //
+        // We would suggest `#[derive(Default)]` if `field` has a default value, regardless of what
+        // it is; we don't want to encourage divergent behavior between `Default::default()` and
+        // `..`.
+
+        if let hir::StructTailExpr::Base(_) = tail {
+            // This is *very* niche. We'd only get here if someone wrote
+            // impl Default for Ty {
+            //     fn default() -> Ty {
+            //         Ty { ..something() }
+            //     }
+            // }
+            // where `something()` would have to be a call or path.
+            // We have nothing meaninful to do with this.
+            return;
+        }
+
+        // At least one of the fields with a default value have been overriden in
+        // the `Default` implementation. We suggest removing it and relying on `..`
+        // instead.
+        let any_default_field_given =
+            fields.iter().any(|f| orig_fields.get(&f.ident.name).and_then(|f| f.default).is_some());
+
+        if !any_default_field_given {
+            // None of the default fields were actually provided explicitly, so the manual impl
+            // doesn't override them (the user used `..`), so there's no risk of divergent behavior.
+            return;
+        }
+
+        let Some(local) = parent.as_local() else { return };
+        let hir_id = cx.tcx.local_def_id_to_hir_id(local);
+        let hir::Node::Item(item) = cx.tcx.hir_node(hir_id) else { return };
+        cx.tcx.node_span_lint(DEFAULT_OVERRIDES_DEFAULT_FIELDS, hir_id, item.span, |diag| {
+            mk_lint(diag, orig_fields, fields);
+        });
+    }
+}
+
+fn mk_lint(
+    diag: &mut Diag<'_, ()>,
+    orig_fields: FxHashMap<Symbol, &hir::FieldDef<'_>>,
+    fields: &[hir::ExprField<'_>],
+) {
+    diag.primary_message("`Default` impl doesn't use the declared default field values");
+
+    // For each field in the struct expression
+    //   - if the field in the type has a default value, it should be removed
+    //   - elif the field is an expression that could be a default value, it should be used as the
+    //     field's default value (FIXME: not done).
+    //   - else, we wouldn't touch this field, it would remain in the manual impl
+    let mut removed_all_fields = true;
+    for field in fields {
+        if orig_fields.get(&field.ident.name).and_then(|f| f.default).is_some() {
+            diag.span_label(field.expr.span, "this field has a default value");
+        } else {
+            removed_all_fields = false;
+        }
+    }
+
+    diag.help(if removed_all_fields {
+        "to avoid divergence in behavior between `Struct { .. }` and \
+         `<Struct as Default>::default()`, derive the `Default`"
+    } else {
+        "use the default values in the `impl` with `Struct { mandatory_field, .. }` to avoid them \
+         diverging over time"
+    });
+}
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index d7f0d2a6941..1465c2cff7b 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -41,6 +41,7 @@ mod async_fn_in_trait;
 pub mod builtin;
 mod context;
 mod dangling;
+mod default_could_be_derived;
 mod deref_into_dyn_supertrait;
 mod drop_forget_useless;
 mod early;
@@ -85,6 +86,7 @@ use async_closures::AsyncClosureUsage;
 use async_fn_in_trait::AsyncFnInTrait;
 use builtin::*;
 use dangling::*;
+use default_could_be_derived::DefaultCouldBeDerived;
 use deref_into_dyn_supertrait::*;
 use drop_forget_useless::*;
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
@@ -189,6 +191,7 @@ late_lint_methods!(
         BuiltinCombinedModuleLateLintPass,
         [
             ForLoopsOverFallibles: ForLoopsOverFallibles,
+            DefaultCouldBeDerived: DefaultCouldBeDerived::default(),
             DerefIntoDynSupertrait: DerefIntoDynSupertrait,
             DropForgetUseless: DropForgetUseless,
             ImproperCTypesDeclarations: ImproperCTypesDeclarations,