Document how removing a type's field can be bad and what to do instead

Related to #119645

Author: shepmaster

compiler/rustc_lint_defs/src/builtin.rs

@@ -702,6 +702,20 @@ declare_lint! {
  /// `PhantomData`.
  ///
  /// Otherwise consider removing the unused code.
+ ///
+ /// ### Limitations
+ ///
+ /// Removing fields that are only used for side-effects and never
+ /// read will result in behavioral changes. Examples of this
+ /// include:
+ ///
+ /// - If a field's value performs an action when it is dropped.
+ /// - If a field's type does not implement an auto trait
+ /// (e.g. `Send`, `Sync`, `Unpin`).
+ ///
+ /// For side-effects from dropping field values, this lint should
+ /// be allowed on those fields. For side-effects from containing
+ /// field types, `PhantomData` should be used.
  pub DEAD_CODE,
  Warn,
  "detect unused, unexported items"