pointee_info_at: fix logic for recursing into enums

compiler/rustc_abi/src/lib.rs

@@ -1743,15 +1743,23 @@ pub enum PointerKind {
     Box { unpin: bool, global: bool },
 }
 
-/// Note that this information is advisory only, and backends are free to ignore it.
-/// It can only be used to encode potential optimizations, but no critical information.
+/// Encodes extra information we have about a pointer.
+/// Note that this information is advisory only, and backends are free to ignore it:
+/// if the information is wrong, that can cause UB, but if the information is absent,
+/// that must always be okay.
 #[derive(Copy, Clone, Debug)]
 pub struct PointeeInfo {
-    pub size: Size,
-    pub align: Align,
     /// If this is `None`, then this is a raw pointer, so size and alignment are not guaranteed to
     /// be reliable.
     pub safe: Option<PointerKind>,
+    /// If `safe` is `Some`, then the pointer is either null or dereferenceable for this many bytes.
+    /// On a function argument, "dereferenceable" here means "dereferenceable for the entire duration
+    /// of this function call", i.e. it is UB for the memory that this pointer points to to be freed
+    /// while this function is still running.
+    /// The size can be zero if the pointer is not dereferenceable.
+    pub size: Size,
+    /// If `safe` is `Some`, then the pointer is aligned as indicated.
+    pub align: Align,
 }
 
 impl<FieldIdx: Idx, VariantIdx: Idx> LayoutData<FieldIdx, VariantIdx> {