fix error for unsized packed struct field

Author: RalfJung

src/librustc/traits/error_reporting.rs

@@ -1472,11 +1472,16 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             ObligationCauseCode::StructInitializerSized => {
                 err.note("structs must have a statically known size to be initialized");
             }
-            ObligationCauseCode::FieldSized(ref item) => {
+            ObligationCauseCode::FieldSized { adt_kind: ref item, last } => {
                 match *item {
                     AdtKind::Struct => {
-                        err.note("only the last field of a struct may have a dynamically \
-                                  sized type");
+                        if last {
+                            err.note("the last field of a packed struct may only have a \
+                                      dynamically sized type if it does not need drop to be run");
+                        } else {
+                            err.note("only the last field of a struct may have a dynamically \
+                                      sized type");
+                        }
                     }
                     AdtKind::Union => {
                         err.note("no field of a union may have a dynamically sized type");

src/librustc/traits/mod.rs

@@ -192,8 +192,8 @@ pub enum ObligationCauseCode<'tcx> {
     /// [T,..n] --> T must be Copy
     RepeatVec,
 
-    /// Types of fields (other than the last) in a struct must be sized.
-    FieldSized(AdtKind),
+    /// Types of fields (other than the last, except for packed structs) in a struct must be sized.
+    FieldSized { adt_kind: AdtKind, last: bool },
 
     /// Constant expressions must be sized.
     ConstSized,

src/librustc/traits/structural_impls.rs

@@ -206,7 +206,7 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
             super::SizedReturnType => Some(super::SizedReturnType),
             super::SizedYieldType => Some(super::SizedYieldType),
             super::RepeatVec => Some(super::RepeatVec),
-            super::FieldSized(item) => Some(super::FieldSized(item)),
+            super::FieldSized { adt_kind, last } => Some(super::FieldSized { adt_kind, last }),
             super::ConstSized => Some(super::ConstSized),
             super::SharedStatic => Some(super::SharedStatic),
             super::BuiltinDerivedObligation(ref cause) => {

src/librustc_typeck/check/wfcheck.rs

@@ -258,25 +258,35 @@ fn check_type_defn<'a, 'tcx, F>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                     ty.needs_drop(fcx_tcx, fcx_tcx.param_env(def_id))
                 }
             };
-            let unsized_len = if
+            let all_sized =
                 all_sized ||
                 variant.fields.is_empty() ||
-                needs_drop_copy()
-            {
+                needs_drop_copy();
+            let unsized_len = if all_sized {
                 0
             } else {
                 1
             };
-            for field in &variant.fields[..variant.fields.len() - unsized_len] {
+            for (idx, field) in variant.fields[..variant.fields.len() - unsized_len]
+                .iter()
+                .enumerate()
+            {
+                let last = idx == variant.fields.len() - 1;
                 fcx.register_bound(
                     field.ty,
                     fcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
-                    traits::ObligationCause::new(field.span,
-                                                    fcx.body_id,
-                                                    traits::FieldSized(match item.node.adt_kind() {
-                                                    Some(i) => i,
-                                                    None => bug!(),
-                                                    })));
+                    traits::ObligationCause::new(
+                        field.span,
+                        fcx.body_id,
+                        traits::FieldSized {
+                            adt_kind: match item.node.adt_kind() {
+                                Some(i) => i,
+                                None => bug!(),
+                            },
+                            last
+                        }
+                    )
+                );
             }
 
             // All field types must be well-formed.

src/test/ui/issues/issue-27060-2.stderr

@@ -7,7 +7,7 @@ LL |     data: T, //~ ERROR the size for values of type
    = help: the trait `std::marker::Sized` is not implemented for `T`
    = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = help: consider adding a `where T: std::marker::Sized` bound
-   = note: only the last field of a struct may have a dynamically sized type
+   = note: the last field of a packed struct may only have a dynamically sized type if it does not need drop to be run
 
 error: aborting due to previous error