Ban #[unsafe_no_drop_flag] on zero-sized types.

Author: eddyb

src/librustc/ty/layout.rs

@@ -882,13 +882,19 @@ impl<'a, 'gcx, 'tcx> Layout {
                 // won't actually be in the location we say it is because it'll be after
                 // the unsized field. Several other pieces of code assume that the unsized
                 // field is definitely the last one.
-                if def.dtor_kind().has_drop_flag() &&
+                let dtor_kind = def.dtor_kind();
+                if dtor_kind.has_drop_flag() &&
                    ty.is_sized(tcx, &infcx.parameter_environment, DUMMY_SP) {
                     st.extend(dl, Some(Ok(&Scalar {
                         value: Int(I8),
                         non_zero: false
                     })).into_iter(), ty)?;
                 }
+                if dtor_kind == ty::TraitDtor(false) && st.min_size().bytes() == 0 {
+                    let span = tcx.map.def_id_span(def.did, DUMMY_SP);
+                    tcx.sess.span_fatal(span, &format!(
+                        "cannot have a destructor for zero-sized type `{}`", ty));
+                }
                 Univariant {
                     variant: st,
                     non_zero: Some(def.did) == tcx.lang_items.non_zero()
@@ -898,7 +904,8 @@ impl<'a, 'gcx, 'tcx> Layout {
                 let hint = *tcx.lookup_repr_hints(def.did).get(0)
                     .unwrap_or(&attr::ReprAny);
 
-                let dtor = def.dtor_kind().has_drop_flag();
+                let dtor_kind = def.dtor_kind();
+                let dtor = dtor_kind.has_drop_flag();
                 let drop_flag = if dtor {
                     Some(Scalar { value: Int(I8), non_zero: false })
                 } else {
@@ -953,6 +960,11 @@ impl<'a, 'gcx, 'tcx> Layout {
                     });
                     let mut st = Struct::new(dl, false);
                     st.extend(dl, fields.chain(drop_flag.iter().map(Ok)), ty)?;
+                    if dtor_kind == ty::TraitDtor(false) && st.min_size().bytes() == 0 {
+                        let span = tcx.map.def_id_span(def.did, DUMMY_SP);
+                        tcx.sess.span_fatal(span, &format!(
+                            "cannot have a destructor for zero-sized type `{}`", ty));
+                    }
                     return Ok(Univariant { variant: st, non_zero: false });
                 }
 

src/librustc/ty/mod.rs

@@ -117,7 +117,7 @@ pub struct CrateAnalysis<'a> {
     pub glob_map: Option<hir::GlobMap>,
 }
 
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, PartialEq, Eq)]
 pub enum DtorKind {
     NoDtor,
     TraitDtor(bool)

src/librustc_trans/type_of.rs

@@ -231,6 +231,11 @@ pub fn in_memory_type_of<'a, 'tcx>(cx: &CrateContext<'a, 'tcx>, t: Ty<'tcx>) ->
         return llty;
     }
 
+    // FIXME(eddyb) use the layout to actually compute the LLVM type.
+    let _layout = cx.tcx().normalizing_infer_ctxt(ProjectionMode::Any).enter(|infcx| {
+        t.layout(&infcx)
+    });
+
     let mut llty = match t.sty {
       ty::TyBool => Type::bool(cx),
       ty::TyChar => Type::char(cx),

src/test/run-pass/zero-size-type-destructors.rs renamed to src/test/compile-fail/zero-size-type-destructors.rs

@@ -8,16 +8,14 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(rustc_attrs, unsafe_no_drop_flag)]
-
-// ignore-pretty : (#23623) problems when  ending with // comments
+#![feature(unsafe_no_drop_flag)]
 
 static mut destructions : isize = 3;
 
-#[rustc_no_mir] // FIXME #29855 MIR doesn't handle all drops correctly.
 pub fn foo() {
     #[unsafe_no_drop_flag]
     struct Foo;
+    //~^ ERROR cannot have a destructor for zero-sized type `foo::Foo`
 
     impl Drop for Foo {
         fn drop(&mut self) {