Do not ICE when combining unsized locals and async

src/librustc/mir/interpret/error.rs

@@ -137,6 +137,7 @@ impl<'tcx> ConstEvalErr<'tcx> {
     ) -> Result<DiagnosticBuilder<'tcx>, ErrorHandled> {
         let must_error = match self.error {
             err_inval!(Layout(LayoutError::Unknown(_))) |
+            err_inval!(Layout(LayoutError::Unsized(_))) |
             err_inval!(TooGeneric) =>
                 return Err(ErrorHandled::TooGeneric),
             err_inval!(TypeckError) =>

src/librustc/ty/layout.rs

@@ -155,12 +155,16 @@ pub const FAT_PTR_EXTRA: usize = 1;
 #[derive(Copy, Clone, Debug, RustcEncodable, RustcDecodable)]
 pub enum LayoutError<'tcx> {
     Unknown(Ty<'tcx>),
-    SizeOverflow(Ty<'tcx>)
+    SizeOverflow(Ty<'tcx>),
+    Unsized(Ty<'tcx>),
 }
 
 impl<'tcx> fmt::Display for LayoutError<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {
+            LayoutError::Unsized(ty) => {
+                write!(f, "the type `{:?}` has unknown size", ty)
+            }
             LayoutError::Unknown(ty) => {
                 write!(f, "the type `{:?}` has an unknown layout", ty)
             }
@@ -745,7 +749,11 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     let mut abi = Abi::Aggregate { sized: true };
                     let index = VariantIdx::new(0);
                     for field in &variants[index] {
-                        assert!(!field.is_unsized());
+                        if field.is_unsized() {
+                            // Instead of asserting, return an error because this can happen with
+                            // generators/async-await and unsized locals.
+                            return Err(LayoutError::Unsized(field.ty));
+                        }
                         align = align.max(field.align);
 
                         // If all non-ZST fields have the same ABI, forward this ABI
@@ -2492,6 +2500,7 @@ impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for LayoutError<'tcx> {
         mem::discriminant(self).hash_stable(hcx, hasher);
 
         match *self {
+            Unsized(t) |
             Unknown(t) |
             SizeOverflow(t) => t.hash_stable(hcx, hasher)
         }

src/librustc_lint/types.rs

@@ -999,6 +999,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for VariantSizeDifferences {
             let ty = cx.tcx.erase_regions(&t);
             let layout = match cx.layout_of(ty) {
                 Ok(layout) => layout,
+                Err(ty::layout::LayoutError::Unsized(_)) |
                 Err(ty::layout::LayoutError::Unknown(_)) => return,
                 Err(err @ ty::layout::LayoutError::SizeOverflow(_)) => {
                     bug!("failed to get layout for `{}`: {}", t, err);

src/librustc_mir/transform/const_prop.rs

@@ -669,28 +669,43 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
             let place_ty: Ty<'tcx> = place
                 .ty(&self.local_decls, self.tcx)
                 .ty;
-            if let Ok(place_layout) = self.tcx.layout_of(self.param_env.and(place_ty)) {
-                if let Some(value) = self.const_prop(rval, place_layout, statement.source_info) {
-                    if let Place {
-                        base: PlaceBase::Local(local),
-                        projection: box [],
-                    } = *place {
-                        trace!("checking whether {:?} can be stored to {:?}", value, local);
-                        if self.can_const_prop[local] {
-                            trace!("storing {:?} to {:?}", value, local);
-                            assert!(self.get_const(local).is_none());
-                            self.set_const(local, value);
-
-                            if self.should_const_prop() {
-                                self.replace_with_const(
-                                    rval,
-                                    value,
-                                    statement.source_info,
-                                );
+
+            match self.tcx.layout_of(self.param_env.and(place_ty)) {
+                Ok(place_layout) => {
+                    if let Some(value) = self.const_prop(
+                        rval,
+                        place_layout,
+                        statement.source_info,
+                    ) {
+                        if let Place {
+                            base: PlaceBase::Local(local),
+                            projection: box [],
+                        } = *place {
+                            trace!("checking whether {:?} can be stored to {:?}", value, local);
+                            if self.can_const_prop[local] {
+                                trace!("storing {:?} to {:?}", value, local);
+                                assert!(self.get_const(local).is_none());
+                                self.set_const(local, value);
+
+                                if self.should_const_prop() {
+                                    self.replace_with_const(
+                                        rval,
+                                        value,
+                                        statement.source_info,
+                                    );
                             }
                         }
                     }
                 }
+                Err(LayoutError::Unsized(_ty)) => {
+                    let sp = statement.source_info.span; // async fn block
+                    self.tcx.sess.struct_span_err(
+                        sp,
+                        "unsized values can't be used in `async` functions",
+                    ).span_label(sp, "contains unsized values")
+                        .emit();
+                }
+                Err(_) => {}
             }
         }
         self.super_statement(statement, location);

src/test/ui/async-await/unsized-locals.rs

@@ -0,0 +1,26 @@
+// edition:2018
+
+#![feature(unsized_locals)]
+#![feature(gen_future)]
+
+use std::future::poll_with_tls_context;
+use std::pin::Pin;
+use std::fmt::Display;
+
+async fn foo2() {}
+
+async fn foo(x: Box<dyn Display>) { //~ ERROR unsized values can't be used in `async` functions
+    let x = *x;
+    foo2().await;
+    println!("hello {}", &x);
+}
+
+fn main() {
+    let mut a = foo(Box::new(5));
+    let b = unsafe {
+        Pin::new_unchecked(&mut a)
+    };
+    match poll_with_tls_context(b) {
+        _ => ()
+    };
+}

src/test/ui/async-await/unsized-locals.stderr

@@ -0,0 +1,13 @@
+error: unsized values can't be used in `async` functions
+  --> $DIR/unsized-locals.rs:12:35
+   |
+LL |   async fn foo(x: Box<dyn Display>) {
+   |  ___________________________________^
+LL | |     let x = *x;
+LL | |     foo2().await;
+LL | |     println!("hello {}", &x);
+LL | | }
+   | |_^ contains unsized values
+
+error: aborting due to previous error
+