Auto merge of #34443 - eddyb:sized-matters, r=arielb1

Disallow constants and statics from having unsized types.

This is a `[breaking-change]` which fixes #34390 by banning unsized `const` and `static`, e.g.:
```rust
const A: [i32] = *(&[0, 1, 2] as &[i32]);
static B: str = *"foo";
```

This was not intentionally allowed, and other than for `static` since some versions ago, it ICE'd.
If you've been taking advantage of this with `static`, you should be able to just use references instead.

Author: bors

src/librustc/traits/error_reporting.rs

@@ -908,6 +908,9 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 err.note("only the last field of a struct or enum variant \
                           may have a dynamically sized type");
             }
+            ObligationCauseCode::ConstSized => {
+                err.note("constant expressions must have a statically known size");
+            }
             ObligationCauseCode::SharedStatic => {
                 err.note("shared static variables must have a type that implements `Sync`");
             }

src/librustc/traits/mod.rs

@@ -127,6 +127,9 @@ pub enum ObligationCauseCode<'tcx> {
     // Types of fields (other than the last) in a struct must be sized.
     FieldSized,
 
+    // Constant expressions must be sized.
+    ConstSized,
+
     // static items must have `Sync` type
     SharedStatic,
 

src/librustc_typeck/check/mod.rs

@@ -1157,6 +1157,7 @@ fn check_const<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>,
         let rty = ccx.tcx.node_id_to_type(id);
         let fcx = FnCtxt::new(&inh, ty::FnConverging(rty), e.id);
         let declty = fcx.tcx.lookup_item_type(ccx.tcx.map.local_def_id(id)).ty;
+        fcx.require_type_is_sized(declty, e.span, traits::ConstSized);
         fcx.check_const_with_ty(sp, e, declty);
     });
 }

src/test/compile-fail/const-unsized.rs

@@ -0,0 +1,35 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::fmt::Debug;
+
+const CONST_0: Debug+Sync = *(&0 as &(Debug+Sync));
+//~^ ERROR `std::fmt::Debug + Sync + 'static: std::marker::Sized` is not satisfied
+//~| NOTE does not have a constant size known at compile-time
+//~| NOTE constant expressions must have a statically known size
+
+const CONST_FOO: str = *"foo";
+//~^ ERROR `str: std::marker::Sized` is not satisfied
+//~| NOTE does not have a constant size known at compile-time
+//~| NOTE constant expressions must have a statically known size
+
+static STATIC_1: Debug+Sync = *(&1 as &(Debug+Sync));
+//~^ ERROR `std::fmt::Debug + Sync + 'static: std::marker::Sized` is not satisfied
+//~| NOTE does not have a constant size known at compile-time
+//~| NOTE constant expressions must have a statically known size
+
+static STATIC_BAR: str = *"bar";
+//~^ ERROR `str: std::marker::Sized` is not satisfied
+//~| NOTE does not have a constant size known at compile-time
+//~| NOTE constant expressions must have a statically known size
+
+fn main() {
+    println!("{:?} {:?} {:?} {:?}", &CONST_0, &CONST_FOO, &STATIC_1, &STATIC_BAR);
+}

src/test/compile-fail/issue-24446.rs

@@ -11,6 +11,8 @@
 fn main() {
     static foo: Fn() -> u32 = || -> u32 {
         //~^ ERROR: mismatched types
+        //~| ERROR: `std::ops::Fn() -> u32 + 'static: std::marker::Sized` is not satisfied
+
         0
     };
 }