closures cannot be constants

During type collection, error if a closures is found in constant
position, catching that before they go causing ICEs.

Fixes rust-lang#50600.
Fixes rust-lang#48838.

src/librustc_mir/interpret/const_eval.rs

@@ -444,9 +444,18 @@ pub fn const_eval_provider<'a, 'tcx>(
     }
 
     if let Some(id) = tcx.hir.as_local_node_id(def_id) {
-        let tables = tcx.typeck_tables_of(def_id);
         let span = tcx.def_span(def_id);
+
+        // Closures in constant position can hit this in type collection.
+        if !tcx.has_typeck_tables(def_id) {
+            return Err(ConstEvalErr {
+                kind: Lrc::new(TypeckError),
+                span,
+            });
+        }
 
+        let tables = tcx.typeck_tables_of(def_id);
+
         // Do match-check before building MIR
         if tcx.check_match(def_id).is_err() {
             return Err(ConstEvalErr {

src/librustc_typeck/collect.rs

@@ -1085,12 +1085,16 @@ fn type_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
         NodeField(field) => icx.to_ty(&field.ty),
 
-        NodeExpr(&hir::Expr { node: hir::ExprClosure(.., gen), .. }) => {
+        NodeExpr(&hir::Expr { node: hir::ExprClosure(.., gen), span, .. }) => {
             if gen.is_some() {
                 let hir_id = tcx.hir.node_to_hir_id(node_id);
                 return tcx.typeck_tables_of(def_id).node_id_to_type(hir_id);
             }
 
+            if !tcx.has_typeck_tables(def_id) {
+                span_err!(tcx.sess, span, E0912, "closures cannot be constants");
+            }
+
             let substs = ty::ClosureSubsts {
                 substs: Substs::for_item(
                     tcx,

src/librustc_typeck/diagnostics.rs

@@ -4770,4 +4770,5 @@ register_diagnostics! {
     E0641, // cannot cast to/from a pointer with an unknown kind
     E0645, // trait aliases not finished
     E0907, // type inside generator must be known in this context
+    E0912, // closures cannot be constants
 }

src/test/ui/const-eval/issue-50600.rs

@@ -0,0 +1,20 @@
+// Copyright 2018 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.
+
+// Testing that these do not ICE.
+
+struct Foo ([u8; |x: u8| { }]);
+//~^ ERROR closures cannot be constants
+enum Functions {
+    Square = |x:i32| { },
+    //~^ ERROR closures cannot be constants
+}
+
+fn main() {}

src/test/ui/const-eval/issue-50600.stderr

@@ -0,0 +1,15 @@
+error[E0912]: closures cannot be constants
+  --> $DIR/issue-50600.rs:13:18
+   |
+LL | struct Foo ([u8; |x: u8| { }]);
+   |                  ^^^^^^^^^^^
+
+error[E0912]: closures cannot be constants
+  --> $DIR/issue-50600.rs:16:14
+   |
+LL |     Square = |x:i32| { },
+   |              ^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0912`.