Fix never-type rvalue ICE

src/librustc_mir/build/block.rs

@@ -143,7 +143,17 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
         if let Some(expr) = expr {
             unpack!(block = this.into(destination, block, expr));
         } else {
-            this.cfg.push_assign_unit(block, source_info, destination);
+            // If a block has no trailing expression, then it is given an implicit return type.
+            // This return type is usually `()`, unless the block is diverging, in which case the
+            // return type is `!`. For the unit type, we need to actually return the unit, but in
+            // the case of `!`, no return value is required, as the block will never return.
+            let tcx = this.hir.tcx();
+            let ty = destination.ty(&this.local_decls, tcx).to_ty(tcx);
+            if ty.is_nil() {
+                // We only want to assign an implicit `()` as the return value of the block if the
+                // block does not diverge. (Otherwise, we may try to assign a unit to a `!`-type.)
+                this.cfg.push_assign_unit(block, source_info, destination);
+            }
         }
         // Finally, we pop all the let scopes before exiting out from the scope of block
         // itself.

src/librustc_mir/build/expr/into.rs

@@ -272,7 +272,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
             ExprKind::Continue { .. } |
             ExprKind::Break { .. } |
             ExprKind::InlineAsm { .. } |
-            ExprKind::Return {.. } => {
+            ExprKind::Return { .. } => {
                 unpack!(block = this.stmt_expr(block, expr));
                 this.cfg.push_assign_unit(block, source_info, destination);
                 block.unit()

src/test/run-pass/never-type-rvalues.rs

@@ -0,0 +1,46 @@
+// 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.
+
+#![feature(never_type)]
+#![allow(dead_code)]
+#![allow(path_statements)]
+#![allow(unreachable_patterns)]
+
+fn never_direct(x: !) {
+    x;
+}
+
+fn never_ref_pat(ref x: !) {
+    *x;
+}
+
+fn never_ref(x: &!) {
+    let &y = x;
+    y;
+}
+
+fn never_pointer(x: *const !) {
+    unsafe {
+        *x;
+    }
+}
+
+fn never_slice(x: &[!]) {
+    x[0];
+}
+
+fn never_match(x: Result<(), !>) {
+    match x {
+        Ok(_) => {},
+        Err(_) => {},
+    }
+}
+
+pub fn main() { }