Auto merge of #52400 - RalfJung:match, r=oli-obk

CTFE: use binary_op to compare integer with match disriminant

This is needed to unblock rust-lang/miri#401: There is code in the Windows initialization functions that uses `match` to test whether a pointer is NULL.

I will add a testcase in miri; I was not sure where to add a testcase in Rust itself.

r? @oli-obk

Author: bors

src/librustc_mir/interpret/terminator/mod.rs

@@ -1,10 +1,10 @@
 use rustc::mir;
 use rustc::ty::{self, Ty};
-use rustc::ty::layout::LayoutOf;
+use rustc::ty::layout::{LayoutOf, Size};
 use syntax::codemap::Span;
 use rustc_target::spec::abi::Abi;
 
-use rustc::mir::interpret::EvalResult;
+use rustc::mir::interpret::{EvalResult, Scalar};
 use super::{EvalContext, Place, Machine, ValTy};
 
 use rustc_data_structures::indexed_vec::Idx;
@@ -41,13 +41,18 @@ impl<'a, 'mir, 'tcx, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M> {
                 let discr_prim = self.value_to_scalar(discr_val)?;
                 let discr_layout = self.layout_of(discr_val.ty).unwrap();
                 trace!("SwitchInt({:?}, {:#?})", discr_prim, discr_layout);
-                let discr_prim = discr_prim.to_bits(discr_layout.size)?;
 
                 // Branch to the `otherwise` case by default, if no match is found.
                 let mut target_block = targets[targets.len() - 1];
 
                 for (index, &const_int) in values.iter().enumerate() {
-                    if discr_prim == const_int {
+                    // Compare using binary_op
+                    let const_int = Scalar::Bits { bits: const_int, defined: 128 };
+                    let res = self.binary_op(mir::BinOp::Eq,
+                        discr_prim, discr_val.ty,
+                        const_int, discr_val.ty
+                    )?;
+                    if res.0.to_bits(Size::from_bytes(1))? != 0 {
                         target_block = targets[index];
                         break;
                     }

src/test/ui/const-eval/match-test-ptr-null.rs

@@ -0,0 +1,22 @@
+// 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.
+
+fn main() {
+    // Make sure match uses the usual pointer comparison code path -- i.e., it should complain
+    // that pointer comparison is disallowed, not that parts of a pointer are accessed as raw
+    // bytes.
+    let _: [u8; 0] = [4; { //~ ERROR could not evaluate repeat length
+        match &1 as *const i32 as usize { //~ ERROR raw pointers cannot be cast to integers
+            0 => 42, //~ ERROR constant contains unimplemented expression type
+            //~^ NOTE "pointer arithmetic or comparison" needs an rfc before being allowed
+            n => n,
+        }
+    }];
+}

src/test/ui/const-eval/match-test-ptr-null.stderr

@@ -0,0 +1,30 @@
+error[E0018]: raw pointers cannot be cast to integers in constants
+  --> $DIR/match-test-ptr-null.rs:16:15
+   |
+LL |         match &1 as *const i32 as usize { //~ ERROR raw pointers cannot be cast to integers
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/match-test-ptr-null.rs:17:13
+   |
+LL |             0 => 42, //~ ERROR constant contains unimplemented expression type
+   |             ^
+
+error[E0080]: could not evaluate repeat length
+  --> $DIR/match-test-ptr-null.rs:15:26
+   |
+LL |       let _: [u8; 0] = [4; { //~ ERROR could not evaluate repeat length
+   |  __________________________^
+LL | |         match &1 as *const i32 as usize { //~ ERROR raw pointers cannot be cast to integers
+LL | |             0 => 42, //~ ERROR constant contains unimplemented expression type
+   | |             - "pointer arithmetic or comparison" needs an rfc before being allowed inside constants
+LL | |             //~^ NOTE "pointer arithmetic or comparison" needs an rfc before being allowed
+LL | |             n => n,
+LL | |         }
+LL | |     }];
+   | |_____^
+
+error: aborting due to 3 previous errors
+
+Some errors occurred: E0018, E0019, E0080.
+For more information about an error, try `rustc --explain E0018`.