rustc_trans: do not attempt to truncate an i1 const to i1.

src/librustc_trans/mir/constant.rs

@@ -415,8 +415,11 @@ impl<'a, 'tcx> MirConstContext<'a, 'tcx> {
                                           Value(base));
                             }
                             if projected_ty.is_bool() {
-                                unsafe {
+                                let i1_type = Type::i1(self.ccx);
-                                    val = llvm::LLVMConstTrunc(val, Type::i1(self.ccx).to_ref());
+                                if val_ty(val) != i1_type {
+                                    unsafe {
+                                        val = llvm::LLVMConstTrunc(val, i1_type.to_ref());
+                                    }
                                 }
                             }
                             (Base::Value(val), extra)

src/test/run-pass/issue-41744.rs

@@ -0,0 +1,16 @@
+// Copyright 2017 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.
+
+trait Tc {}
+impl Tc for bool {}
+
+fn main() {
+    let _: &[&Tc] = &[&true];
+}