Auto merge of #46894 - detrumi:fix-const-eval-trait, r=eddyb

Const-eval array lengths in rustdoc.

Fixes #46727
r? @eddyb

Big thanks to @eddyb for helping me figure this out.

src/librustdoc/clean/mod.rs

@@ -33,7 +33,6 @@ use rustc::middle::resolve_lifetime as rl;
 use rustc::middle::lang_items;
 use rustc::hir::def::{Def, CtorKind};
 use rustc::hir::def_id::{CrateNum, DefId, CRATE_DEF_INDEX, LOCAL_CRATE};
-use rustc::traits::Reveal;
 use rustc::ty::subst::Substs;
 use rustc::ty::{self, Ty, AdtKind};
 use rustc::middle::stability;
@@ -2062,7 +2061,7 @@ impl Clean<Type> for hir::Ty {
             TySlice(ref ty) => Slice(box ty.clean(cx)),
             TyArray(ref ty, n) => {
                 let def_id = cx.tcx.hir.body_owner_def_id(n);
-                let param_env = ty::ParamEnv::empty(Reveal::UserFacing);
+                let param_env = cx.tcx.param_env(def_id);
                 let substs = Substs::identity_for_item(cx.tcx, def_id);
                 let n = cx.tcx.const_eval(param_env.and((def_id, substs))).unwrap();
                 let n = if let ConstVal::Integral(ConstInt::Usize(n)) = n.val {
@@ -2191,6 +2190,11 @@ impl<'tcx> Clean<Type> for Ty<'tcx> {
             ty::TyStr => Primitive(PrimitiveType::Str),
             ty::TySlice(ty) => Slice(box ty.clean(cx)),
             ty::TyArray(ty, n) => {
+                let mut n = cx.tcx.lift(&n).unwrap();
+                if let ConstVal::Unevaluated(def_id, substs) = n.val {
+                    let param_env = cx.tcx.param_env(def_id);
+                    n = cx.tcx.const_eval(param_env.and((def_id, substs))).unwrap()
+                };
                 let n = if let ConstVal::Integral(ConstInt::Usize(n)) = n.val {
                     n.to_string()
                 } else if let ConstVal::Unevaluated(def_id, _) = n.val {

src/test/rustdoc/auxiliary/issue-46727.rs

@@ -0,0 +1,17 @@
+// 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.
+
+// compile-flags: -Cmetadata=aux
+
+pub trait Foo {}
+
+pub struct Bar<T> { x: T }
+
+impl<T> Foo for Bar<[T; 1 + 1 + 1]> {}

src/test/rustdoc/issue-46727.rs

@@ -0,0 +1,17 @@
+// 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.
+
+// aux-build:issue-46727.rs
+
+extern crate issue_46727;
+
+// @has issue_46727/trait.Foo.html
+// @has - '//code' 'impl<T> Foo for Bar<[T; 3]>'
+pub use issue_46727::{Foo, Bar};