External spans: Added a test for #38875.

A bug has been discovered and fixed in the process.

src/librustc_metadata/decoder.rs

@@ -765,7 +765,7 @@ impl<'a, 'tcx> CrateMetadata {
         assert!(!self.is_proc_macro(id));
         let ast = self.entry(id).ast.unwrap();
         let def_id = self.local_def_id(id);
-        let body = ast.decode(self).body.decode(self);
+        let body = ast.decode((self, tcx)).body.decode((self, tcx));
         tcx.hir.intern_inlined_body(def_id, body)
     }
 

src/test/ui/issue-38875/auxiliary/issue_38875_b.rs

@@ -0,0 +1,11 @@
+// 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.
+
+pub const FOO: usize = *&0;

src/test/ui/issue-38875/issue_38875.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_38875_b.rs
+
+extern crate issue_38875_b;
+
+fn main() {
+    let test_x = [0; issue_38875_b::FOO];
+}

src/test/ui/issue-38875/issue_38875.stderr

@@ -0,0 +1,14 @@
+error[E0080]: constant evaluation error
+  --> $DIR/auxiliary/issue_38875_b.rs:11:24
+   |
+11 | pub const FOO: usize = *&0;
+   |                        ^^^ unimplemented constant expression: deref operation
+   |
+note: for repeat count here
+  --> $DIR/issue_38875.rs:16:22
+   |
+16 |     let test_x = [0; issue_38875_b::FOO];
+   |                      ^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error(s)
+