Clarify the usage of "hints" in const_eval.

The "hint" mechanism is essentially used as a workaround to compute
types for expressions which have not yet been type-checked. This
commit clarifies that usage, and limits the effects to the places
where it is currently necessary.

Fixes #26210.

src/test/compile-fail/lint-exceeding-bitshifts.rs

@@ -8,11 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(negate_unsigned)]
 #![deny(exceeding_bitshifts)]
 #![allow(unused_variables)]
 #![allow(dead_code)]
-#![feature(num_bits_bytes, negate_unsigned)]
+#![feature(num_bits_bytes)]
 
 fn main() {
       let n = 1u8 << 7;
@@ -60,4 +59,7 @@ fn main() {
 
       let n = 1_isize << std::isize::BITS; //~ ERROR: bitshift exceeds the type's number of bits
       let n = 1_usize << std::usize::BITS; //~ ERROR: bitshift exceeds the type's number of bits
+
+
+      let n = 1i8<<(1isize+-1);
 }

src/test/compile-fail/match-range-fail-2.rs

@@ -0,0 +1,23 @@
+// Copyright 2015 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() {
+    match 5 {
+        6 ... 1 => { }
+        _ => { }
+    };
+    //~^^^ ERROR lower range bound must be less than or equal to upper
+
+    match 5u64 {
+        0xFFFF_FFFF_FFFF_FFFF ... 1 => { }
+        _ => { }
+    };
+    //~^^^ ERROR lower range bound must be less than or equal to upper
+}

src/test/compile-fail/match-range-fail.rs

@@ -9,12 +9,6 @@
 // except according to those terms.
 
 fn main() {
-    match 5 {
-        6 ... 1 => { }
-        _ => { }
-    };
-    //~^^^ ERROR lower range bound must be less than or equal to upper
-
     match "wow" {
         "bar" ... "foo" => { }
     };