Auto merge of #33214 - oli-obk:const_err_var_exprs, r=eddyb

report `const_err` on all expressions that can fail

also a drive-by fix for reporting an "overflow in shift *left*" when shifting an `i64` *right*

This increases the warning noise for shifting by more than the bitwidth and for `-T::MIN`. I can silence the bitwidth warnings explicitly and fix the const evaluator to make sure `--$expr` is treated exactly like `$expr` (which is kinda wrong, but mathematically right).

r? @eddyb

src/test/compile-fail/const-err-early.rs

@@ -18,5 +18,5 @@ pub const D: u8 = 42u8 - (42u8 + 1); //~ ERROR attempted to subtract with overfl
 pub const E: u8 = [5u8][1]; //~ ERROR index out of bounds
 
 fn main() {
-    let _e = [6u8][1];
+    let _e = [6u8][1]; //~ ERROR: array index out of bounds
 }

src/test/compile-fail/const-err.rs

@@ -8,8 +8,12 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// these errors are not actually "const_err", they occur in trans/consts
+// and are unconditional warnings that can't be denied or allowed
+
 #![feature(rustc_attrs)]
 #![allow(exceeding_bitshifts)]
+#![allow(const_err)]
 
 fn black_box<T>(_: T) {
     unimplemented!()
@@ -21,7 +25,7 @@ fn main() {
     //~^ WARN attempted to negate with overflow
     let b = 200u8 + 200u8 + 200u8;
     //~^ WARN attempted to add with overflow
-    //~^^ WARN attempted to add with overflow
+    //~| WARN attempted to add with overflow
     let c = 200u8 * 4;
     //~^ WARN attempted to multiply with overflow
     let d = 42u8 - (42u8 + 1);

src/test/compile-fail/const-err2.rs

@@ -0,0 +1,34 @@
+// Copyright 2012 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.
+
+#![feature(rustc_attrs)]
+#![allow(exceeding_bitshifts)]
+#![deny(const_err)]
+
+fn black_box<T>(_: T) {
+    unimplemented!()
+}
+
+fn main() {
+    let a = -std::i8::MIN;
+    //~^ ERROR attempted to negate with overflow
+    let b = 200u8 + 200u8 + 200u8;
+    //~^ ERROR attempted to add with overflow
+    //~| ERROR attempted to add with overflow
+    let c = 200u8 * 4;
+    //~^ ERROR attempted to multiply with overflow
+    let d = 42u8 - (42u8 + 1);
+    //~^ ERROR attempted to subtract with overflow
+    let _e = [5u8][1];
+    black_box(a);
+    black_box(b);
+    black_box(c);
+    black_box(d);
+}

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

@@ -53,6 +53,7 @@ fn main() {
       let n = n << 8; //~ ERROR: bitshift exceeds the type's number of bits
 
       let n = 1u8 << -8; //~ ERROR: bitshift exceeds the type's number of bits
+      //~^ WARN: attempted to shift by a negative amount
 
       let n = 1u8 << (4+3);
       let n = 1u8 << (4+4); //~ ERROR: bitshift exceeds the type's number of bits

src/test/compile-fail/lint-type-overflow2.rs

@@ -10,6 +10,7 @@
 //
 
 #![deny(overflowing_literals)]
+#![deny(const_err)]
 
 #[allow(unused_variables)]
 fn main() {