make const_err a hard error

Author: RalfJung

clippy_lints/src/indexing_slicing.rs

@@ -19,7 +19,6 @@ declare_clippy_lint! {
     ///
     /// ### Example
     /// ```rust,no_run
-    /// # #![allow(const_err)]
     /// let x = [1, 2, 3, 4];
     ///
     /// x[9];

tests/ui/crashes/ice-9463.rs

@@ -1,4 +1,4 @@
-#![deny(arithmetic_overflow, const_err)]
+#![deny(arithmetic_overflow)]
 fn main() {
     let _x = -1_i32 >> -1;
     let _y = 1u32 >> 10000000000000u32;

tests/ui/crashes/ice-9463.stderr

@@ -7,7 +7,7 @@ LL |     let _x = -1_i32 >> -1;
 note: the lint level is defined here
   --> $DIR/ice-9463.rs:1:9
    |
-LL | #![deny(arithmetic_overflow, const_err)]
+LL | #![deny(arithmetic_overflow)]
    |         ^^^^^^^^^^^^^^^^^^^
 
 error: this arithmetic operation will overflow

tests/ui/indexing_slicing_index.rs

@@ -3,7 +3,7 @@
 // We also check the out_of_bounds_indexing lint here, because it lints similar things and
 // we want to avoid false positives.
 #![warn(clippy::out_of_bounds_indexing)]
-#![allow(const_err, unconditional_panic, clippy::no_effect, clippy::unnecessary_operation)]
+#![allow(unconditional_panic, clippy::no_effect, clippy::unnecessary_operation)]
 
 const ARR: [i32; 2] = [1, 2];
 const REF: &i32 = &ARR[idx()]; // Ok, should not produce stderr.

tests/ui/indexing_slicing_index.stderr

@@ -59,6 +59,12 @@ LL |     v[M];
    |
    = help: consider using `.get(n)` or `.get_mut(n)` instead
 
-error: aborting due to 8 previous errors
+error[E0080]: evaluation of constant value failed
+  --> $DIR/indexing_slicing_index.rs:10:24
+   |
+LL | const REF_ERR: &i32 = &ARR[idx4()]; // Ok, let rustc handle const contexts.
+   |                        ^^^^^^^^^^^ index out of bounds: the length is 2 but the index is 4
+
+error: aborting due to 9 previous errors
 
 For more information about this error, try `rustc --explain E0080`.

tests/ui/out_of_bounds_indexing/issue-3102.rs

@@ -1,5 +1,5 @@
 #![warn(clippy::out_of_bounds_indexing)]
-#![allow(clippy::no_effect, const_err)]
+#![allow(clippy::no_effect)]
 
 fn main() {
     let x = [1, 2, 3, 4];

tests/ui/out_of_bounds_indexing/simple.rs

@@ -1,5 +1,5 @@
 #![warn(clippy::out_of_bounds_indexing)]
-#![allow(clippy::no_effect, clippy::unnecessary_operation, const_err)]
+#![allow(clippy::no_effect, clippy::unnecessary_operation)]
 
 fn main() {
     let x = [1, 2, 3, 4];