rustc_const_eval: track the length and index in IndexOutOfBounds.

src/test/compile-fail/array_const_index-0.rs

@@ -10,7 +10,7 @@
 
 const A: &'static [i32] = &[];
 const B: i32 = (&A)[1];
-//~^ ERROR: array index out of bounds
+//~^ ERROR index out of bounds: the len is 0 but the index is 1
 
 fn main() {
     let _ = B;

src/test/compile-fail/array_const_index-1.rs

@@ -9,7 +9,8 @@
 // except according to those terms.
 
 const A: [i32; 0] = [];
-const B: i32 = A[1]; //~ ERROR: array index out of bounds
+const B: i32 = A[1];
+//~^ ERROR index out of bounds: the len is 0 but the index is 1
 
 fn main() {
     let _ = B;

src/test/compile-fail/const-array-oob.rs

@@ -8,13 +8,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// ignore-tidy-linelength
+
 #![feature(const_indexing)]
 
 const FOO: [u32; 3] = [1, 2, 3];
 const BAR: u32 = FOO[5]; // no error, because the error below occurs before regular const eval
 
 const BLUB: [u32; FOO[4]] = [5, 6];
-//~^ ERROR array length constant evaluation error: array index out of bounds [E0250]
+//~^ ERROR array length constant evaluation error: index out of bounds: the len is 3 but the index is 4 [E0250]
 
 fn main() {
     let _ = BAR;

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

@@ -15,8 +15,10 @@ pub const A: i8 = -std::i8::MIN; //~ ERROR attempted to negate with overflow
 pub const B: u8 = 200u8 + 200u8; //~ ERROR attempted to add with overflow
 pub const C: u8 = 200u8 * 4; //~ ERROR attempted to multiply with overflow
 pub const D: u8 = 42u8 - (42u8 + 1); //~ ERROR attempted to subtract with overflow
-pub const E: u8 = [5u8][1]; //~ ERROR index out of bounds
+pub const E: u8 = [5u8][1];
+//~^ ERROR index out of bounds: the len is 1 but the index is 1
 
 fn main() {
-    let _e = [6u8][1]; //~ ERROR: array index out of bounds
+    let _e = [6u8][1];
+    //~^ ERROR index out of bounds: the len is 1 but the index is 1
 }

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

@@ -20,8 +20,8 @@ fn black_box<T>(_: T) {
 
 // Make sure that the two uses get two errors.
 const FOO: u8 = [5u8][1];
-//~^ ERROR array index out of bounds
-//~^^ ERROR array index out of bounds
+//~^ ERROR index out of bounds: the len is 1 but the index is 1
+//~^^ ERROR index out of bounds: the len is 1 but the index is 1
 
 fn main() {
     let a = -std::i8::MIN;
@@ -34,7 +34,7 @@ fn main() {
     let d = 42u8 - (42u8 + 1);
     //~^ WARN attempted to subtract with overflow
     let _e = [5u8][1];
-    //~^ WARN array index out of bounds
+    //~^ WARN index out of bounds: the len is 1 but the index is 1
     black_box(a);
     black_box(b);
     black_box(c);

src/test/compile-fail/const-slice-oob.rs

@@ -9,7 +9,8 @@
 // except according to those terms.
 
 const FOO: &'static[u32] = &[1, 2, 3];
-const BAR: u32 = FOO[5]; //~ ERROR array index out of bounds
+const BAR: u32 = FOO[5];
+//~^ ERROR index out of bounds: the len is 3 but the index is 5
 
 fn main() {
     let _ = BAR;