Rollup merge of #81503 - henryboisdequin:fix-const-fn-arr-err-msg, r=estebank

Suggest to create a new `const` item if the `fn` in the array is a `const fn`

Fixes #73734. If the `fn` in the array repeat expression is a `const fn`, suggest creating a new `const` item. On nightly, suggest creating an inline `const` block. This PR also removes the `suggest_const_in_array_repeat_expressions` as it is no longer necessary.

Example:

```rust
fn main() {
    // Should not compile but hint to create a new const item (stable) or an inline const block (nightly)
    let strings: [String; 5] = [String::new(); 5];
    println!("{:?}", strings);
}

```

Gives this error:

```
error[E0277]: the trait bound `std::string::String: std::marker::Copy` is not satisfied
 --> $DIR/const-fn-in-vec.rs:3:32
  |
2 |     let strings: [String; 5] = [String::new(); 5];
  |                             ^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `String`
  |
  = note: the `Copy` trait is required because the repeated element will be copied
```

With this change, this is the error message:

```
error[E0277]: the trait bound `String: Copy` is not satisfied
  --> $DIR/const-fn-in-vec.rs:3:32
   |
LL |     let strings: [String; 5] = [String::new(); 5];
   |                                ^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
   |
   = help: moving the function call to a new `const` item will resolve the error
```

compiler/rustc_mir/src/transform/promote_consts.rs

@@ -1231,3 +1231,38 @@ pub fn promote_candidates<'tcx>(
 
     promotions
 }
+
+/// This function returns `true` if the function being called in the array
+/// repeat expression is a `const` function.
+crate fn is_const_fn_in_array_repeat_expression<'tcx>(
+    ccx: &ConstCx<'_, 'tcx>,
+    place: &Place<'tcx>,
+    body: &Body<'tcx>,
+) -> bool {
+    match place.as_local() {
+        // rule out cases such as: `let my_var = some_fn(); [my_var; N]`
+        Some(local) if body.local_decls[local].is_user_variable() => return false,
+        None => return false,
+        _ => {}
+    }
+
+    for block in body.basic_blocks() {
+        if let Some(Terminator { kind: TerminatorKind::Call { func, destination, .. }, .. }) =
+            &block.terminator
+        {
+            if let Operand::Constant(box Constant { literal: ty::Const { ty, .. }, .. }) = func {
+                if let ty::FnDef(def_id, _) = *ty.kind() {
+                    if let Some((destination_place, _)) = destination {
+                        if destination_place == place {
+                            if is_const_fn(ccx.tcx, def_id) {
+                                return true;
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    false
+}