do not lint on indexing inside const contexts

Author: tesuji

clippy_lints/src/no_effect.rs

@@ -1,7 +1,9 @@
 use clippy_utils::diagnostics::{span_lint_hir, span_lint_hir_and_then};
 use clippy_utils::source::snippet_opt;
 use clippy_utils::ty::has_drop;
-use clippy_utils::{any_parent_is_automatically_derived, is_lint_allowed, path_to_local, peel_blocks};
+use clippy_utils::{
+    any_parent_is_automatically_derived, is_inside_always_const_context, is_lint_allowed, path_to_local, peel_blocks,
+};
 use rustc_errors::Applicability;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::{
@@ -265,6 +267,9 @@ fn check_unnecessary_operation(cx: &LateContext<'_>, stmt: &Stmt<'_>) {
         && reduced.iter().all(|e| e.span.ctxt() == ctxt)
     {
         if let ExprKind::Index(..) = &expr.kind {
+            if is_inside_always_const_context(cx.tcx, expr.hir_id) {
+                return;
+            }
             let snippet =
                 if let (Some(arr), Some(func)) = (snippet_opt(cx, reduced[0].span), snippet_opt(cx, reduced[1].span)) {
                     format!("assert!({}.len() > {});", &arr, &func)

tests/ui/unnecessary_operation.fixed

@@ -114,12 +114,12 @@ fn main() {
         break 'label
     };
     let () = const {
-        assert!([42, 55].len() > get_usize());
+        [42, 55][get_usize()];
     };
 }
 
 const _: () = {
-    assert!([42, 55].len() > get_usize());
+    [42, 55][get_usize()];
 };
 
 const fn foo() {

tests/ui/unnecessary_operation.stderr

@@ -119,23 +119,11 @@ LL | |         s: String::from("blah"),
 LL | |     };
    | |______^ help: statement can be reduced to: `String::from("blah");`
 
-error: unnecessary operation
-  --> tests/ui/unnecessary_operation.rs:121:9
-   |
-LL |         [42, 55][get_usize()];
-   |         ^^^^^^^^^^^^^^^^^^^^^^ help: statement can be written as: `assert!([42, 55].len() > get_usize());`
-
-error: unnecessary operation
-  --> tests/ui/unnecessary_operation.rs:126:5
-   |
-LL |     [42, 55][get_usize()];
-   |     ^^^^^^^^^^^^^^^^^^^^^^ help: statement can be written as: `assert!([42, 55].len() > get_usize());`
-
 error: unnecessary operation
   --> tests/ui/unnecessary_operation.rs:130:5
    |
 LL |     [42, 55][get_usize()];
    |     ^^^^^^^^^^^^^^^^^^^^^^ help: statement can be written as: `assert!([42, 55].len() > get_usize());`
 
-error: aborting due to 22 previous errors
+error: aborting due to 20 previous errors