warn about rust_2021_compatibility and fix warning

warning: changes to closure capture in Rust 2021 will affect drop order
   --> clippy_utils/src/lib.rs:995:26
    |
995 |     v.allow_closure.then(|| v.captures)
    |                          ^^^----------
    |                             |
    |                             in Rust 2018, this closure captures all of `v`, but in Rust 2021, it will only capture `v.captures`
996 | }
    | - in Rust 2018, `v` is dropped here, but in Rust 2021, only `v.captures` will be dropped here as part of the closure
    |
note: the lint level is defined here
   --> clippy_utils/src/lib.rs:16:9
    |
16  | #![warn(rust_2021_compatibility)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^
    = note: `#[warn(rust_2021_incompatible_closure_captures)]` implied by `#[warn(rust_2021_compatibility)]`
    = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2021/disjoint-capture-in-closures.html>
help: add a dummy let to cause `v` to be fully captured
    |
995 |     v.allow_closure.then(|| { let _ = &v; v.captures })
    |                             +++++++++++++            +

clippy_dev/src/lib.rs

@@ -1,5 +1,6 @@
 #![feature(once_cell)]
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2021_compatibility)]
 // warn on lints, that are included in `rust-lang/rust`s bootstrap
 #![warn(rust_2018_idioms, unused_lifetimes)]
 

clippy_dev/src/main.rs

@@ -1,4 +1,5 @@
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2021_compatibility)]
 // warn on lints, that are included in `rust-lang/rust`s bootstrap
 #![warn(rust_2018_idioms, unused_lifetimes)]
 

clippy_lints/src/lib.rs

@@ -12,6 +12,7 @@
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
 #![allow(clippy::missing_docs_in_private_items, clippy::must_use_candidate)]
 #![warn(trivial_casts, trivial_numeric_casts)]
+#![warn(rust_2021_compatibility)]
 // warn on lints, that are included in `rust-lang/rust`s bootstrap
 #![warn(rust_2018_idioms, unused_lifetimes)]
 // warn on rustc internal lints

clippy_utils/src/lib.rs

@@ -8,10 +8,12 @@
 #![allow(clippy::missing_errors_doc, clippy::missing_panics_doc, clippy::must_use_candidate)]
 // warn on the same lints as `clippy_lints`
 #![warn(trivial_casts, trivial_numeric_casts)]
+#![warn(rust_2021_compatibility)]
 // warn on lints, that are included in `rust-lang/rust`s bootstrap
 #![warn(rust_2018_idioms, unused_lifetimes)]
 // warn on rustc internal lints
 #![warn(rustc::internal)]
+#![warn(rust_2021_compatibility)]
 
 // FIXME: switch to something more ergonomic here, once available.
 // (Currently there is no way to opt into sysroot crates without `extern crate`.)
@@ -990,7 +992,10 @@ pub fn can_move_expr_to_closure(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) ->
         captures: HirIdMap::default(),
     };
     v.visit_expr(expr);
-    v.allow_closure.then(|| v.captures)
+    v.allow_closure.then(|| {
+        let _ = &v;
+        v.captures
+    })
 }
 
 /// Returns the method names and argument list of nested method call expressions that make up

src/driver.rs

@@ -1,6 +1,7 @@
 #![feature(rustc_private)]
 #![feature(once_cell)]
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2021_compatibility)]
 // warn on lints, that are included in `rust-lang/rust`s bootstrap
 #![warn(rust_2018_idioms, unused_lifetimes)]
 // warn on rustc internal lints

src/main.rs

@@ -1,4 +1,5 @@
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2021_compatibility)]
 // warn on lints, that are included in `rust-lang/rust`s bootstrap
 #![warn(rust_2018_idioms, unused_lifetimes)]
 

tests/compile-test.rs

@@ -1,6 +1,7 @@
 #![feature(test)] // compiletest_rs requires this attribute
 #![feature(once_cell)]
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2021_compatibility)]
 #![warn(rust_2018_idioms, unused_lifetimes)]
 
 use compiletest_rs as compiletest;

tests/dogfood.rs

@@ -7,6 +7,7 @@
 #![cfg(not(windows))]
 #![feature(once_cell)]
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2021_compatibility)]
 #![warn(rust_2018_idioms, unused_lifetimes)]
 
 use std::lazy::SyncLazy;

tests/fmt.rs

@@ -1,4 +1,5 @@
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2021_compatibility)]
 #![warn(rust_2018_idioms, unused_lifetimes)]
 
 use std::path::PathBuf;

tests/integration.rs

@@ -1,5 +1,6 @@
 #![cfg(feature = "integration")]
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2021_compatibility)]
 #![warn(rust_2018_idioms, unused_lifetimes)]
 
 use std::env;

tests/lint_message_convention.rs

@@ -1,4 +1,5 @@
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2021_compatibility)]
 #![warn(rust_2018_idioms, unused_lifetimes)]
 
 use std::ffi::OsStr;

tests/missing-test-files.rs

@@ -1,4 +1,5 @@
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2021_compatibility)]
 #![warn(rust_2018_idioms, unused_lifetimes)]
 #![allow(clippy::assertions_on_constants)]
 

tests/versioncheck.rs

@@ -1,4 +1,5 @@
 #![cfg_attr(feature = "deny-warnings", deny(warnings))]
+#![warn(rust_2021_compatibility)]
 #![warn(rust_2018_idioms, unused_lifetimes)]
 #![allow(clippy::single_match_else)]