Fix missing_const_for_thread_local false positive on targets without native #[thread_local]

clippy_lints/src/missing_const_for_thread_local.rs

@@ -122,6 +122,11 @@ impl<'tcx> LateLintPass<'tcx> for MissingConstForThreadLocal {
             && let ExprKind::Block(block, _) = body.value.kind
             && let Some(unpeeled) = block.expr
             && let ret_expr = peel_blocks(unpeeled)
+            // The initializer is already a `const` block: there is nothing to suggest.
+            // On targets without `#[thread_local]` (e.g. x86_64-pc-windows-gnu), the
+            // generated init fn is not a `const fn` even for `const` initializers,
+            // so the `is_const_fn` check above does not cover this case.
+            && !matches!(ret_expr.kind, ExprKind::ConstBlock(_))
             // A common pattern around threadlocal! is to make the value unreachable
             // to force an initialization before usage
             // https://github.com/rust-lang/rust-clippy/issues/12637

tests/ui/missing_const_for_thread_local.fixed

@@ -69,6 +69,14 @@ fn issue_12637() {
     println!("{}", STATE_12637_UNREACHABLE.get());
 }
 
+fn issue_17566() {
+    // On targets without native `#[thread_local]`, a `const` initializer expands to a
+    // non-`const` init fn, which used to slip past the `is_const_fn` guard and lint here.
+    thread_local! {
+        static STATE_17566: Cell<Option<u8>> = const { Cell::new(None) };
+    }
+}
+
 #[clippy::msrv = "1.58"]
 fn f() {
     thread_local! {

tests/ui/missing_const_for_thread_local.rs

@@ -69,6 +69,14 @@ fn issue_12637() {
     println!("{}", STATE_12637_UNREACHABLE.get());
 }
 
+fn issue_17566() {
+    // On targets without native `#[thread_local]`, a `const` initializer expands to a
+    // non-`const` init fn, which used to slip past the `is_const_fn` guard and lint here.
+    thread_local! {
+        static STATE_17566: Cell<Option<u8>> = const { Cell::new(None) };
+    }
+}
+
 #[clippy::msrv = "1.58"]
 fn f() {
     thread_local! {