Do not lint intrinsics as empty loops (#15201)

Fix #15200

changelog: [`empty_loop`]: fix false positive on intrinsic function
declaration

Author: dswij

clippy_lints/src/loops/empty_loop.rs

@@ -1,11 +1,22 @@
 use super::EMPTY_LOOP;
 use clippy_utils::diagnostics::span_lint_and_help;
-use clippy_utils::{is_in_panic_handler, is_no_std_crate};
+use clippy_utils::{is_in_panic_handler, is_no_std_crate, sym};
 
-use rustc_hir::{Block, Expr};
+use rustc_hir::{Block, Expr, ItemKind, Node};
 use rustc_lint::LateContext;
 
 pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, loop_block: &Block<'_>) {
+    let parent_hir_id = cx.tcx.parent_hir_id(expr.hir_id);
+    if let Node::Item(parent_node) = cx.tcx.hir_node(parent_hir_id)
+        && matches!(parent_node.kind, ItemKind::Fn { .. })
+        && let attrs = cx.tcx.hir_attrs(parent_hir_id)
+        && attrs.iter().any(|attr| attr.has_name(sym::rustc_intrinsic))
+    {
+        // Intrinsic functions are expanded into an empty loop when lowering the AST
+        // to simplify the job of later passes which might expect any function to have a body.
+        return;
+    }
+
     if loop_block.stmts.is_empty() && loop_block.expr.is_none() && !is_in_panic_handler(cx, expr) {
         let msg = "empty `loop {}` wastes CPU cycles";
         let help = if is_no_std_crate(cx) {

tests/ui/empty_loop_intrinsic.rs

@@ -0,0 +1,13 @@
+//@check-pass
+
+#![warn(clippy::empty_loop)]
+#![feature(intrinsics)]
+#![feature(rustc_attrs)]
+
+// From issue #15200
+#[rustc_intrinsic]
+#[rustc_nounwind]
+/// # Safety
+pub const unsafe fn simd_insert<T, U>(x: T, idx: u32, val: U) -> T;
+
+fn main() {}