Enable empty_loop lint for no_std crates

We skip the lint if the `loop {}` is in the `#[panic_handler]` as the
main recommendation we give is to panic, which obviously isn't
possible in a panic handler.

Signed-off-by: Joe Richey <joerichey@google.com>

clippy_lints/src/loops.rs

@@ -4,10 +4,10 @@ use crate::utils::sugg::Sugg;
 use crate::utils::usage::{is_unused, mutated_variables};
 use crate::utils::{
     contains_name, get_enclosing_block, get_parent_expr, get_trait_def_id, has_iter_method, higher, implements_trait,
-    indent_of, is_integer_const, is_no_std_crate, is_refutable, is_type_diagnostic_item, last_path_segment,
-    match_trait_method, match_type, match_var, multispan_sugg, qpath_res, single_segment_path, snippet,
-    snippet_with_applicability, snippet_with_macro_callsite, span_lint, span_lint_and_help, span_lint_and_sugg,
-    span_lint_and_then, sugg, SpanlessEq,
+    indent_of, is_in_panic_handler, is_integer_const, is_no_std_crate, is_refutable, is_type_diagnostic_item,
+    last_path_segment, match_trait_method, match_type, match_var, multispan_sugg, qpath_res, single_segment_path,
+    snippet, snippet_with_applicability, snippet_with_macro_callsite, span_lint, span_lint_and_help,
+    span_lint_and_sugg, span_lint_and_then, sugg, SpanlessEq,
 };
 use if_chain::if_chain;
 use rustc_ast::ast;
@@ -543,17 +543,15 @@ impl<'tcx> LateLintPass<'tcx> for Loops {
         // (also matches an explicit "match" instead of "if let")
         // (even if the "match" or "if let" is used for declaration)
         if let ExprKind::Loop(ref block, _, LoopSource::Loop) = expr.kind {
-            // also check for empty `loop {}` statements
-            // TODO(issue #6161): Enable for no_std crates (outside of #[panic_handler])
-            if block.stmts.is_empty() && block.expr.is_none() && !is_no_std_crate(cx.tcx.hir().krate()) {
-                span_lint_and_help(
-                    cx,
-                    EMPTY_LOOP,
-                    expr.span,
-                    "empty `loop {}` wastes CPU cycles",
-                    None,
-                    "You should either use `panic!()` or add `std::thread::sleep(..);` to the loop body.",
-                );
+            // also check for empty `loop {}` statements, skipping those in #[panic_handler]
+            if block.stmts.is_empty() && 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.tcx.hir().krate()) {
+                    "You should either use `panic!()` or add a call pausing or sleeping the thread to the loop body."
+                } else {
+                    "You should either use `panic!()` or add `std::thread::sleep(..);` to the loop body."
+                };
+                span_lint_and_help(cx, EMPTY_LOOP, expr.span, msg, None, help);
             }
 
             // extract the expression from the first statement (if any) in a block

clippy_lints/src/utils/mod.rs

@@ -468,6 +468,13 @@ pub fn is_entrypoint_fn(cx: &LateContext<'_>, def_id: DefId) -> bool {
         .map_or(false, |(entry_fn_def_id, _)| def_id == entry_fn_def_id.to_def_id())
 }
 
+/// Returns `true` if the expression is in the program's `#[panic_handler]`.
+pub fn is_in_panic_handler(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
+    let parent = cx.tcx.hir().get_parent_item(e.hir_id);
+    let def_id = cx.tcx.hir().local_def_id(parent).to_def_id();
+    Some(def_id) == cx.tcx.lang_items().panic_impl()
+}
+
 /// Gets the name of the item the expression is in, if available.
 pub fn get_item_name(cx: &LateContext<'_>, expr: &Expr<'_>) -> Option<Symbol> {
     let parent_id = cx.tcx.hir().get_parent_item(expr.hir_id);

tests/ui/empty_loop_no_std.rs

@@ -10,13 +10,18 @@ use core::panic::PanicInfo;
 
 #[start]
 fn main(argc: isize, argv: *const *const u8) -> isize {
+    // This should trigger the lint
     loop {}
 }
 
 #[panic_handler]
 fn panic(_info: &PanicInfo) -> ! {
+    // This should NOT trigger the lint
     loop {}
 }
 
 #[lang = "eh_personality"]
-extern "C" fn eh_personality() {}
+extern "C" fn eh_personality() {
+    // This should also trigger the lint
+    loop {}
+}

tests/ui/empty_loop_no_std.stderr

@@ -0,0 +1,19 @@
+error: empty `loop {}` wastes CPU cycles
+  --> $DIR/empty_loop_no_std.rs:14:5
+   |
+LL |     loop {}
+   |     ^^^^^^^
+   |
+   = note: `-D clippy::empty-loop` implied by `-D warnings`
+   = help: You should either use `panic!()` or add a call pausing or sleeping the thread to the loop body.
+
+error: empty `loop {}` wastes CPU cycles
+  --> $DIR/empty_loop_no_std.rs:26:5
+   |
+LL |     loop {}
+   |     ^^^^^^^
+   |
+   = help: You should either use `panic!()` or add a call pausing or sleeping the thread to the loop body.
+
+error: aborting due to 2 previous errors
+