Implement lint for black_boxing ZSTs

compiler/rustc_lint/messages.ftl

@@ -58,6 +58,13 @@ lint_builtin_allow_internal_unsafe =
 lint_builtin_anonymous_params = anonymous parameters are deprecated and will be removed in the next edition
     .suggestion = try naming the parameter or explicitly ignoring it
 
+lint_builtin_black_box_zst_call = use of `black_box` on the zero-sized type `{$ty}` has no effect
+    .label = zero-sized value passed here
+
+lint_builtin_black_box_zst_help = if this is a function, coerce it to a function pointer first
+
+lint_builtin_black_box_zst_note = zero-sized values have no runtime representation, so the compiler can ignore the call
+
 lint_builtin_clashing_extern_diff_name = `{$this}` redeclares `{$orig}` with a different signature
     .previous_decl_label = `{$orig}` previously declared here
     .mismatch_label = this signature doesn't match the previous declaration

compiler/rustc_lint/src/builtin.rs

@@ -50,16 +50,17 @@ use rustc_trait_selection::traits::{self};
 
 use crate::errors::BuiltinEllipsisInclusiveRangePatterns;
 use crate::lints::{
-    BuiltinAnonymousParams, BuiltinConstNoMangle, BuiltinDerefNullptr, BuiltinDoubleNegations,
-    BuiltinDoubleNegationsAddParens, BuiltinEllipsisInclusiveRangePatternsLint,
-    BuiltinExplicitOutlives, BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote,
-    BuiltinIncompleteFeatures, BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures,
-    BuiltinKeywordIdents, BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc,
-    BuiltinMutablesTransmutes, BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns,
-    BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds, BuiltinTypeAliasBounds,
-    BuiltinUngatedAsyncFnTrackCaller, BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub,
-    BuiltinUnreachablePub, BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment,
-    BuiltinUnusedDocCommentSub, BuiltinWhileTrue, InvalidAsmLabel,
+    BuiltinAnonymousParams, BuiltinBlackBoxZstCall, BuiltinConstNoMangle, BuiltinDerefNullptr,
+    BuiltinDoubleNegations, BuiltinDoubleNegationsAddParens,
+    BuiltinEllipsisInclusiveRangePatternsLint, BuiltinExplicitOutlives,
+    BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote, BuiltinIncompleteFeatures,
+    BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures, BuiltinKeywordIdents,
+    BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc, BuiltinMutablesTransmutes,
+    BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns, BuiltinSpecialModuleNameUsed,
+    BuiltinTrivialBounds, BuiltinTypeAliasBounds, BuiltinUngatedAsyncFnTrackCaller,
+    BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub,
+    BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment, BuiltinUnusedDocCommentSub,
+    BuiltinWhileTrue, InvalidAsmLabel,
 };
 use crate::{
     EarlyContext, EarlyLintPass, LateContext, LateLintPass, Level, LintContext,
@@ -3110,6 +3111,74 @@ impl<'tcx> LateLintPass<'tcx> for AsmLabels {
     }
 }
 
+declare_lint! {
+    /// The `black_box_zst_calls` lint detects calls to `core::hint::black_box`
+    /// with a zero-sized type (ZST).
+    ///
+    /// Zero-sized types (like `()`, unit structs, or function items) have no
+    /// runtime representation, so `black_box` cannot block the optimizer because
+    /// there is no value to hide.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(black_box_zst_calls)]
+    /// use std::hint::black_box;
+    ///
+    /// fn main() {
+    ///     // This does nothing because `()` has no size.
+    ///     black_box(());
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// `black_box` is intended to treat a value as "unknown" to the optimizer.
+    /// However, if the value has size 0, the optimizer knows exactly what it is
+    /// (it's nothing!) and can optimize around it anyway.
+    pub BLACK_BOX_ZST_CALLS,
+    Warn,
+    "usage of `black_box` with zero-sized types"
+}
+
+declare_lint_pass!(BlackBoxZstCalls => [BLACK_BOX_ZST_CALLS]);
+
+impl<'tcx> LateLintPass<'tcx> for BlackBoxZstCalls {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &hir::Expr<'tcx>) {
+        let hir::ExprKind::Call(callee, args) = expr.kind else { return };
+        if args.len() != 1 {
+            return;
+        }
+
+        let hir::ExprKind::Path(ref qpath) = callee.kind else { return };
+        let Some(def_id) = cx.qpath_res(qpath, callee.hir_id).opt_def_id() else { return };
+
+        if !cx.tcx.is_diagnostic_item(sym::black_box, def_id) {
+            return;
+        }
+
+        let arg = &args[0];
+        let arg_ty = cx.typeck_results().expr_ty_adjusted(arg);
+
+        if !is_zst(cx, arg_ty) {
+            return;
+        }
+
+        let ty_name = with_no_trimmed_paths!(arg_ty.to_string());
+        cx.emit_span_lint(
+            BLACK_BOX_ZST_CALLS,
+            expr.span,
+            BuiltinBlackBoxZstCall { arg_span: arg.span, ty: ty_name },
+        );
+    }
+}
+
+fn is_zst<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
+    cx.tcx.layout_of(cx.typing_env().as_query_input(ty)).is_ok_and(|layout| layout.is_zst())
+}
+
 declare_lint! {
     /// The `special_module_name` lint detects module
     /// declarations for files that have a special meaning.

compiler/rustc_lint/src/lib.rs

@@ -201,6 +201,7 @@ late_lint_methods!(
             InvalidFromUtf8: InvalidFromUtf8,
             VariantSizeDifferences: VariantSizeDifferences,
             PathStatements: PathStatements,
+            BlackBoxZstCalls: BlackBoxZstCalls,
             LetUnderscore: LetUnderscore,
             InvalidReferenceCasting: InvalidReferenceCasting,
             ImplicitAutorefs: ImplicitAutorefs,

compiler/rustc_lint/src/lints.rs

@@ -118,6 +118,16 @@ pub(crate) struct BuiltinNonShorthandFieldPatterns {
     pub prefix: &'static str,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_builtin_black_box_zst_call)]
+#[note(lint_builtin_black_box_zst_note)]
+#[help(lint_builtin_black_box_zst_help)]
+pub(crate) struct BuiltinBlackBoxZstCall {
+    #[label]
+    pub arg_span: Span,
+    pub ty: String,
+}
+
 #[derive(LintDiagnostic)]
 pub(crate) enum BuiltinUnsafe {
     #[diag(lint_builtin_allow_internal_unsafe)]

library/core/src/hint.rs

@@ -478,6 +478,7 @@ pub fn spin_loop() {
 #[inline]
 #[stable(feature = "bench_black_box", since = "1.66.0")]
 #[rustc_const_stable(feature = "const_black_box", since = "1.86.0")]
+#[rustc_diagnostic_item = "black_box"]
 pub const fn black_box<T>(dummy: T) -> T {
     crate::intrinsics::black_box(dummy)
 }

library/std/src/sys/backtrace.rs

@@ -182,6 +182,7 @@ where
     let result = f();
 
     // prevent this frame from being tail-call optimised away
+    #[allow(black_box_zst_calls)]
     crate::hint::black_box(());
 
     result

tests/ui/lint/lint-black-box-zst-call.rs

@@ -0,0 +1,13 @@
+#![deny(black_box_zst_calls)]
+
+use std::hint::black_box;
+
+fn add(a: u32, b: u32) -> u32 {
+    a + b
+}
+
+fn main() {
+    let add_bb = black_box(add);
+    //~^ ERROR use of `black_box` on the zero-sized type
+    let _ = add_bb(1, 2);
+}

tests/ui/lint/lint-black-box-zst-call.stderr

@@ -0,0 +1,18 @@
+error: use of `black_box` on the zero-sized type `fn(u32, u32) -> u32 {add}` has no effect
+  --> $DIR/lint-black-box-zst-call.rs:10:18
+   |
+LL |     let add_bb = black_box(add);
+   |                  ^^^^^^^^^^---^
+   |                            |
+   |                            zero-sized value passed here
+   |
+   = note: zero-sized values have no runtime representation, so the compiler can ignore the call
+   = help: if this is a function, coerce it to a function pointer first
+note: the lint level is defined here
+  --> $DIR/lint-black-box-zst-call.rs:1:9
+   |
+LL | #![deny(black_box_zst_calls)]
+   |         ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+