Auto merge of #118833 - Urgau:lint_function_pointer_comparisons, r=<try>

Add lint against function pointer comparisons

This is kind of a follow-up to #117758 where we added a lint against wide pointer comparisons for being ambiguous and unreliable; well function pointer comparisons are also unreliable. We should IMO follow a similar logic and warn people about it.

-----

## `unpredictable_function_pointer_comparisons`

*warn-by-default*

The `unpredictable_function_pointer_comparisons` lint checks comparison of function pointer as the operands.

### Example

```rust
fn foo() {}
let a = foo as fn();

let _ = a == foo;
```

### Explanation

Function pointers comparisons do not produce meaningful result since they are never guaranteed to be unique and could vary between different code generation units. Furthermore different function could have the same address after being merged together.

----

This PR also uplift the very similar `clippy::fn_address_comparisons` lint, which only linted on if one of the operand was an `ty::FnDef` while this PR lints proposes to lint on all `ty::FnPtr` and `ty::FnDef`.

`@rustbot` labels +I-lang-nominated

Author: bors

compiler/rustc_lint/messages.ftl

@@ -538,6 +538,10 @@ lint_unknown_lint =
 lint_unknown_tool_in_scoped_lint = unknown tool name `{$tool_name}` found in scoped lint: `{$tool_name}::{$lint_name}`
     .help = add `#![register_tool({$tool_name})]` to the crate root
 
+lint_unpredictable_fn_pointer_comparisons = function pointer comparisons do not produce meaningful results since their addresses are not guaranteed to be unique
+    .note_duplicated_fn = the address of the same function can very between different codegen units
+    .note_deduplicated_fn = furthermore, different functions could have the same address after being merged together
+
 lint_unsupported_group = `{$lint_group}` lint group is not supported with ´--force-warn´
 
 lint_untranslatable_diag = diagnostics should be created using translatable messages

compiler/rustc_lint/src/lints.rs

@@ -1644,6 +1644,12 @@ pub enum AmbiguousWidePointerComparisonsAddrSuggestion<'a> {
     },
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_unpredictable_fn_pointer_comparisons)]
+#[note(lint_note_duplicated_fn)]
+#[note(lint_note_deduplicated_fn)]
+pub struct UnpredictableFunctionPointerComparisons;
+
 pub struct ImproperCTypes<'a> {
     pub ty: Ty<'a>,
     pub desc: &'a str,

compiler/rustc_lint/src/types.rs

@@ -7,7 +7,8 @@ use crate::{
         InvalidNanComparisonsSuggestion, OnlyCastu8ToChar, OverflowingBinHex,
         OverflowingBinHexSign, OverflowingBinHexSignBitSub, OverflowingBinHexSub, OverflowingInt,
         OverflowingIntHelp, OverflowingLiteral, OverflowingUInt, RangeEndpointOutOfRange,
-        UnusedComparisons, UseInclusiveRange, VariantSizeDifferencesDiag,
+        UnpredictableFunctionPointerComparisons, UnusedComparisons, UseInclusiveRange,
+        VariantSizeDifferencesDiag,
     },
 };
 use crate::{LateContext, LateLintPass, LintContext};
@@ -169,6 +170,32 @@ declare_lint! {
     "detects ambiguous wide pointer comparisons"
 }
 
+declare_lint! {
+    /// The `unpredictable_function_pointer_comparisons` lint checks comparison
+    /// of function pointer as the operands.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// fn foo() {}
+    /// # let a = foo as fn();
+    ///
+    /// let _ = a == foo;
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Function pointers comparisons do not produce meaningful result since
+    /// they are never guaranteed to be unique and could vary between different
+    /// code generation units. Furthermore, different functions could have the
+    /// same address after being merged together.
+    UNPREDICTABLE_FUNCTION_POINTER_COMPARISONS,
+    Deny,
+    "detects unpredictable function pointer comparisons"
+}
+
 #[derive(Copy, Clone)]
 pub struct TypeLimits {
     /// Id of the last visited negated expression
@@ -181,7 +208,8 @@ impl_lint_pass!(TypeLimits => [
     UNUSED_COMPARISONS,
     OVERFLOWING_LITERALS,
     INVALID_NAN_COMPARISONS,
-    AMBIGUOUS_WIDE_POINTER_COMPARISONS
+    AMBIGUOUS_WIDE_POINTER_COMPARISONS,
+    UNPREDICTABLE_FUNCTION_POINTER_COMPARISONS
 ]);
 
 impl TypeLimits {
@@ -758,6 +786,30 @@ fn lint_wide_pointer<'tcx>(
     );
 }
 
+fn lint_fn_pointer<'tcx>(
+    cx: &LateContext<'tcx>,
+    e: &'tcx hir::Expr<'tcx>,
+    _binop: hir::BinOpKind,
+    l: &'tcx hir::Expr<'tcx>,
+    r: &'tcx hir::Expr<'tcx>,
+) {
+    let Some(l_ty) = cx.typeck_results().expr_ty_opt(l) else { return };
+    let Some(r_ty) = cx.typeck_results().expr_ty_opt(r) else { return };
+
+    let l_ty = l_ty.peel_refs();
+    let r_ty = r_ty.peel_refs();
+
+    if !l_ty.is_fn() || !r_ty.is_fn() {
+        return;
+    }
+
+    cx.emit_spanned_lint(
+        UNPREDICTABLE_FUNCTION_POINTER_COMPARISONS,
+        e.span,
+        UnpredictableFunctionPointerComparisons,
+    );
+}
+
 impl<'tcx> LateLintPass<'tcx> for TypeLimits {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx hir::Expr<'tcx>) {
         match e.kind {
@@ -775,6 +827,7 @@ impl<'tcx> LateLintPass<'tcx> for TypeLimits {
                     } else {
                         lint_nan(cx, e, binop, l, r);
                         lint_wide_pointer(cx, e, binop.node, l, r);
+                        lint_fn_pointer(cx, e, binop.node, l, r);
                     }
                 }
             }
@@ -786,13 +839,15 @@ impl<'tcx> LateLintPass<'tcx> for TypeLimits {
                     && let Some(binop) = partialeq_binop(diag_item) =>
             {
                 lint_wide_pointer(cx, e, binop, l, r);
+                lint_fn_pointer(cx, e, binop, l, r);
             }
             hir::ExprKind::MethodCall(_, l, [r], _)
                 if let Some(def_id) = cx.typeck_results().type_dependent_def_id(e.hir_id)
                     && let Some(diag_item) = cx.tcx.get_diagnostic_name(def_id)
                     && let Some(binop) = partialeq_binop(diag_item) =>
             {
                 lint_wide_pointer(cx, e, binop, l, r);
+                lint_fn_pointer(cx, e, binop, l, r);
             }
             _ => {}
         };

library/core/tests/ptr.rs

@@ -303,6 +303,7 @@ fn test_const_nonnull_new() {
 #[test]
 #[cfg(unix)] // printf may not be available on other platforms
 #[allow(deprecated)] // For SipHasher
+#[cfg_attr(not(bootstrap), allow(unpredictable_function_pointer_comparisons))]
 pub fn test_variadic_fnptr() {
     use core::ffi;
     use core::hash::{Hash, SipHasher};

src/tools/clippy/clippy_lints/src/declared_lints.rs

@@ -683,7 +683,6 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::unit_types::LET_UNIT_VALUE_INFO,
     crate::unit_types::UNIT_ARG_INFO,
     crate::unit_types::UNIT_CMP_INFO,
-    crate::unnamed_address::FN_ADDRESS_COMPARISONS_INFO,
     crate::unnecessary_box_returns::UNNECESSARY_BOX_RETURNS_INFO,
     crate::unnecessary_map_on_constructor::UNNECESSARY_MAP_ON_CONSTRUCTOR_INFO,
     crate::unnecessary_owned_empty_strings::UNNECESSARY_OWNED_EMPTY_STRINGS_INFO,

src/tools/clippy/clippy_lints/src/lib.rs

@@ -328,7 +328,6 @@ mod unicode;
 mod uninit_vec;
 mod unit_return_expecting_ord;
 mod unit_types;
-mod unnamed_address;
 mod unnecessary_box_returns;
 mod unnecessary_map_on_constructor;
 mod unnecessary_owned_empty_strings;
@@ -862,7 +861,6 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_early_pass(|| Box::new(option_env_unwrap::OptionEnvUnwrap));
     store.register_late_pass(move |_| Box::new(wildcard_imports::WildcardImports::new(warn_on_all_wildcard_imports)));
     store.register_late_pass(|_| Box::<redundant_pub_crate::RedundantPubCrate>::default());
-    store.register_late_pass(|_| Box::new(unnamed_address::UnnamedAddress));
     store.register_late_pass(|_| Box::<dereference::Dereferencing<'_>>::default());
     store.register_late_pass(|_| Box::new(option_if_let_else::OptionIfLetElse));
     store.register_late_pass(|_| Box::new(future_not_send::FutureNotSend));

src/tools/clippy/clippy_lints/src/renamed_lints.rs

@@ -59,4 +59,5 @@ pub static RENAMED_LINTS: &[(&str, &str)] = &[
     ("clippy::unknown_clippy_lints", "unknown_lints"),
     ("clippy::unused_label", "unused_labels"),
     ("clippy::vtable_address_comparisons", "ambiguous_wide_pointer_comparisons"),
+    ("clippy::fn_address_comparisons", "unpredictable_function_pointer_comparisons"),
 ];