Rollup merge of #134586 - Urgau:fn-ptr-lint-option, r=compiler-errors

Also lint on option of function pointer comparisons

This PR is the first part of #134536, ie. the linting on `Option<{fn ptr}>` in the `unpredictable_function_pointer_comparisons` lint, which isn't part of the lang nomination that the second part is going trough, and so should be able to be approved independently.

Related to https://github.com/rust-lang/rust/issues/134527
r? `@compiler-errors`

compiler/rustc_lint/src/types.rs

@@ -4,7 +4,7 @@ use std::ops::ControlFlow;
 use rustc_abi::{BackendRepr, ExternAbi, TagEncoding, Variants, WrappingRange};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::DiagMessage;
-use rustc_hir::{Expr, ExprKind};
+use rustc_hir::{Expr, ExprKind, LangItem};
 use rustc_middle::bug;
 use rustc_middle::ty::layout::{LayoutOf, SizeSkeleton};
 use rustc_middle::ty::{
@@ -444,7 +444,25 @@ fn lint_fn_pointer<'tcx>(
     let (l_ty, l_ty_refs) = peel_refs(l_ty);
     let (r_ty, r_ty_refs) = peel_refs(r_ty);
 
-    if !l_ty.is_fn() || !r_ty.is_fn() {
+    if l_ty.is_fn() && r_ty.is_fn() {
+        // both operands are function pointers, fallthrough
+    } else if let ty::Adt(l_def, l_args) = l_ty.kind()
+        && let ty::Adt(r_def, r_args) = r_ty.kind()
+        && cx.tcx.is_lang_item(l_def.did(), LangItem::Option)
+        && cx.tcx.is_lang_item(r_def.did(), LangItem::Option)
+        && let Some(l_some_arg) = l_args.get(0)
+        && let Some(r_some_arg) = r_args.get(0)
+        && l_some_arg.expect_ty().is_fn()
+        && r_some_arg.expect_ty().is_fn()
+    {
+        // both operands are `Option<{function ptr}>`
+        return cx.emit_span_lint(
+            UNPREDICTABLE_FUNCTION_POINTER_COMPARISONS,
+            e.span,
+            UnpredictableFunctionPointerComparisons::Warn,
+        );
+    } else {
+        // types are not function pointers, nothing to do
         return;
     }
 

tests/ui/lint/fn-ptr-comparisons-some.rs

@@ -0,0 +1,17 @@
+// This test checks that we lint on Option of fn ptr.
+//
+// https://github.com/rust-lang/rust/issues/134527.
+//
+//@ check-pass
+
+unsafe extern "C" fn func() {}
+
+type FnPtr = unsafe extern "C" fn();
+
+fn main() {
+    let _ = Some::<FnPtr>(func) == Some(func as unsafe extern "C" fn());
+    //~^ WARN function pointer comparisons
+
+    // Undecided as of https://github.com/rust-lang/rust/pull/134536
+    assert_eq!(Some::<FnPtr>(func), Some(func as unsafe extern "C" fn()));
+}

tests/ui/lint/fn-ptr-comparisons-some.stderr

@@ -0,0 +1,13 @@
+warning: function pointer comparisons do not produce meaningful results since their addresses are not guaranteed to be unique
+  --> $DIR/fn-ptr-comparisons-some.rs:12:13
+   |
+LL |     let _ = Some::<FnPtr>(func) == Some(func as unsafe extern "C" fn());
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the address of the same function can vary between different codegen units
+   = note: furthermore, different functions could have the same address after being merged together
+   = note: for more information visit <https://doc.rust-lang.org/nightly/core/ptr/fn.fn_addr_eq.html>
+   = note: `#[warn(unpredictable_function_pointer_comparisons)]` on by default
+
+warning: 1 warning emitted
+