Add warn-by-default lint against unpredictable fn pointer comparisons

compiler/rustc_lint/src/lints.rs

@@ -1810,6 +1810,42 @@ pub(crate) enum AmbiguousWidePointerComparisonsAddrSuggestion<'a> {
     },
 }
 
+#[derive(LintDiagnostic)]
+pub(crate) enum UnpredictableFunctionPointerComparisons<'a> {
+    #[diag(lint_unpredictable_fn_pointer_comparisons)]
+    #[note(lint_note_duplicated_fn)]
+    #[note(lint_note_deduplicated_fn)]
+    #[note(lint_note_visit_fn_addr_eq)]
+    Suggestion {
+        #[subdiagnostic]
+        sugg: UnpredictableFunctionPointerComparisonsSuggestion<'a>,
+    },
+    #[diag(lint_unpredictable_fn_pointer_comparisons)]
+    #[note(lint_note_duplicated_fn)]
+    #[note(lint_note_deduplicated_fn)]
+    #[note(lint_note_visit_fn_addr_eq)]
+    Warn,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    lint_fn_addr_eq_suggestion,
+    style = "verbose",
+    applicability = "maybe-incorrect"
+)]
+pub(crate) struct UnpredictableFunctionPointerComparisonsSuggestion<'a> {
+    pub ne: &'a str,
+    pub cast_right: String,
+    pub deref_left: &'a str,
+    pub deref_right: &'a str,
+    #[suggestion_part(code = "{ne}std::ptr::fn_addr_eq({deref_left}")]
+    pub left: Span,
+    #[suggestion_part(code = ", {deref_right}")]
+    pub middle: Span,
+    #[suggestion_part(code = "{cast_right})")]
+    pub right: Span,
+}
+
 pub(crate) struct ImproperCTypes<'a> {
     pub ty: Ty<'a>,
     pub desc: &'a str,