Rollup merge of #105484 - nbdd0121:upcast, r=compiler-errors

Implement allow-by-default `multiple_supertrait_upcastable` lint

The lint detects when an object-safe trait has multiple supertraits.

Enabled in libcore and liballoc as they are low-level enough that many embedded programs will use them.

r? `@nikomatsakis`

compiler/rustc_lint/src/lib.rs

@@ -61,6 +61,7 @@ mod late;
 mod let_underscore;
 mod levels;
 mod methods;
+mod multiple_supertrait_upcastable;
 mod non_ascii_idents;
 mod non_fmt_panic;
 mod nonstandard_style;
@@ -95,6 +96,7 @@ use hidden_unicode_codepoints::*;
 use internal::*;
 use let_underscore::*;
 use methods::*;
+use multiple_supertrait_upcastable::*;
 use non_ascii_idents::*;
 use non_fmt_panic::NonPanicFmt;
 use nonstandard_style::*;
@@ -229,6 +231,7 @@ late_lint_methods!(
             InvalidAtomicOrdering: InvalidAtomicOrdering,
             NamedAsmLabels: NamedAsmLabels,
             OpaqueHiddenInferredBound: OpaqueHiddenInferredBound,
+            MultipleSupertraitUpcastable: MultipleSupertraitUpcastable,
         ]
     ]
 );

compiler/rustc_lint/src/multiple_supertrait_upcastable.rs

@@ -0,0 +1,63 @@
+use crate::{LateContext, LateLintPass, LintContext};
+
+use rustc_errors::DelayDm;
+use rustc_hir as hir;
+use rustc_span::sym;
+
+declare_lint! {
+    /// The `multiple_supertrait_upcastable` lint detects when an object-safe trait has multiple
+    /// supertraits.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// trait A {}
+    /// trait B {}
+    ///
+    /// #[warn(multiple_supertrait_upcastable)]
+    /// trait C: A + B {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// To support upcasting with multiple supertraits, we need to store multiple vtables and this
+    /// can result in extra space overhead, even if no code actually uses upcasting.
+    /// This lint allows users to identify when such scenarios occur and to decide whether the
+    /// additional overhead is justified.
+    pub MULTIPLE_SUPERTRAIT_UPCASTABLE,
+    Allow,
+    "detect when an object-safe trait has multiple supertraits",
+    @feature_gate = sym::multiple_supertrait_upcastable;
+}
+
+declare_lint_pass!(MultipleSupertraitUpcastable => [MULTIPLE_SUPERTRAIT_UPCASTABLE]);
+
+impl<'tcx> LateLintPass<'tcx> for MultipleSupertraitUpcastable {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
+        let def_id = item.owner_id.to_def_id();
+        if let hir::ItemKind::Trait(_, _, _, _, _) = item.kind
+            && cx.tcx.is_object_safe(def_id)
+        {
+            let direct_super_traits_iter = cx.tcx
+                    .super_predicates_of(def_id)
+                    .predicates
+                    .into_iter()
+                    .filter_map(|(pred, _)| pred.to_opt_poly_trait_pred());
+            if direct_super_traits_iter.count() > 1 {
+                cx.struct_span_lint(
+                    MULTIPLE_SUPERTRAIT_UPCASTABLE,
+                    cx.tcx.def_span(def_id),
+                    DelayDm(|| {
+                        format!(
+                            "`{}` is object-safe and has multiple supertraits",
+                            item.ident,
+                        )
+                    }),
+                    |diag| diag,
+                );
+            }
+        }
+    }
+}