Reintroduce multiple_supertrait_upcastable lint

compiler/rustc_error_messages/locales/en-US/lint.ftl

@@ -100,6 +100,8 @@ lint_cstring_ptr = getting the inner pointer of a temporary `CString`
     .note = pointers do not have a lifetime; when calling `as_ptr` the `CString` will be deallocated at the end of the statement because nothing is referencing it as far as the type system is concerned
     .help = for more information, see https://doc.rust-lang.org/reference/destructors.html
 
+lint_multple_supertrait_upcastable = `{$ident}` is object-safe and has multiple supertraits
+
 lint_identifier_non_ascii_char = identifier contains non-ASCII characters
 
 lint_identifier_uncommon_codepoints = identifier contains uncommon Unicode codepoints

compiler/rustc_feature/src/active.rs

@@ -160,6 +160,8 @@ declare_features! (
     (active, intrinsics, "1.0.0", None, None),
     /// Allows using `#[lang = ".."]` attribute for linking items to special compiler logic.
     (active, lang_items, "1.0.0", None, None),
+    /// Allows the `multiple_supertrait_upcastable` lint.
+    (active, multiple_supertrait_upcastable, "CURRENT_RUSTC_VERSION", None, None),
     /// Allows using `#[omit_gdb_pretty_printer_section]`.
     (active, omit_gdb_pretty_printer_section, "1.5.0", None, None),
     /// Allows using `#[prelude_import]` on glob `use` items.

compiler/rustc_lint/src/lib.rs

@@ -64,6 +64,7 @@ mod let_underscore;
 mod levels;
 mod lints;
 mod methods;
+mod multiple_supertrait_upcastable;
 mod non_ascii_idents;
 mod non_fmt_panic;
 mod nonstandard_style;
@@ -98,6 +99,7 @@ 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::*;
@@ -232,6 +234,7 @@ late_lint_methods!(
             InvalidAtomicOrdering: InvalidAtomicOrdering,
             NamedAsmLabels: NamedAsmLabels,
             OpaqueHiddenInferredBound: OpaqueHiddenInferredBound,
+            MultipleSupertraitUpcastable: MultipleSupertraitUpcastable,
         ]
     ]
 );

compiler/rustc_lint/src/lints.rs

@@ -917,6 +917,13 @@ pub struct CStringPtr {
     pub unwrap: Span,
 }
 
+// multiple_supertrait_upcastable.rs
+#[derive(LintDiagnostic)]
+#[diag(lint_multple_supertrait_upcastable)]
+pub struct MultipleSupertraitUpcastable {
+    pub ident: Ident,
+}
+
 // non_ascii_idents.rs
 #[derive(LintDiagnostic)]
 #[diag(lint_identifier_non_ascii_char)]

compiler/rustc_lint/src/multiple_supertrait_upcastable.rs

@@ -0,0 +1,60 @@
+use crate::{LateContext, LateLintPass, LintContext};
+
+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();
+        // NOTE(nbdd0121): use `object_safety_violations` instead of `check_is_object_safe` because
+        // the latter will report `where_clause_object_safety` lint.
+        if let hir::ItemKind::Trait(_, _, _, _, _) = item.kind
+            && cx.tcx.object_safety_violations(def_id).is_empty()
+        {
+            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.emit_spanned_lint(
+                    MULTIPLE_SUPERTRAIT_UPCASTABLE,
+                    cx.tcx.def_span(def_id),
+                    crate::lints::MultipleSupertraitUpcastable {
+                        ident: item.ident
+                    },
+                );
+            }
+        }
+    }
+}

compiler/rustc_span/src/symbol.rs

@@ -955,6 +955,7 @@ symbols! {
         mul,
         mul_assign,
         mul_with_overflow,
+        multiple_supertrait_upcastable,
         must_not_suspend,
         must_use,
         naked,

library/alloc/src/lib.rs

@@ -87,6 +87,7 @@
 #![warn(missing_debug_implementations)]
 #![warn(missing_docs)]
 #![allow(explicit_outlives_requirements)]
+#![cfg_attr(not(bootstrap), warn(multiple_supertrait_upcastable))]
 //
 // Library features:
 #![feature(alloc_layout_extra)]
@@ -195,6 +196,7 @@
 #![feature(c_unwind)]
 #![feature(with_negative_coherence)]
 #![cfg_attr(test, feature(panic_update_hook))]
+#![cfg_attr(not(bootstrap), feature(multiple_supertrait_upcastable))]
 //
 // Rustdoc features:
 #![feature(doc_cfg)]

library/core/src/error.rs

@@ -28,6 +28,7 @@ use crate::fmt::{Debug, Display};
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "Error")]
 #[rustc_has_incoherent_inherent_impls]
+#[cfg_attr(not(bootstrap), allow(multiple_supertrait_upcastable))]
 pub trait Error: Debug + Display {
     /// The lower-level source of this error, if any.
     ///

library/core/src/lib.rs

@@ -95,6 +95,7 @@
 #![warn(missing_docs)]
 #![allow(explicit_outlives_requirements)]
 #![allow(incomplete_features)]
+#![cfg_attr(not(bootstrap), warn(multiple_supertrait_upcastable))]
 //
 // Library features:
 #![feature(const_align_offset)]
@@ -235,6 +236,7 @@
 #![feature(unsized_fn_params)]
 #![feature(asm_const)]
 #![feature(const_transmute_copy)]
+#![cfg_attr(not(bootstrap), feature(multiple_supertrait_upcastable))]
 //
 // Target features:
 #![feature(arm_target_feature)]

tests/ui/feature-gates/feature-gate-multiple_supertrait_upcastable.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+#![deny(multiple_supertrait_upcastable)]
+//~^ WARNING unknown lint: `multiple_supertrait_upcastable`
+//~| WARNING unknown lint: `multiple_supertrait_upcastable`
+//~| WARNING unknown lint: `multiple_supertrait_upcastable`
+#![warn(multiple_supertrait_upcastable)]
+//~^ WARNING unknown lint: `multiple_supertrait_upcastable`
+//~| WARNING unknown lint: `multiple_supertrait_upcastable`
+//~| WARNING unknown lint: `multiple_supertrait_upcastable`
+
+fn main() {}

tests/ui/feature-gates/feature-gate-multiple_supertrait_upcastable.stderr

@@ -0,0 +1,57 @@
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:3:1
+   |
+LL | #![deny(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+   = note: `#[warn(unknown_lints)]` on by default
+
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:7:1
+   |
+LL | #![warn(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:3:1
+   |
+LL | #![deny(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:7:1
+   |
+LL | #![warn(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:3:1
+   |
+LL | #![deny(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+
+warning: unknown lint: `multiple_supertrait_upcastable`
+  --> $DIR/feature-gate-multiple_supertrait_upcastable.rs:7:1
+   |
+LL | #![warn(multiple_supertrait_upcastable)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `multiple_supertrait_upcastable` lint is unstable
+   = help: add `#![feature(multiple_supertrait_upcastable)]` to the crate attributes to enable
+
+warning: 6 warnings emitted
+

tests/ui/traits/trait-upcasting/multiple_supertrait_upcastable.rs

@@ -0,0 +1,10 @@
+#![feature(multiple_supertrait_upcastable)]
+#![deny(multiple_supertrait_upcastable)]
+
+trait A {}
+trait B {}
+
+trait C: A + B {}
+//~^ ERROR `C` is object-safe and has multiple supertraits
+
+fn main() {}

tests/ui/traits/trait-upcasting/multiple_supertrait_upcastable.stderr

@@ -0,0 +1,14 @@
+error: `C` is object-safe and has multiple supertraits
+  --> $DIR/multiple_supertrait_upcastable.rs:7:1
+   |
+LL | trait C: A + B {}
+   | ^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/multiple_supertrait_upcastable.rs:2:9
+   |
+LL | #![deny(multiple_supertrait_upcastable)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+