Rollup merge of #106248 - dtolnay:revertupcastlint, r=jackh726

Revert "Implement allow-by-default `multiple_supertrait_upcastable` lint"

This is a clean revert of #105484.

I confirmed that reverting that PR fixes the regression reported in #106247. ~~I can't say I understand what this code is doing, but maybe it can be re-landed with a different implementation.~~ **Edit:** https://github.com/rust-lang/rust/issues/106247#issuecomment-1367174384 has an explanation of why #105484 ends up surfacing spurious `where_clause_object_safety` errors. The implementation of `where_clause_object_safety` assumes we only check whether a trait is object safe when somebody actually uses that trait with `dyn`. However the implementation of `multiple_supertrait_upcastable` added in the problematic PR involves checking *every* trait for whether it is object-safe.

FYI `@nbdd0121` `@compiler-errors`

12 files changed, 9 insertions, 167 deletions

diff --git a/compiler/rustc_feature/src/active.rs b/compiler/rustc_feature/src/active.rs
index 34fc62e0549..deb37bdebda 100644
--- a/compiler/rustc_feature/src/active.rs
+++ b/compiler/rustc_feature/src/active.rs
@@ -160,8 +160,6 @@ 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.
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index 44ee4172675..1275d6f223c 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -61,7 +61,6 @@ mod late;
 mod let_underscore;
 mod levels;
 mod methods;
-mod multiple_supertrait_upcastable;
 mod non_ascii_idents;
 mod non_fmt_panic;
 mod nonstandard_style;
@@ -96,7 +95,6 @@ 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::*;
@@ -231,7 +229,6 @@ late_lint_methods!(
             InvalidAtomicOrdering: InvalidAtomicOrdering,
             NamedAsmLabels: NamedAsmLabels,
             OpaqueHiddenInferredBound: OpaqueHiddenInferredBound,
-            MultipleSupertraitUpcastable: MultipleSupertraitUpcastable,
         ]
     ]
 );
diff --git a/compiler/rustc_lint/src/multiple_supertrait_upcastable.rs b/compiler/rustc_lint/src/multiple_supertrait_upcastable.rs
deleted file mode 100644
index 5861b826b1c..00000000000
--- a/compiler/rustc_lint/src/multiple_supertrait_upcastable.rs
+++ /dev/null
@@ -1,63 +0,0 @@
-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,
-                );
-            }
-        }
-    }
-}
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index c450c4da9a8..85510fa2c66 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -944,7 +944,6 @@ symbols! {
         mul,
         mul_assign,
         mul_with_overflow,
-        multiple_supertrait_upcastable,
         must_not_suspend,
         must_use,
         naked,
diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index f6cbac005ff..9857f0516ba 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -87,7 +87,6 @@
 #![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)]
@@ -192,7 +191,6 @@
 #![feature(unsized_fn_params)]
 #![feature(c_unwind)]
 #![feature(with_negative_coherence)]
-#![cfg_attr(not(bootstrap), feature(multiple_supertrait_upcastable))]
 //
 // Rustdoc features:
 #![feature(doc_cfg)]
diff --git a/library/core/src/error.rs b/library/core/src/error.rs
index d2fac23ff18..7152300abcb 100644
--- a/library/core/src/error.rs
+++ b/library/core/src/error.rs
@@ -28,7 +28,6 @@ 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.
     ///
diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index 890d7df8e17..0e3fef4ead3 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -95,7 +95,6 @@
 #![warn(missing_docs)]
 #![allow(explicit_outlives_requirements)]
 #![allow(incomplete_features)]
-#![cfg_attr(not(bootstrap), warn(multiple_supertrait_upcastable))]
 //
 // Library features:
 #![feature(const_align_offset)]
@@ -232,7 +231,6 @@
 #![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)]
diff --git a/src/test/ui/feature-gates/feature-gate-multiple_supertrait_upcastable.rs b/src/test/ui/feature-gates/feature-gate-multiple_supertrait_upcastable.rs
deleted file mode 100644
index 0467dea621b..00000000000
--- a/src/test/ui/feature-gates/feature-gate-multiple_supertrait_upcastable.rs
+++ /dev/null
@@ -1,12 +0,0 @@
-// 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() {}
diff --git a/src/test/ui/feature-gates/feature-gate-multiple_supertrait_upcastable.stderr b/src/test/ui/feature-gates/feature-gate-multiple_supertrait_upcastable.stderr
deleted file mode 100644
index 1f725f35417..00000000000
--- a/src/test/ui/feature-gates/feature-gate-multiple_supertrait_upcastable.stderr
+++ /dev/null
@@ -1,57 +0,0 @@
-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
-
diff --git a/src/test/ui/object-safety/issue-106247.rs b/src/test/ui/object-safety/issue-106247.rs
new file mode 100644
index 00000000000..64bf59e5d3a
--- /dev/null
+++ b/src/test/ui/object-safety/issue-106247.rs
@@ -0,0 +1,9 @@
+// check-pass
+
+#![deny(where_clauses_object_safety)]
+
+pub trait Trait {
+    fn method(&self) where Self: Sync;
+}
+
+fn main() {}
diff --git a/src/test/ui/traits/trait-upcasting/multiple_supertrait_upcastable.rs b/src/test/ui/traits/trait-upcasting/multiple_supertrait_upcastable.rs
deleted file mode 100644
index 3c6ab86e4c6..00000000000
--- a/src/test/ui/traits/trait-upcasting/multiple_supertrait_upcastable.rs
+++ /dev/null
@@ -1,10 +0,0 @@
-#![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() {}
diff --git a/src/test/ui/traits/trait-upcasting/multiple_supertrait_upcastable.stderr b/src/test/ui/traits/trait-upcasting/multiple_supertrait_upcastable.stderr
deleted file mode 100644
index ad80a009ece..00000000000
--- a/src/test/ui/traits/trait-upcasting/multiple_supertrait_upcastable.stderr
+++ /dev/null
@@ -1,14 +0,0 @@
-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
-