From 81b1810cd7b5f1cb74d498f20fa0ef5887420d33 Mon Sep 17 00:00:00 2001
From: Deadbeef <ent3rm4n@gmail.com>
Date: Sat, 30 Jul 2022 18:01:03 +0000
Subject: [PATCH] Require `#[const_trait]` for `const` `impl`s

---
 compiler/rustc_passes/src/check_const.rs | 22 ++++++++++++++++++++++
 library/core/src/convert/mod.rs          |  1 +
 library/core/src/default.rs              |  1 +
 library/core/src/ops/index.rs            |  2 ++
 library/core/src/slice/index.rs          |  1 +
 5 files changed, 27 insertions(+)

diff --git a/compiler/rustc_passes/src/check_const.rs b/compiler/rustc_passes/src/check_const.rs
index 70518284cf9..4062862ad74 100644
--- a/compiler/rustc_passes/src/check_const.rs
+++ b/compiler/rustc_passes/src/check_const.rs
@@ -192,6 +192,28 @@ impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
     }
 
     fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
+        let tcx = self.tcx;
+        if let hir::ItemKind::Impl(hir::Impl {
+            constness: hir::Constness::Const,
+            of_trait: Some(trait_ref),
+            ..
+        }) = item.kind
+        {
+            let def_id = trait_ref.trait_def_id().unwrap();
+            let source_map = tcx.sess.source_map();
+            if !tcx.has_attr(def_id, sym::const_trait) {
+                tcx.sess
+                    .struct_span_err(
+                        source_map.guess_head_span(item.span),
+                        "const `impl`s must be for traits marked with `#[const_trait]`",
+                    )
+                    .span_note(
+                        source_map.guess_head_span(tcx.def_span(def_id)),
+                        "this trait must be annotated with `#[const_trait]`",
+                    )
+                    .emit();
+            }
+        }
         intravisit::walk_item(self, item);
     }
 
diff --git a/library/core/src/convert/mod.rs b/library/core/src/convert/mod.rs
index 05637c16622..95900828451 100644
--- a/library/core/src/convert/mod.rs
+++ b/library/core/src/convert/mod.rs
@@ -368,6 +368,7 @@ pub trait Into<T>: Sized {
     all(_Self = "&str", T = "std::string::String"),
     note = "to coerce a `{T}` into a `{Self}`, use `&*` as a prefix",
 ))]
+#[const_trait]
 pub trait From<T>: Sized {
     /// Converts to this type from the input type.
     #[lang = "from"]
diff --git a/library/core/src/default.rs b/library/core/src/default.rs
index b53cd6074b5..d96b53de0a3 100644
--- a/library/core/src/default.rs
+++ b/library/core/src/default.rs
@@ -99,6 +99,7 @@
 /// ```
 #[cfg_attr(not(test), rustc_diagnostic_item = "Default")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[const_trait]
 pub trait Default: Sized {
     /// Returns the "default value" for a type.
     ///
diff --git a/library/core/src/ops/index.rs b/library/core/src/ops/index.rs
index e2e569cb7ea..5e3dc48b6ca 100644
--- a/library/core/src/ops/index.rs
+++ b/library/core/src/ops/index.rs
@@ -55,6 +55,7 @@
 #[doc(alias = "]")]
 #[doc(alias = "[")]
 #[doc(alias = "[]")]
+#[const_trait]
 pub trait Index<Idx: ?Sized> {
     /// The returned type after indexing.
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -163,6 +164,7 @@ see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#ind
 #[doc(alias = "[")]
 #[doc(alias = "]")]
 #[doc(alias = "[]")]
+#[const_trait]
 pub trait IndexMut<Idx: ?Sized>: Index<Idx> {
     /// Performs the mutable indexing (`container[index]`) operation.
     ///
diff --git a/library/core/src/slice/index.rs b/library/core/src/slice/index.rs
index 3403a5a86f7..3d99bb7f9a0 100644
--- a/library/core/src/slice/index.rs
+++ b/library/core/src/slice/index.rs
@@ -158,6 +158,7 @@ mod private_slice_index {
     message = "the type `{T}` cannot be indexed by `{Self}`",
     label = "slice indices are of type `usize` or ranges of `usize`"
 )]
+#[const_trait]
 pub unsafe trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
     /// The output type returned by methods.
     #[stable(feature = "slice_get_slice", since = "1.28.0")]