diff --git a/library/core/src/future/future.rs b/library/core/src/future/future.rs
index 71b9464efd288..af2e422e8a00c 100644
--- a/library/core/src/future/future.rs
+++ b/library/core/src/future/future.rs
@@ -28,7 +28,7 @@ use crate::task::{Context, Poll};
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 #[stable(feature = "futures_api", since = "1.36.0")]
 #[lang = "future_trait"]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     label = "`{Self}` is not a future",
     message = "`{Self}` is not a future",
     note = "{Self} must be a future or must implement `IntoFuture` to be awaited"
diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index 07720f235989b..9f8ae7a09f094 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -218,6 +218,7 @@
 #![feature(const_trait_impl)]
 #![feature(decl_macro)]
 #![feature(deprecated_suggestion)]
+#![feature(diagnostic_namespace)]
 #![feature(doc_cfg)]
 #![feature(doc_cfg_hide)]
 #![feature(doc_notable_trait)]
diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
index 99762bccd18f2..69d54f0640780 100644
--- a/library/core/src/marker.rs
+++ b/library/core/src/marker.rs
@@ -75,7 +75,7 @@ macro marker_impls {
 /// [ub]: ../../reference/behavior-considered-undefined.html
 #[stable(feature = "rust1", since = "1.0.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "Send")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "`{Self}` cannot be sent between threads safely",
     label = "`{Self}` cannot be sent between threads safely"
 )]
@@ -134,7 +134,7 @@ unsafe impl<T: Sync + ?Sized> Send for &T {}
 #[doc(alias = "?", alias = "?Sized")]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[lang = "sized"]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "the size for values of type `{Self}` cannot be known at compilation time",
     label = "doesn't have a size known at compile-time"
 )]
@@ -205,7 +205,7 @@ pub trait Unsize<T: ?Sized> {
 /// [RFC1445]: https://github.com/rust-lang/rfcs/blob/master/text/1445-restrict-constants-in-patterns.md
 /// [issue 63438]: https://github.com/rust-lang/rust/issues/63438
 #[unstable(feature = "structural_match", issue = "31434")]
-#[rustc_on_unimplemented(message = "the type `{Self}` does not `#[derive(PartialEq)]`")]
+#[diagnostic::on_unimplemented(message = "the type `{Self}` does not `#[derive(PartialEq)]`")]
 #[lang = "structural_peq"]
 pub trait StructuralPartialEq {
     // Empty.
@@ -273,7 +273,7 @@ marker_impls! {
 /// of the two derives (`#[derive(PartialEq)]` and `#[derive(Eq)]`) and check
 /// that both of them are present as part of structural-match checking.
 #[unstable(feature = "structural_match", issue = "31434")]
-#[rustc_on_unimplemented(message = "the type `{Self}` does not `#[derive(Eq)]`")]
+#[diagnostic::on_unimplemented(message = "the type `{Self}` does not `#[derive(Eq)]`")]
 #[lang = "structural_teq"]
 pub trait StructuralEq {
     // Empty.
@@ -941,7 +941,7 @@ marker_impls! {
 /// [Pin]: crate::pin::Pin
 /// [`pin` module]: crate::pin
 #[stable(feature = "pin", since = "1.33.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     note = "consider using the `pin!` macro\nconsider using `Box::pin` if you need to access the pinned value outside of the current scope",
     message = "`{Self}` cannot be unpinned"
 )]
@@ -989,7 +989,7 @@ pub trait Destruct {}
 /// for any user type.
 #[unstable(feature = "tuple_trait", issue = "none")]
 #[lang = "tuple_trait"]
-#[rustc_on_unimplemented(message = "`{Self}` is not a tuple")]
+#[diagnostic::on_unimplemented(message = "`{Self}` is not a tuple")]
 #[rustc_deny_explicit_impl(implement_via_object = false)]
 pub trait Tuple {}
 
@@ -999,7 +999,7 @@ pub trait Tuple {}
 /// `*const ()` automatically implement this trait.
 #[unstable(feature = "pointer_like_trait", issue = "none")]
 #[lang = "pointer_like"]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "`{Self}` needs to have the same ABI as a pointer",
     label = "`{Self}` needs to be a pointer-like type"
 )]
@@ -1013,7 +1013,7 @@ pub trait PointerLike {}
 /// are `StructuralPartialEq`.
 #[lang = "const_param_ty"]
 #[unstable(feature = "adt_const_params", issue = "95174")]
-#[rustc_on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
+#[diagnostic::on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
 #[allow(multiple_supertrait_upcastable)]
 pub trait ConstParamTy: StructuralEq + StructuralPartialEq + Eq {}
 
diff --git a/library/core/src/ops/arith.rs b/library/core/src/ops/arith.rs
index 1773fdbf37cc7..bb3cdde66d173 100644
--- a/library/core/src/ops/arith.rs
+++ b/library/core/src/ops/arith.rs
@@ -307,7 +307,7 @@ sub_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "mul"]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot multiply `{Self}` by `{Rhs}`",
     label = "no implementation for `{Self} * {Rhs}`"
 )]
@@ -441,7 +441,7 @@ mul_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "div"]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot divide `{Self}` by `{Rhs}`",
     label = "no implementation for `{Self} / {Rhs}`"
 )]
@@ -543,7 +543,7 @@ div_impl_float! { f32 f64 }
 /// ```
 #[lang = "rem"]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot calculate the remainder of `{Self}` divided by `{Rhs}`",
     label = "no implementation for `{Self} % {Rhs}`"
 )]
@@ -729,7 +729,7 @@ neg_impl! { isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "add_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot add-assign `{Rhs}` to `{Self}`",
     label = "no implementation for `{Self} += {Rhs}`"
 )]
@@ -796,7 +796,7 @@ add_assign_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "sub_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot subtract-assign `{Rhs}` from `{Self}`",
     label = "no implementation for `{Self} -= {Rhs}`"
 )]
@@ -854,7 +854,7 @@ sub_assign_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "mul_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot multiply-assign `{Self}` by `{Rhs}`",
     label = "no implementation for `{Self} *= {Rhs}`"
 )]
@@ -912,7 +912,7 @@ mul_assign_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "div_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot divide-assign `{Self}` by `{Rhs}`",
     label = "no implementation for `{Self} /= {Rhs}`"
 )]
@@ -973,7 +973,7 @@ div_assign_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "rem_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "cannot calculate and assign the remainder of `{Self}` divided by `{Rhs}`",
     label = "no implementation for `{Self} %= {Rhs}`"
 )]
diff --git a/library/core/src/ops/bit.rs b/library/core/src/ops/bit.rs
index c70f4a3da2ed8..6984100e498e8 100644
--- a/library/core/src/ops/bit.rs
+++ b/library/core/src/ops/bit.rs
@@ -137,7 +137,7 @@ impl Not for ! {
 #[lang = "bitand"]
 #[doc(alias = "&")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} & {Rhs}`",
     label = "no implementation for `{Self} & {Rhs}`"
 )]
@@ -237,7 +237,7 @@ bitand_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 #[lang = "bitor"]
 #[doc(alias = "|")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} | {Rhs}`",
     label = "no implementation for `{Self} | {Rhs}`"
 )]
@@ -337,7 +337,7 @@ bitor_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 #[lang = "bitxor"]
 #[doc(alias = "^")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} ^ {Rhs}`",
     label = "no implementation for `{Self} ^ {Rhs}`"
 )]
@@ -436,7 +436,7 @@ bitxor_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 #[lang = "shl"]
 #[doc(alias = "<<")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} << {Rhs}`",
     label = "no implementation for `{Self} << {Rhs}`"
 )]
@@ -554,7 +554,7 @@ shl_impl_all! { u8 u16 u32 u64 u128 usize i8 i16 i32 i64 isize i128 }
 #[lang = "shr"]
 #[doc(alias = ">>")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} >> {Rhs}`",
     label = "no implementation for `{Self} >> {Rhs}`"
 )]
@@ -681,7 +681,7 @@ shr_impl_all! { u8 u16 u32 u64 u128 usize i8 i16 i32 i64 i128 isize }
 #[lang = "bitand_assign"]
 #[doc(alias = "&=")]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} &= {Rhs}`",
     label = "no implementation for `{Self} &= {Rhs}`"
 )]
@@ -752,7 +752,7 @@ bitand_assign_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 #[lang = "bitor_assign"]
 #[doc(alias = "|=")]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} |= {Rhs}`",
     label = "no implementation for `{Self} |= {Rhs}`"
 )]
@@ -823,7 +823,7 @@ bitor_assign_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 #[lang = "bitxor_assign"]
 #[doc(alias = "^=")]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} ^= {Rhs}`",
     label = "no implementation for `{Self} ^= {Rhs}`"
 )]
@@ -892,7 +892,7 @@ bitxor_assign_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 #[lang = "shl_assign"]
 #[doc(alias = "<<=")]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} <<= {Rhs}`",
     label = "no implementation for `{Self} <<= {Rhs}`"
 )]
@@ -974,7 +974,7 @@ shl_assign_impl_all! { u8 u16 u32 u64 u128 usize i8 i16 i32 i64 i128 isize }
 #[lang = "shr_assign"]
 #[doc(alias = ">>=")]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "no implementation for `{Self} >>= {Rhs}`",
     label = "no implementation for `{Self} >>= {Rhs}`"
 )]
diff --git a/library/core/src/ops/index.rs b/library/core/src/ops/index.rs
index 6ceee46372980..37d9a28fb99c0 100644
--- a/library/core/src/ops/index.rs
+++ b/library/core/src/ops/index.rs
@@ -47,7 +47,7 @@
 /// assert_eq!(nucleotide_count[Nucleotide::T], 12);
 /// ```
 #[lang = "index"]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "the type `{Self}` cannot be indexed by `{Idx}`",
     label = "`{Self}` cannot be indexed by `{Idx}`"
 )]
diff --git a/library/core/src/panic/unwind_safe.rs b/library/core/src/panic/unwind_safe.rs
index 6a53909a8f128..37859212c0ee3 100644
--- a/library/core/src/panic/unwind_safe.rs
+++ b/library/core/src/panic/unwind_safe.rs
@@ -83,7 +83,7 @@ use crate::task::{Context, Poll};
 /// implemented for any closed over variables passed to `catch_unwind`.
 #[stable(feature = "catch_unwind", since = "1.9.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "unwind_safe_trait")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "the type `{Self}` may not be safely transferred across an unwind boundary",
     label = "`{Self}` may not be safely transferred across an unwind boundary"
 )]
@@ -99,7 +99,7 @@ pub auto trait UnwindSafe {}
 /// [`UnwindSafe`] trait, for more information see that documentation.
 #[stable(feature = "catch_unwind", since = "1.9.0")]
 #[cfg_attr(not(test), rustc_diagnostic_item = "ref_unwind_safe_trait")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "the type `{Self}` may contain interior mutability and a reference may not be safely \
                transferrable across a catch_unwind boundary",
     label = "`{Self}` may contain interior mutability and a reference may not be safely \
diff --git a/tests/ui/proc-macro/meta-macro-hygiene.stdout b/tests/ui/proc-macro/meta-macro-hygiene.stdout
index 6d10cc604c27f..3672a3590fd7a 100644
--- a/tests/ui/proc-macro/meta-macro-hygiene.stdout
+++ b/tests/ui/proc-macro/meta-macro-hygiene.stdout
@@ -50,6 +50,9 @@ crate0::{{expn1}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt:
 crate0::{{expn2}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Bang, "produce_it")
 crate0::{{expn3}}: parent: crate0::{{expn2}}, call_site_ctxt: #3, def_site_ctxt: #0, kind: Macro(Bang, "meta_macro::print_def_site")
 crate0::{{expn4}}: parent: crate0::{{expn3}}, call_site_ctxt: #4, def_site_ctxt: #0, kind: Macro(Bang, "$crate::dummy")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "derive")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "derive")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Bang, "include")
diff --git a/tests/ui/proc-macro/nonterminal-token-hygiene.stdout b/tests/ui/proc-macro/nonterminal-token-hygiene.stdout
index 5c70e780f7489..d3c2c46ac75cf 100644
--- a/tests/ui/proc-macro/nonterminal-token-hygiene.stdout
+++ b/tests/ui/proc-macro/nonterminal-token-hygiene.stdout
@@ -73,6 +73,9 @@ crate0::{{expn1}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt:
 crate0::{{expn2}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Bang, "outer")
 crate0::{{expn3}}: parent: crate0::{{expn2}}, call_site_ctxt: #3, def_site_ctxt: #3, kind: Macro(Bang, "inner")
 crate0::{{expn4}}: parent: crate0::{{expn3}}, call_site_ctxt: #5, def_site_ctxt: #0, kind: Macro(Bang, "print_bang")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
+crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "diagnostic::on_unimplemented")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "derive")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Attr, "derive")
 crate1::{{expnNNN}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Bang, "include")