diff --git a/compiler/rustc_feature/src/active.rs b/compiler/rustc_feature/src/active.rs
index c13fe2ae280..04912fe4096 100644
--- a/compiler/rustc_feature/src/active.rs
+++ b/compiler/rustc_feature/src/active.rs
@@ -622,6 +622,7 @@ pub const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::lazy_normalization_consts,
     sym::specialization,
     sym::inline_const,
+    sym::repr128,
 ];
 
 /// Some features are not allowed to be used together at the same time, if
diff --git a/src/test/ui/enum-discriminant/discriminant_size.rs b/src/test/ui/enum-discriminant/discriminant_size.rs
index 4cede8c2a2d..b939a70dfc5 100644
--- a/src/test/ui/enum-discriminant/discriminant_size.rs
+++ b/src/test/ui/enum-discriminant/discriminant_size.rs
@@ -1,5 +1,6 @@
 // run-pass
 #![feature(core_intrinsics, repr128)]
+//~^ WARN the feature `repr128` is incomplete
 
 use std::intrinsics::discriminant_value;
 
diff --git a/src/test/ui/enum-discriminant/discriminant_size.stderr b/src/test/ui/enum-discriminant/discriminant_size.stderr
new file mode 100644
index 00000000000..efc7d998466
--- /dev/null
+++ b/src/test/ui/enum-discriminant/discriminant_size.stderr
@@ -0,0 +1,11 @@
+warning: the feature `repr128` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/discriminant_size.rs:2:29
+   |
+LL | #![feature(core_intrinsics, repr128)]
+   |                             ^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #56071 <https://github.com/rust-lang/rust/issues/56071> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/enum-discriminant/issue-70509-partial_eq.rs b/src/test/ui/enum-discriminant/issue-70509-partial_eq.rs
index 4e2cc89948a..ae389e11466 100644
--- a/src/test/ui/enum-discriminant/issue-70509-partial_eq.rs
+++ b/src/test/ui/enum-discriminant/issue-70509-partial_eq.rs
@@ -1,5 +1,6 @@
 // run-pass
 #![feature(repr128, arbitrary_enum_discriminant)]
+//~^ WARN the feature `repr128` is incomplete
 
 #[derive(PartialEq, Debug)]
 #[repr(i128)]
diff --git a/src/test/ui/enum-discriminant/issue-70509-partial_eq.stderr b/src/test/ui/enum-discriminant/issue-70509-partial_eq.stderr
new file mode 100644
index 00000000000..5bf6ea56ebc
--- /dev/null
+++ b/src/test/ui/enum-discriminant/issue-70509-partial_eq.stderr
@@ -0,0 +1,11 @@
+warning: the feature `repr128` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-70509-partial_eq.rs:2:12
+   |
+LL | #![feature(repr128, arbitrary_enum_discriminant)]
+   |            ^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #56071 <https://github.com/rust-lang/rust/issues/56071> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/enum-discriminant/repr128.rs b/src/test/ui/enum-discriminant/repr128.rs
index eefbc44f585..00021a07b37 100644
--- a/src/test/ui/enum-discriminant/repr128.rs
+++ b/src/test/ui/enum-discriminant/repr128.rs
@@ -1,5 +1,6 @@
 // run-pass
 #![feature(repr128, core_intrinsics, discriminant_kind)]
+//~^ WARN the feature `repr128` is incomplete
 
 use std::intrinsics::discriminant_value;
 use std::marker::DiscriminantKind;
diff --git a/src/test/ui/enum-discriminant/repr128.stderr b/src/test/ui/enum-discriminant/repr128.stderr
new file mode 100644
index 00000000000..88adfb1742d
--- /dev/null
+++ b/src/test/ui/enum-discriminant/repr128.stderr
@@ -0,0 +1,11 @@
+warning: the feature `repr128` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/repr128.rs:2:12
+   |
+LL | #![feature(repr128, core_intrinsics, discriminant_kind)]
+   |            ^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #56071 <https://github.com/rust-lang/rust/issues/56071> for more information
+
+warning: 1 warning emitted
+
diff --git a/src/test/ui/issues/issue-43398.rs b/src/test/ui/issues/issue-43398.rs
index f0b762c6254..581db033f92 100644
--- a/src/test/ui/issues/issue-43398.rs
+++ b/src/test/ui/issues/issue-43398.rs
@@ -2,6 +2,7 @@
 
 #![feature(core_intrinsics)]
 #![feature(repr128)]
+//~^ WARN the feature `repr128` is incomplete
 
 #[repr(i128)]
 enum Big { A, B }
diff --git a/src/test/ui/issues/issue-43398.stderr b/src/test/ui/issues/issue-43398.stderr
new file mode 100644
index 00000000000..9a394153bf6
--- /dev/null
+++ b/src/test/ui/issues/issue-43398.stderr
@@ -0,0 +1,11 @@
+warning: the feature `repr128` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/issue-43398.rs:4:12
+   |
+LL | #![feature(repr128)]
+   |            ^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #56071 <https://github.com/rust-lang/rust/issues/56071> for more information
+
+warning: 1 warning emitted
+