diff --git a/alloc/src/macros.rs b/alloc/src/macros.rs
index 0f767df6063a3..d5ca5c4ed27a2 100644
--- a/alloc/src/macros.rs
+++ b/alloc/src/macros.rs
@@ -41,18 +41,18 @@
 #[allow_internal_unstable(rustc_attrs, liballoc_internals)]
 macro_rules! vec {
     () => (
-        $crate::__rust_force_expr!($crate::vec::Vec::new())
+        $crate::vec::Vec::new()
     );
     ($elem:expr; $n:expr) => (
-        $crate::__rust_force_expr!($crate::vec::from_elem($elem, $n))
+        $crate::vec::from_elem($elem, $n)
     );
     ($($x:expr),+ $(,)?) => (
-        $crate::__rust_force_expr!(<[_]>::into_vec(
+        <[_]>::into_vec(
             // This rustc_box is not required, but it produces a dramatic improvement in compile
             // time when constructing arrays with many elements.
             #[rustc_box]
             $crate::boxed::Box::new([$($x),+])
-        ))
+        )
     );
 }
 
@@ -126,13 +126,3 @@ macro_rules! format {
         res
     }}
 }
-
-/// Force AST node to an expression to improve diagnostics in pattern position.
-#[doc(hidden)]
-#[macro_export]
-#[unstable(feature = "liballoc_internals", issue = "none", reason = "implementation detail")]
-macro_rules! __rust_force_expr {
-    ($e:expr) => {
-        $e
-    };
-}
diff --git a/std/src/sys_common/wtf8.rs b/std/src/sys_common/wtf8.rs
index 708f62f476e73..117a3e23044ea 100644
--- a/std/src/sys_common/wtf8.rs
+++ b/std/src/sys_common/wtf8.rs
@@ -480,7 +480,7 @@ impl Wtf8Buf {
     #[inline]
     pub(crate) fn extend_from_slice(&mut self, other: &[u8]) {
         self.bytes.extend_from_slice(other);
-        self.is_known_utf8 = self.is_known_utf8 || self.next_surrogate(0).is_none();
+        self.is_known_utf8 = false;
     }
 }
 
diff --git a/std/src/sys_common/wtf8/tests.rs b/std/src/sys_common/wtf8/tests.rs
index 6a1cc41a8fb04..b57c99a8452a1 100644
--- a/std/src/sys_common/wtf8/tests.rs
+++ b/std/src/sys_common/wtf8/tests.rs
@@ -725,3 +725,27 @@ fn wtf8_utf8_boundary_between_surrogates() {
     string.push(CodePoint::from_u32(0xD800).unwrap());
     check_utf8_boundary(&string, 3);
 }
+
+#[test]
+fn wobbled_wtf8_plus_bytes_isnt_utf8() {
+    let mut string: Wtf8Buf = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
+    assert!(!string.is_known_utf8);
+    string.extend_from_slice(b"some utf-8");
+    assert!(!string.is_known_utf8);
+}
+
+#[test]
+fn wobbled_wtf8_plus_str_isnt_utf8() {
+    let mut string: Wtf8Buf = unsafe { Wtf8::from_bytes_unchecked(b"\xED\xA0\x80").to_owned() };
+    assert!(!string.is_known_utf8);
+    string.push_str("some utf-8");
+    assert!(!string.is_known_utf8);
+}
+
+#[test]
+fn unwobbly_wtf8_plus_utf8_is_utf8() {
+    let mut string: Wtf8Buf = Wtf8Buf::from_str("hello world");
+    assert!(string.is_known_utf8);
+    string.push_str("some utf-8");
+    assert!(string.is_known_utf8);
+}