diff --git a/bazel/cc_toolchains/clang_cc_toolchain_config.bzl b/bazel/cc_toolchains/clang_cc_toolchain_config.bzl
index 702c0107d8da..a9670d6b23dd 100644
--- a/bazel/cc_toolchains/clang_cc_toolchain_config.bzl
+++ b/bazel/cc_toolchains/clang_cc_toolchain_config.bzl
@@ -444,7 +444,7 @@ def _impl(ctx):
         flag_sets = [flag_set(
             actions = all_compile_actions + all_link_actions,
             flag_groups = [flag_group(flags = [
-                "-fsanitize=address,undefined",
+                "-fsanitize=address,undefined,nullability",
                 "-fsanitize-address-use-after-scope",
                 # We don't need the recovery behavior of UBSan as we expect
                 # builds to be clean. Not recoverying is a bit cheaper.
@@ -453,10 +453,6 @@ def _impl(ctx):
                 # and combined with line numbers is unlikely to result in many
                 # ambiguities.
                 "-fsanitize-undefined-strip-path-components=-1",
-                # Force some expensive UBSan checks to the cheaper trap mode.
-                # The dedicated debugging message is unlikely to be critical for
-                # these.
-                "-fsanitize-trap=alignment,bool,null,return,unreachable",
                 # Needed due to clang AST issues, such as in
                 # clang/AST/Redeclarable.h line 199.
                 "-fno-sanitize=vptr",
diff --git a/executable_semantics/common/ptr.h b/executable_semantics/common/ptr.h
index c971e5635459..b3cc5f174189 100644
--- a/executable_semantics/common/ptr.h
+++ b/executable_semantics/common/ptr.h
@@ -11,12 +11,11 @@ namespace Carbon {
 
 // A non-nullable pointer. Written as `Nonnull<T*>` instead of `T*`.
 //
-// Note LLVM primarily enforces the attribute on function calls that can be
-// proven to be called with nullptr; in other places, this is essentially a
-// comment.
+// Sanitizers enforce this dynamically on assignment, return, and when passing
+// as an argument. Static analysis will also track erroneous uses of `nullptr`.
 template <typename T,
           typename std::enable_if_t<std::is_pointer_v<T>>* = nullptr>
-using Nonnull = T _Nonnull __attribute__((nonnull));
+using Nonnull = T _Nonnull;
 
 }  // namespace Carbon