diff --git a/.ci_extras/pin-crate-vers-msrv.sh b/.ci_extras/pin-crate-vers-msrv.sh
index d05ec46d..cbf65bb4 100755
--- a/.ci_extras/pin-crate-vers-msrv.sh
+++ b/.ci_extras/pin-crate-vers-msrv.sh
@@ -4,3 +4,4 @@ set -eux
 
 # Pin some dependencies to specific versions for the MSRV.
 # cargo update -p <crate> --precise <version>
+cargo update -p actix-rt --precise 2.9.0
diff --git a/.vscode/settings.json b/.vscode/settings.json
index 17072aa5..a8aa663c 100644
--- a/.vscode/settings.json
+++ b/.vscode/settings.json
@@ -10,6 +10,7 @@
         "ahash",
         "armv",
         "benmanes",
+        "cfgs",
         "CHECKME",
         "circleci",
         "CLFU",
@@ -33,6 +34,7 @@
         "getrandom",
         "hashbrown",
         "Hasher",
+        "kani",
         "Kawano",
         "mapref",
         "Miri",
diff --git a/build.rs b/build.rs
index 72a59c0e..e99050f6 100644
--- a/build.rs
+++ b/build.rs
@@ -1,3 +1,15 @@
+#![allow(unexpected_cfgs)] // for `#[cfg(rustver)]` in this build.rs.
+
+const ALLOWED_CFG_NAMES: &[&str] = &[
+    "armv5te",
+    "beta_clippy",
+    "circleci",
+    "kani",
+    "mips",
+    "rustver",
+    "trybuild",
+];
+
 #[cfg(rustver)]
 fn main() {
     use rustc_version::version;
@@ -6,7 +18,18 @@ fn main() {
         "cargo:rustc-env=RUSTC_SEMVER={}.{}",
         version.major, version.minor
     );
+
+    allow_cfgs(ALLOWED_CFG_NAMES);
 }
 
 #[cfg(not(rustver))]
-fn main() {}
+fn main() {
+    allow_cfgs(ALLOWED_CFG_NAMES);
+}
+
+/// Tells `rustc` to allow `#[cfg(...)]` with the given names.
+fn allow_cfgs(names: &[&str]) {
+    for name in names.iter() {
+        println!("cargo:rustc-check-cfg=cfg({name})");
+    }
+}