diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 91c1d7a37593..b9159c0ee226 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -186,30 +186,30 @@ jobs:
           - target: aarch64-linux-android
           - target: aarch64-unknown-linux-musl
           - target: aarch64-unknown-linux-musl
-            env: { RUST_LIBC_UNSTABLE_MUSL_V1_2_3: 1 }
+            env: { TEST_MUSL_V1_2_3: 1 }
             artifact-tag: new-musl
           - target: arm-linux-androideabi
           - target: arm-unknown-linux-gnueabihf
           - target: arm-unknown-linux-musleabihf
           - target: arm-unknown-linux-musleabihf
-            env: { RUST_LIBC_UNSTABLE_MUSL_V1_2_3: 1 }
+            env: { TEST_MUSL_V1_2_3: 1 }
             artifact-tag: new-musl
           # FIXME(#4297): Disabled due to spurious failue
           # - target: i686-linux-android
           - target: i686-unknown-linux-musl
           - target: i686-unknown-linux-musl
-            env: { RUST_LIBC_UNSTABLE_MUSL_V1_2_3: 1 }
+            env: { TEST_MUSL_V1_2_3: 1 }
             artifact-tag: new-musl
           - target: loongarch64-unknown-linux-gnu
           - target: loongarch64-unknown-linux-musl
           - target: loongarch64-unknown-linux-musl
-            env: { RUST_LIBC_UNSTABLE_MUSL_V1_2_3: 1 }
+            env: { TEST_MUSL_V1_2_3: 1 }
             artifact-tag: new-musl
           - target: powerpc64-unknown-linux-gnu
           - target: powerpc64le-unknown-linux-gnu
           - target: powerpc64le-unknown-linux-musl
           - target: powerpc64le-unknown-linux-musl
-            env: { RUST_LIBC_UNSTABLE_MUSL_V1_2_3: 1 }
+            env: { TEST_MUSL_V1_2_3: 1 }
             artifact-tag: new-musl
           - target: riscv64gc-unknown-linux-gnu
           - target: s390x-unknown-linux-gnu
@@ -223,7 +223,7 @@ jobs:
           # - target: x86_64-unknown-linux-gnux32
           - target: x86_64-unknown-linux-musl
           - target: x86_64-unknown-linux-musl
-            env: { RUST_LIBC_UNSTABLE_MUSL_V1_2_3: 1 }
+            env: { TEST_MUSL_V1_2_3: 1 }
             artifact-tag: new-musl
           # FIXME: It seems some items in `src/unix/mod.rs` aren't defined on redox actually.
           # - target: x86_64-unknown-redox
diff --git a/build.rs b/build.rs
index cbebca6207f2..0a016899c3bf 100644
--- a/build.rs
+++ b/build.rs
@@ -70,9 +70,8 @@ fn main() {
     //
     // On CI, we detect the actual FreeBSD version and match its ABI exactly,
     // running tests to ensure that the ABI is correct.
-    println!("cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_FREEBSD_VERSION");
     // Allow overriding the default version for testing
-    let which_freebsd = if let Ok(version) = env::var("RUST_LIBC_UNSTABLE_FREEBSD_VERSION") {
+    let which_freebsd = if let Ok(version) = env::var("CARGO_CFG_LIBC_UNSTABLE_FREEBSD_VERSION") {
         let vers = version.parse().unwrap();
         println!("cargo:warning=setting FreeBSD version to {vers}");
         vers
@@ -104,8 +103,7 @@ fn main() {
         _ => (),
     }
 
-    let mut musl_v1_2_3 = env_flag("RUST_LIBC_UNSTABLE_MUSL_V1_2_3");
-    println!("cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_MUSL_V1_2_3");
+    let mut musl_v1_2_3 = env_flag("CARGO_CFG_LIBC_UNSTABLE_MUSL_V1_2_3");
 
     // OpenHarmony uses a fork of the musl libc
     let musl = target_env == "musl" || target_env == "ohos";
@@ -123,13 +121,6 @@ fn main() {
         }
     }
 
-    let linux_time_bits64 = env::var("RUST_LIBC_UNSTABLE_LINUX_TIME_BITS64").is_ok();
-    println!("cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_LINUX_TIME_BITS64");
-    if linux_time_bits64 {
-        set_cfg("linux_time_bits64");
-    }
-    println!("cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS");
-    println!("cargo:rerun-if-env-changed=RUST_LIBC_UNSTABLE_GNU_TIME_BITS");
     if target_env == "gnu"
         && target_os == "linux"
         && target_ptr_width == "32"
@@ -138,25 +129,33 @@ fn main() {
     {
         let defaultbits = "32".to_string();
         let (timebits, filebits) = match (
-            env::var("RUST_LIBC_UNSTABLE_GNU_TIME_BITS"),
-            env::var("RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS"),
+            env::var("CARGO_CFG_LIBC_UNSTABLE_GNU_TIME_BITS"),
+            env::var("CARGO_CFG_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS"),
         ) {
-            (Ok(_), Ok(_)) => panic!("Do not set both RUST_LIBC_UNSTABLE_GNU_TIME_BITS and RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS"),
+            (Ok(_), Ok(_)) => panic!(
+                "Do not set both `libc_unstable_gnu_time_bits` and \
+                `libc_unstable_gnu_file_offset_bits`"
+            ),
             (Err(_), Err(_)) => (defaultbits.clone(), defaultbits.clone()),
             (Ok(tb), Err(_)) if tb == "64" => (tb.clone(), tb.clone()),
             (Ok(tb), Err(_)) if tb == "32" => (tb, defaultbits.clone()),
-            (Ok(_), Err(_)) => panic!("Invalid value for RUST_LIBC_UNSTABLE_GNU_TIME_BITS, must be 32 or 64"),
+            (Ok(_), Err(_)) => {
+                panic!("Invalid value for libc_unstable_gnu_time_bits, must be 32 or 64")
+            }
             (Err(_), Ok(fb)) if fb == "32" || fb == "64" => (defaultbits.clone(), fb),
-            (Err(_), Ok(_)) => panic!("Invalid value for RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS, must be 32 or 64"),
+            (Err(_), Ok(_)) => {
+                panic!("Invalid value for `libc_unstable_gnu_file_offset_bits`, must be 32 or 64")
+            }
         };
         let valid_bits = ["32", "64"];
         assert!(
             valid_bits.contains(&filebits.as_str()) && valid_bits.contains(&timebits.as_str()),
-            "Invalid value for RUST_LIBC_UNSTABLE_GNU_TIME_BITS or RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS, must be 32, 64 or unset"
+            "Invalid value for `libc_unstable_gnu_time_bits` or `libc_unstable_gnu_file_offset_bits`, \
+            must be 32, 64 or unset"
         );
         assert!(
             !(filebits == "32" && timebits == "64"),
-            "RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS must be 64 or unset if RUST_LIBC_UNSTABLE_GNU_TIME_BITS is 64"
+            "`libc_unstable_gnu_file_offset_bits` must be 64 or unset if `libc_unstable_gnu_time_bits` is 64"
         );
         if timebits == "64" {
             set_cfg("linux_time_bits64");
diff --git a/ci/run-docker.sh b/ci/run-docker.sh
index 838ba10543a7..47c1e7213e60 100755
--- a/ci/run-docker.sh
+++ b/ci/run-docker.sh
@@ -37,7 +37,8 @@ run() {
     )
 
     if [[ "$run_target" = *"musl"* ]]; then
-        if [ -n "${RUST_LIBC_UNSTABLE_MUSL_V1_2_3:-}" ]; then
+        if [ -n "${TEST_MUSL_V1_2_3:-}" ]; then
+            export RUSTFLAGS="$RUSTFLAGS libc_unstable_musl_v1_2_3"
             build_args+=("--build-arg=MUSL_VERSION=new")
         else
             build_args+=("--build-arg=MUSL_VERSION=old")
@@ -62,9 +63,6 @@ run() {
         --env RUSTFLAGS \
         --env RUSTDOCFLAGS \
         --env RUST_BACKTRACE \
-        --env RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS \
-        --env RUST_LIBC_UNSTABLE_GNU_TIME_BITS \
-        --env RUST_LIBC_UNSTABLE_MUSL_V1_2_3 \
         --env CARGO_TERM_COLOR \
         --env CARGO_TERM_VERBOSE \
         --env CARGO_HOME=/cargo \
diff --git a/ci/run.sh b/ci/run.sh
index 96e5750945df..f44a461c3fd1 100755
--- a/ci/run.sh
+++ b/ci/run.sh
@@ -51,7 +51,7 @@ $cmd --features extra_traits -- $test_flags
 
 if [ "$env" = "gnu" ] && [ "$bits" = "32" ]; then
     # shellcheck disable=SC2086
-    RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS=64 $cmd -- $test_flags
+    RUSTFLAGS="$RUSTFLAGS libc_unstable_gnu_file_offset_bits='64'" $cmd -- $test_flags
     # shellcheck disable=SC2086
-    RUST_LIBC_UNSTABLE_GNU_TIME_BITS=64 $cmd -- $test_flags
+    RUSTFLAGS="$RUSTFLAGS libc_unstable_gnu_time_bits='64'" $cmd -- $test_flags
 fi
diff --git a/ci/verify-build.py b/ci/verify-build.py
index 831c1cdb3d3e..fa4f0dc500e3 100755
--- a/ci/verify-build.py
+++ b/ci/verify-build.py
@@ -242,8 +242,15 @@ def run(
     args: Sequence[str | Path],
     *,
     env: Optional[dict[str, str]] = None,
+    extra_rustflags: Optional[str] = None,
     check: bool = True,
 ) -> sp.CompletedProcess:
+    if env is None:
+        env = os.environ.copy()
+
+    if extra_rustflags is not None:
+        env["RUSTFLAGS"] = env.get("RUSTFLAGS", "") + " " + extra_rustflags
+
     xtrace(args, env=env)
     return sp.run(args, env=env, check=check)
 
@@ -358,7 +365,7 @@ def test_target(cfg: Cfg, target: Target) -> TargetResult:
     """Run tests for a single target."""
     start = time.time()
     env = os.environ.copy()
-    env.setdefault("RUSTFLAGS", "")
+    rustflags = ""
 
     tname = target.name
     target_cfg = check_output(["rustc", "--print=cfg", "--target", tname])
@@ -383,7 +390,7 @@ def test_target(cfg: Cfg, target: Target) -> TargetResult:
         cmd += ["-Zbuild-std=core"]
         # FIXME: With `the build-std` feature, `compiler_builtins` emits a lot of
         # lint warnings.
-        env["RUSTFLAGS"] += " -Aimproper_ctypes_definitions"
+        rustflags += " -Aimproper_ctypes_definitions"
     else:
         run(["rustup", "target", "add", tname, "--toolchain", cfg.toolchain_name])
 
@@ -393,13 +400,16 @@ def test_target(cfg: Cfg, target: Target) -> TargetResult:
 
     if "gnu" in target_env and target_bits == "32":
         # Equivalent of _FILE_OFFSET_BITS=64
-        run(cmd, env=env | {"RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS": "64"})
+        run(
+            cmd,
+            extra_rustflags=rustflags + " libc_unstable_gnu_file_offset_bits='64'",
+        )
         # Equivalent of _TIME_BITS=64
-        run(cmd, env=env | {"RUST_LIBC_UNSTABLE_GNU_TIME_BITS": "64"})
+        run(cmd, extra_rustflags=rustflags + " libc_unstable_gnu_time_bits='64'")
 
     if "musl" in target_env:
         # Check with breaking changes from musl, including 64-bit time_t on 32-bit
-        run(cmd, env=env | {"RUST_LIBC_UNSTABLE_MUSL_V1_2_3": "1"})
+        run(cmd, extra_rustflags=rustflags + " libc_unstable_musl_v1_2_3")
 
     # Test again without default features, i.e. without `std`
     run([*cmd, "--no-default-features"])
@@ -413,10 +423,16 @@ def test_target(cfg: Cfg, target: Target) -> TargetResult:
     # if on nightly or stable
     if "freebsd" in tname and cfg.toolchain >= Toolchain.STABLE:
         for version in FREEBSD_VERSIONS:
-            run(cmd, env=env | {"RUST_LIBC_UNSTABLE_FREEBSD_VERSION": str(version)})
+
+            run(
+                cmd,
+                extra_rustflags=rustflags
+                + f" libc_unstable_freebsd_version = '{str(version)}'",
+            )
             run(
                 [*cmd, "--no-default-features"],
-                env=env | {"RUST_LIBC_UNSTABLE_FREEBSD_VERSION": str(version)},
+                extra_rustflags=rustflags
+                + f" libc_unstable_freebsd_version = '{str(version)}'",
             )
 
     if cfg.skip_semver:
diff --git a/libc-test/build.rs b/libc-test/build.rs
index 1cbacfde9d97..c8f0a8c0697e 100644
--- a/libc-test/build.rs
+++ b/libc-test/build.rs
@@ -2275,7 +2275,7 @@ fn test_freebsd(target: &str) {
     assert!(target.contains("freebsd"));
     let mut cfg = ctest_cfg();
 
-    let freebsd_ver = if let Ok(version) = env::var("RUST_LIBC_UNSTABLE_FREEBSD_VERSION") {
+    let freebsd_ver = if let Ok(version) = env::var("CARGO_CFG_LIBC_UNSTABLE_FREEBSD_VERSION") {
         let vers = version.parse().unwrap();
         println!("cargo:warning=setting FreeBSD version to {vers}");
         Some(vers)
@@ -3607,25 +3607,33 @@ fn config_gnu_bits(target: &str, cfg: &mut ctest::TestGenerator) {
     {
         let defaultbits = "32".to_string();
         let (timebits, filebits) = match (
-            env::var("RUST_LIBC_UNSTABLE_GNU_TIME_BITS"),
-            env::var("RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS"),
+            env::var("CARGO_CFG_LIBC_UNSTABLE_GNU_TIME_BITS"),
+            env::var("CARGO_CFG_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS"),
         ) {
-            (Ok(_), Ok(_)) => panic!("Do not set both RUST_LIBC_UNSTABLE_GNU_TIME_BITS and RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS"),
+            (Ok(_), Ok(_)) => panic!(
+                "Do not set both `libc_unstable_gnu_time_bits` and \
+                `libc_unstable_gnu_file_offset_bits`"
+            ),
             (Err(_), Err(_)) => (defaultbits.clone(), defaultbits.clone()),
             (Ok(tb), Err(_)) if tb == "64" => (tb.clone(), tb.clone()),
             (Ok(tb), Err(_)) if tb == "32" => (tb, defaultbits.clone()),
-            (Ok(_), Err(_)) => panic!("Invalid value for RUST_LIBC_UNSTABLE_GNU_TIME_BITS, must be 32 or 64"),
+            (Ok(_), Err(_)) => {
+                panic!("Invalid value for libc_unstable_gnu_time_bits, must be 32 or 64")
+            }
             (Err(_), Ok(fb)) if fb == "32" || fb == "64" => (defaultbits.clone(), fb),
-            (Err(_), Ok(_)) => panic!("Invalid value for RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS, must be 32 or 64"),
+            (Err(_), Ok(_)) => {
+                panic!("Invalid value for `libc_unstable_gnu_file_offset_bits`, must be 32 or 64")
+            }
         };
         let valid_bits = ["32", "64"];
         assert!(
             valid_bits.contains(&filebits.as_str()) && valid_bits.contains(&timebits.as_str()),
-            "Invalid value for RUST_LIBC_UNSTABLE_GNU_TIME_BITS or RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS, must be 32, 64 or unset"
+            "Invalid value for `libc_unstable_gnu_time_bits` or `libc_unstable_gnu_file_offset_bits`, \
+            must be 32, 64 or unset"
         );
         assert!(
             !(filebits == "32" && timebits == "64"),
-            "RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS must be 64 or unset if RUST_LIBC_UNSTABLE_GNU_TIME_BITS is 64"
+            "`libc_unstable_gnu_file_offset_bits` must be 64 or unset if `libc_unstable_gnu_time_bits` is 64"
         );
         if timebits == "64" {
             cfg.define("_TIME_BITS", Some("64"));
@@ -3692,7 +3700,7 @@ fn test_linux(target: &str) {
     let mips64 = target.contains("mips64");
     let mips32 = mips && !mips64;
 
-    let musl_v1_2_3 = env::var("RUST_LIBC_UNSTABLE_MUSL_V1_2_3").is_ok();
+    let musl_v1_2_3 = env::var("CARGO_CFG_LIBC_UNSTABLE_MUSL_V1_2_3").is_ok();
     if musl_v1_2_3 {
         assert!(musl);
     }