diff --git a/ci/azure.yml b/ci/azure.yml
index bb8f220956..f1269b0426 100644
--- a/ci/azure.yml
+++ b/ci/azure.yml
@@ -163,7 +163,7 @@ jobs:
         #  TARGET: i686-pc-windows-gnu
         #i686-pc-windows-msvc:
         #  TARGET: i686-pc-windows-msvc
-    
+
   - job: StyleAndDocs
     pool:
       vmImage: ubuntu-16.04
@@ -186,7 +186,16 @@ jobs:
     steps:
       - template: azure-install-rust.yml
       - script: cargo test --manifest-path crates/stdarch-verify/Cargo.toml
-        displayName: Automatic verification  
+        displayName: Automatic verification
+
+  - job: EnvOverride
+    dependsOn: StyleAndDocs
+    pool:
+      vmImage: ubuntu-16.04
+    steps:
+      - template: azure-install-rust.yml
+      - script: RUST_STD_DETECT_UNSTABLE=avx cargo test --manifest-path crates/std_detect/Cargo.toml env_override_no_avx
+        displayName: std_detect env override
 
 #  - job: GameBoyAdvance
 #    dependsOn: StyleAndDocs
diff --git a/crates/std_detect/Cargo.toml b/crates/std_detect/Cargo.toml
index acfcc116f4..857d8a5b9b 100644
--- a/crates/std_detect/Cargo.toml
+++ b/crates/std_detect/Cargo.toml
@@ -31,6 +31,7 @@ auxv = "0.3.3"
 cupid = "0.6.0"
 
 [features]
-default = [ "std_detect_dlsym_getauxval", "std_detect_file_io" ]
+default = [ "std_detect_dlsym_getauxval", "std_detect_file_io", "std_detect_env_override" ]
 std_detect_file_io = []
-std_detect_dlsym_getauxval = [ "libc" ]
\ No newline at end of file
+std_detect_dlsym_getauxval = [ "libc" ]
+std_detect_env_override = []
diff --git a/crates/std_detect/src/detect/cache.rs b/crates/std_detect/src/detect/cache.rs
index 92bc4b58d1..9179fbd938 100644
--- a/crates/std_detect/src/detect/cache.rs
+++ b/crates/std_detect/src/detect/cache.rs
@@ -23,6 +23,12 @@ const fn test_bit(x: u64, bit: u32) -> bool {
     x & (1 << bit) != 0
 }
 
+/// Unset the `bit of `x`.
+#[inline]
+const fn unset_bit(x: u64, bit: u32) -> u64 {
+    x & !(1 << bit)
+}
+
 /// Maximum number of features that can be cached.
 const CACHE_CAPACITY: u32 = 63;
 
@@ -37,13 +43,13 @@ impl Default for Initializer {
     }
 }
 
+// NOTE: the `debug_assert!` would catch that we do not add more Features than
+// the one fitting our cache.
 impl Initializer {
     /// Tests the `bit` of the cache.
     #[allow(dead_code)]
     #[inline]
     pub(crate) fn test(self, bit: u32) -> bool {
-        // FIXME: this way of making sure that the cache is large enough is
-        // brittle.
         debug_assert!(
             bit < CACHE_CAPACITY,
             "too many features, time to increase the cache size!"
@@ -54,8 +60,6 @@ impl Initializer {
     /// Sets the `bit` of the cache.
     #[inline]
     pub(crate) fn set(&mut self, bit: u32) {
-        // FIXME: this way of making sure that the cache is large enough is
-        // brittle.
         debug_assert!(
             bit < CACHE_CAPACITY,
             "too many features, time to increase the cache size!"
@@ -63,6 +67,17 @@ impl Initializer {
         let v = self.0;
         self.0 = set_bit(v, bit);
     }
+
+    /// Unsets the `bit` of the cache.
+    #[inline]
+    pub(crate) fn unset(&mut self, bit: u32) {
+        debug_assert!(
+            bit < CACHE_CAPACITY,
+            "too many features, time to increase the cache size!"
+        );
+        let v = self.0;
+        self.0 = unset_bit(v, bit);
+    }
 }
 
 /// This global variable is a cache of the features supported by the CPU.
@@ -143,6 +158,24 @@ impl Cache {
         self.1.store(hi, Ordering::Relaxed);
     }
 }
+cfg_if! {
+    if #[cfg(feature = "std_detect_env_override")] {
+        #[inline(never)]
+        fn initialize(mut value: Initializer) {
+            if let Ok(disable) = crate::env::var("RUST_STD_DETECT_UNSTABLE") {
+                for v in disable.split(" ") {
+                    let _ = super::Feature::from_str(v).map(|v| value.unset(v as u32));
+                }
+            }
+            CACHE.initialize(value);
+        }
+    } else {
+        #[inline]
+        fn initialize(value: Initializer) {
+            CACHE.initialize(value);
+        }
+    }
+}
 
 /// Tests the `bit` of the storage. If the storage has not been initialized,
 /// initializes it with the result of `f()`.
@@ -152,13 +185,17 @@ impl Cache {
 ///
 /// It uses the `Feature` variant to index into this variable as a bitset. If
 /// the bit is set, the feature is enabled, and otherwise it is disabled.
+///
+/// If the feature `std_detect_env_override` is enabled looks for the env
+/// variable `RUST_STD_DETECT_UNSTABLE` and uses its its content to disable
+/// Features that would had been otherwise detected.
 #[inline]
 pub(crate) fn test<F>(bit: u32, f: F) -> bool
 where
     F: FnOnce() -> Initializer,
 {
     if CACHE.is_uninitialized() {
-        CACHE.initialize(f());
+        initialize(f());
     }
     CACHE.test(bit)
 }
diff --git a/crates/std_detect/src/detect/macros.rs b/crates/std_detect/src/detect/macros.rs
index 750cf50aa7..c9200d2f1f 100644
--- a/crates/std_detect/src/detect/macros.rs
+++ b/crates/std_detect/src/detect/macros.rs
@@ -69,6 +69,12 @@ macro_rules! features {
                     Feature::_last => unreachable!(),
                 }
             }
+            pub fn from_str(s: &str) -> Result<Feature, ()> {
+                match s {
+                    $($feature_lit => Ok(Feature::$feature),)*
+                    _ => Err(())
+                }
+            }
         }
 
         /// Each function performs run-time feature detection for a single
diff --git a/crates/std_detect/src/detect/mod.rs b/crates/std_detect/src/detect/mod.rs
index 6389a3fd75..cbbc6afab1 100644
--- a/crates/std_detect/src/detect/mod.rs
+++ b/crates/std_detect/src/detect/mod.rs
@@ -61,6 +61,13 @@ cfg_if! {
             }
             #[doc(hidden)]
             pub mod __is_feature_detected {}
+
+            impl Feature {
+                #[doc(hidden)]
+                pub fn from_str(_s: &str) -> Result<Feature, ()> { Err<()> }
+                #[doc(hidden)]
+                pub fn to_str(self) -> &'static str { "" }
+            }
         }
     }
 }
diff --git a/crates/std_detect/src/detect/os/x86.rs b/crates/std_detect/src/detect/os/x86.rs
index c653771c43..7888776335 100644
--- a/crates/std_detect/src/detect/os/x86.rs
+++ b/crates/std_detect/src/detect/os/x86.rs
@@ -243,120 +243,3 @@ pub(crate) fn detect_features() -> cache::Initializer {
 
     value
 }
-
-#[cfg(test)]
-mod tests {
-    extern crate cupid;
-
-    #[test]
-    fn dump() {
-        println!("aes: {:?}", is_x86_feature_detected!("aes"));
-        println!("pclmulqdq: {:?}", is_x86_feature_detected!("pclmulqdq"));
-        println!("rdrand: {:?}", is_x86_feature_detected!("rdrand"));
-        println!("rdseed: {:?}", is_x86_feature_detected!("rdseed"));
-        println!("tsc: {:?}", is_x86_feature_detected!("tsc"));
-        println!("sse: {:?}", is_x86_feature_detected!("sse"));
-        println!("sse2: {:?}", is_x86_feature_detected!("sse2"));
-        println!("sse3: {:?}", is_x86_feature_detected!("sse3"));
-        println!("ssse3: {:?}", is_x86_feature_detected!("ssse3"));
-        println!("sse4.1: {:?}", is_x86_feature_detected!("sse4.1"));
-        println!("sse4.2: {:?}", is_x86_feature_detected!("sse4.2"));
-        println!("sse4a: {:?}", is_x86_feature_detected!("sse4a"));
-        println!("sha: {:?}", is_x86_feature_detected!("sha"));
-        println!("avx: {:?}", is_x86_feature_detected!("avx"));
-        println!("avx2: {:?}", is_x86_feature_detected!("avx2"));
-        println!("avx512f {:?}", is_x86_feature_detected!("avx512f"));
-        println!("avx512cd {:?}", is_x86_feature_detected!("avx512cd"));
-        println!("avx512er {:?}", is_x86_feature_detected!("avx512er"));
-        println!("avx512pf {:?}", is_x86_feature_detected!("avx512pf"));
-        println!("avx512bw {:?}", is_x86_feature_detected!("avx512bw"));
-        println!("avx512dq {:?}", is_x86_feature_detected!("avx512dq"));
-        println!("avx512vl {:?}", is_x86_feature_detected!("avx512vl"));
-        println!("avx512_ifma {:?}", is_x86_feature_detected!("avx512ifma"));
-        println!("avx512_vbmi {:?}", is_x86_feature_detected!("avx512vbmi"));
-        println!(
-            "avx512_vpopcntdq {:?}",
-            is_x86_feature_detected!("avx512vpopcntdq")
-        );
-        println!("fma: {:?}", is_x86_feature_detected!("fma"));
-        println!("abm: {:?}", is_x86_feature_detected!("abm"));
-        println!("bmi: {:?}", is_x86_feature_detected!("bmi1"));
-        println!("bmi2: {:?}", is_x86_feature_detected!("bmi2"));
-        println!("tbm: {:?}", is_x86_feature_detected!("tbm"));
-        println!("popcnt: {:?}", is_x86_feature_detected!("popcnt"));
-        println!("lzcnt: {:?}", is_x86_feature_detected!("lzcnt"));
-        println!("fxsr: {:?}", is_x86_feature_detected!("fxsr"));
-        println!("xsave: {:?}", is_x86_feature_detected!("xsave"));
-        println!("xsaveopt: {:?}", is_x86_feature_detected!("xsaveopt"));
-        println!("xsaves: {:?}", is_x86_feature_detected!("xsaves"));
-        println!("xsavec: {:?}", is_x86_feature_detected!("xsavec"));
-        println!("cmpxchg16b: {:?}", is_x86_feature_detected!("cmpxchg16b"));
-        println!("adx: {:?}", is_x86_feature_detected!("adx"));
-        println!("rtm: {:?}", is_x86_feature_detected!("rtm"));
-    }
-
-    #[test]
-    fn compare_with_cupid() {
-        let information = cupid::master().unwrap();
-        assert_eq!(is_x86_feature_detected!("aes"), information.aesni());
-        assert_eq!(
-            is_x86_feature_detected!("pclmulqdq"),
-            information.pclmulqdq()
-        );
-        assert_eq!(is_x86_feature_detected!("rdrand"), information.rdrand());
-        assert_eq!(is_x86_feature_detected!("rdseed"), information.rdseed());
-        assert_eq!(is_x86_feature_detected!("tsc"), information.tsc());
-        assert_eq!(is_x86_feature_detected!("sse"), information.sse());
-        assert_eq!(is_x86_feature_detected!("sse2"), information.sse2());
-        assert_eq!(is_x86_feature_detected!("sse3"), information.sse3());
-        assert_eq!(is_x86_feature_detected!("ssse3"), information.ssse3());
-        assert_eq!(is_x86_feature_detected!("sse4.1"), information.sse4_1());
-        assert_eq!(is_x86_feature_detected!("sse4.2"), information.sse4_2());
-        assert_eq!(is_x86_feature_detected!("sse4a"), information.sse4a());
-        assert_eq!(is_x86_feature_detected!("sha"), information.sha());
-        assert_eq!(is_x86_feature_detected!("avx"), information.avx());
-        assert_eq!(is_x86_feature_detected!("avx2"), information.avx2());
-        assert_eq!(is_x86_feature_detected!("avx512f"), information.avx512f());
-        assert_eq!(is_x86_feature_detected!("avx512cd"), information.avx512cd());
-        assert_eq!(is_x86_feature_detected!("avx512er"), information.avx512er());
-        assert_eq!(is_x86_feature_detected!("avx512pf"), information.avx512pf());
-        assert_eq!(is_x86_feature_detected!("avx512bw"), information.avx512bw());
-        assert_eq!(is_x86_feature_detected!("avx512dq"), information.avx512dq());
-        assert_eq!(is_x86_feature_detected!("avx512vl"), information.avx512vl());
-        assert_eq!(
-            is_x86_feature_detected!("avx512ifma"),
-            information.avx512_ifma()
-        );
-        assert_eq!(
-            is_x86_feature_detected!("avx512vbmi"),
-            information.avx512_vbmi()
-        );
-        assert_eq!(
-            is_x86_feature_detected!("avx512vpopcntdq"),
-            information.avx512_vpopcntdq()
-        );
-        assert_eq!(is_x86_feature_detected!("fma"), information.fma());
-        assert_eq!(is_x86_feature_detected!("bmi1"), information.bmi1());
-        assert_eq!(is_x86_feature_detected!("bmi2"), information.bmi2());
-        assert_eq!(is_x86_feature_detected!("popcnt"), information.popcnt());
-        assert_eq!(is_x86_feature_detected!("abm"), information.lzcnt());
-        assert_eq!(is_x86_feature_detected!("tbm"), information.tbm());
-        assert_eq!(is_x86_feature_detected!("lzcnt"), information.lzcnt());
-        assert_eq!(is_x86_feature_detected!("xsave"), information.xsave());
-        assert_eq!(is_x86_feature_detected!("xsaveopt"), information.xsaveopt());
-        assert_eq!(
-            is_x86_feature_detected!("xsavec"),
-            information.xsavec_and_xrstor()
-        );
-        assert_eq!(
-            is_x86_feature_detected!("xsaves"),
-            information.xsaves_xrstors_and_ia32_xss()
-        );
-        assert_eq!(
-            is_x86_feature_detected!("cmpxchg16b"),
-            information.cmpxchg16b(),
-        );
-        assert_eq!(is_x86_feature_detected!("adx"), information.adx(),);
-        assert_eq!(is_x86_feature_detected!("rtm"), information.rtm(),);
-    }
-}
diff --git a/crates/std_detect/src/lib.rs b/crates/std_detect/src/lib.rs
index 7737719c3b..bb70baaf61 100644
--- a/crates/std_detect/src/lib.rs
+++ b/crates/std_detect/src/lib.rs
@@ -24,12 +24,12 @@
 extern crate cfg_if;
 
 cfg_if! {
-    if #[cfg(feature = "std_detect_file_io")] {
+    if #[cfg(any(feature = "std_detect_file_io", feature = "std_detect_env_override"))] {
         #[cfg_attr(test, macro_use(println))]
         extern crate std;
 
         #[allow(unused_imports)]
-        use std::{arch, fs, io, mem, sync};
+        use std::{arch, env, fs, io, mem, sync};
     } else {
         #[cfg(test)]
         #[macro_use(println)]
diff --git a/crates/std_detect/tests/features.rs b/crates/std_detect/tests/features.rs
new file mode 100644
index 0000000000..7fe81d3e7f
--- /dev/null
+++ b/crates/std_detect/tests/features.rs
@@ -0,0 +1,13 @@
+#![feature(stdsimd)]
+#![feature(stdsimd_internal)]
+
+extern crate std_detect;
+
+use std_detect::detect::{features, Feature};
+
+#[test]
+fn features_roundtrip() {
+    for (f, _) in features() {
+        let _ = Feature::from_str(f).unwrap();
+    }
+}
diff --git a/crates/std_detect/tests/x86-specific.rs b/crates/std_detect/tests/x86-specific.rs
new file mode 100644
index 0000000000..cb51649725
--- /dev/null
+++ b/crates/std_detect/tests/x86-specific.rs
@@ -0,0 +1,140 @@
+#![feature(stdsimd)]
+
+extern crate cupid;
+#[macro_use]
+extern crate std_detect;
+
+#[test]
+fn dump() {
+    println!("aes: {:?}", is_x86_feature_detected!("aes"));
+    println!("pclmulqdq: {:?}", is_x86_feature_detected!("pclmulqdq"));
+    println!("rdrand: {:?}", is_x86_feature_detected!("rdrand"));
+    println!("rdseed: {:?}", is_x86_feature_detected!("rdseed"));
+    println!("tsc: {:?}", is_x86_feature_detected!("tsc"));
+    println!("sse: {:?}", is_x86_feature_detected!("sse"));
+    println!("sse2: {:?}", is_x86_feature_detected!("sse2"));
+    println!("sse3: {:?}", is_x86_feature_detected!("sse3"));
+    println!("ssse3: {:?}", is_x86_feature_detected!("ssse3"));
+    println!("sse4.1: {:?}", is_x86_feature_detected!("sse4.1"));
+    println!("sse4.2: {:?}", is_x86_feature_detected!("sse4.2"));
+    println!("sse4a: {:?}", is_x86_feature_detected!("sse4a"));
+    println!("sha: {:?}", is_x86_feature_detected!("sha"));
+    println!("avx: {:?}", is_x86_feature_detected!("avx"));
+    println!("avx2: {:?}", is_x86_feature_detected!("avx2"));
+    println!("avx512f {:?}", is_x86_feature_detected!("avx512f"));
+    println!("avx512cd {:?}", is_x86_feature_detected!("avx512cd"));
+    println!("avx512er {:?}", is_x86_feature_detected!("avx512er"));
+    println!("avx512pf {:?}", is_x86_feature_detected!("avx512pf"));
+    println!("avx512bw {:?}", is_x86_feature_detected!("avx512bw"));
+    println!("avx512dq {:?}", is_x86_feature_detected!("avx512dq"));
+    println!("avx512vl {:?}", is_x86_feature_detected!("avx512vl"));
+    println!("avx512_ifma {:?}", is_x86_feature_detected!("avx512ifma"));
+    println!("avx512_vbmi {:?}", is_x86_feature_detected!("avx512vbmi"));
+    println!(
+        "avx512_vpopcntdq {:?}",
+        is_x86_feature_detected!("avx512vpopcntdq")
+    );
+    println!("fma: {:?}", is_x86_feature_detected!("fma"));
+    println!("abm: {:?}", is_x86_feature_detected!("abm"));
+    println!("bmi: {:?}", is_x86_feature_detected!("bmi1"));
+    println!("bmi2: {:?}", is_x86_feature_detected!("bmi2"));
+    println!("tbm: {:?}", is_x86_feature_detected!("tbm"));
+    println!("popcnt: {:?}", is_x86_feature_detected!("popcnt"));
+    println!("lzcnt: {:?}", is_x86_feature_detected!("lzcnt"));
+    println!("fxsr: {:?}", is_x86_feature_detected!("fxsr"));
+    println!("xsave: {:?}", is_x86_feature_detected!("xsave"));
+    println!("xsaveopt: {:?}", is_x86_feature_detected!("xsaveopt"));
+    println!("xsaves: {:?}", is_x86_feature_detected!("xsaves"));
+    println!("xsavec: {:?}", is_x86_feature_detected!("xsavec"));
+    println!("cmpxchg16b: {:?}", is_x86_feature_detected!("cmpxchg16b"));
+    println!("adx: {:?}", is_x86_feature_detected!("adx"));
+    println!("rtm: {:?}", is_x86_feature_detected!("rtm"));
+}
+
+#[cfg(feature = "std_detect_env_override")]
+#[test]
+fn env_override_no_avx() {
+    if let Ok(disable) = std::env::var("RUST_STD_DETECT_UNSTABLE") {
+        let information = cupid::master().unwrap();
+        for d in disable.split(" ") {
+            match d {
+                "avx" => {
+                    if information.avx() {
+                        assert_ne!(is_x86_feature_detected!("avx"), information.avx())
+                    }
+                }
+                "avx2" => {
+                    if information.avx2() {
+                        assert_ne!(is_x86_feature_detected!("avx2"), information.avx2())
+                    }
+                }
+                _ => {}
+            }
+        }
+    }
+}
+
+#[test]
+fn compare_with_cupid() {
+    let information = cupid::master().unwrap();
+    assert_eq!(is_x86_feature_detected!("aes"), information.aesni());
+    assert_eq!(
+        is_x86_feature_detected!("pclmulqdq"),
+        information.pclmulqdq()
+    );
+    assert_eq!(is_x86_feature_detected!("rdrand"), information.rdrand());
+    assert_eq!(is_x86_feature_detected!("rdseed"), information.rdseed());
+    assert_eq!(is_x86_feature_detected!("tsc"), information.tsc());
+    assert_eq!(is_x86_feature_detected!("sse"), information.sse());
+    assert_eq!(is_x86_feature_detected!("sse2"), information.sse2());
+    assert_eq!(is_x86_feature_detected!("sse3"), information.sse3());
+    assert_eq!(is_x86_feature_detected!("ssse3"), information.ssse3());
+    assert_eq!(is_x86_feature_detected!("sse4.1"), information.sse4_1());
+    assert_eq!(is_x86_feature_detected!("sse4.2"), information.sse4_2());
+    assert_eq!(is_x86_feature_detected!("sse4a"), information.sse4a());
+    assert_eq!(is_x86_feature_detected!("sha"), information.sha());
+    assert_eq!(is_x86_feature_detected!("avx"), information.avx());
+    assert_eq!(is_x86_feature_detected!("avx2"), information.avx2());
+    assert_eq!(is_x86_feature_detected!("avx512f"), information.avx512f());
+    assert_eq!(is_x86_feature_detected!("avx512cd"), information.avx512cd());
+    assert_eq!(is_x86_feature_detected!("avx512er"), information.avx512er());
+    assert_eq!(is_x86_feature_detected!("avx512pf"), information.avx512pf());
+    assert_eq!(is_x86_feature_detected!("avx512bw"), information.avx512bw());
+    assert_eq!(is_x86_feature_detected!("avx512dq"), information.avx512dq());
+    assert_eq!(is_x86_feature_detected!("avx512vl"), information.avx512vl());
+    assert_eq!(
+        is_x86_feature_detected!("avx512ifma"),
+        information.avx512_ifma()
+    );
+    assert_eq!(
+        is_x86_feature_detected!("avx512vbmi"),
+        information.avx512_vbmi()
+    );
+    assert_eq!(
+        is_x86_feature_detected!("avx512vpopcntdq"),
+        information.avx512_vpopcntdq()
+    );
+    assert_eq!(is_x86_feature_detected!("fma"), information.fma());
+    assert_eq!(is_x86_feature_detected!("bmi1"), information.bmi1());
+    assert_eq!(is_x86_feature_detected!("bmi2"), information.bmi2());
+    assert_eq!(is_x86_feature_detected!("popcnt"), information.popcnt());
+    assert_eq!(is_x86_feature_detected!("abm"), information.lzcnt());
+    assert_eq!(is_x86_feature_detected!("tbm"), information.tbm());
+    assert_eq!(is_x86_feature_detected!("lzcnt"), information.lzcnt());
+    assert_eq!(is_x86_feature_detected!("xsave"), information.xsave());
+    assert_eq!(is_x86_feature_detected!("xsaveopt"), information.xsaveopt());
+    assert_eq!(
+        is_x86_feature_detected!("xsavec"),
+        information.xsavec_and_xrstor()
+    );
+    assert_eq!(
+        is_x86_feature_detected!("xsaves"),
+        information.xsaves_xrstors_and_ia32_xss()
+    );
+    assert_eq!(
+        is_x86_feature_detected!("cmpxchg16b"),
+        information.cmpxchg16b(),
+    );
+    assert_eq!(is_x86_feature_detected!("adx"), information.adx(),);
+    assert_eq!(is_x86_feature_detected!("rtm"), information.rtm(),);
+}