diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index caac0969..8ff52d0b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -64,16 +64,16 @@ jobs:
       - uses: actions/checkout@v3
       - uses: dtolnay/rust-toolchain@v1
         with:
-          toolchain: ${{ matrix.msrv }}
-      - run: rustup default ${{ matrix.msrv }}
+          toolchain: nightly
       - uses: dtolnay/rust-toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: ${{ matrix.msrv }}
 
       - name: Install minimal dependencies versions
         run: cargo +nightly update -Z minimal-versions
 
       - run: cargo test --workspace --features full,testing-helpers
+                        -- --skip compile_fail
 
   no_std:
     runs-on: ubuntu-latest
@@ -111,7 +111,6 @@ jobs:
       - uses: dtolnay/rust-toolchain@v1
         with:
           toolchain: ${{ matrix.toolchain }}
-      - run: rustup default ${{ matrix.toolchain }}
 
       - run: cargo test --workspace --features testing-helpers
 
@@ -123,7 +122,6 @@ jobs:
       - uses: dtolnay/rust-toolchain@v1
         with:
           toolchain: nightly
-      - run: rustup default nightly
 
       - name: Install tomljson
         run: |
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 212e635a..5eb67158 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -36,6 +36,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Add `FromStr` derive support for enums that contain variants without fields.
   If you pass the name of the variant to `from_str` it will create the matching
   variant.
+- Use `const fn` in `Constructor` and `IsVariant` derives.
 
 ### Improvements
 
diff --git a/impl/doc/constructor.md b/impl/doc/constructor.md
index 1f195959..43f55c06 100644
--- a/impl/doc/constructor.md
+++ b/impl/doc/constructor.md
@@ -26,7 +26,7 @@ Code like this will be generated:
 ```rust
 # struct MyInts(i32, i32);
 impl MyInts {
-    pub fn new(__0: i32, __1: i32) -> MyInts {
+    pub const fn new(__0: i32, __1: i32) -> MyInts {
         MyInts(__0, __1)
     }
 }
@@ -60,7 +60,7 @@ Code like this will be generated:
 #     y: i32,
 # }
 impl Point2D {
-    pub fn new(x: i32, y: i32) -> Point2D {
+    pub const fn new(x: i32, y: i32) -> Point2D {
         Point2D { x: x, y: y }
     }
 }
diff --git a/impl/doc/is_variant.md b/impl/doc/is_variant.md
index 61092595..9e549dc1 100644
--- a/impl/doc/is_variant.md
+++ b/impl/doc/is_variant.md
@@ -33,10 +33,10 @@ The derive in the above example code generates the following code:
 #     Nothing
 # }
 impl <T> Maybe<T>{
-    pub fn is_just(&self) -> bool {
+    pub const fn is_just(&self) -> bool {
         match self {Self::Just(..) => true, _ => false}
     }
-    pub fn is_nothing(&self) -> bool {
+    pub const fn is_nothing(&self) -> bool {
         match self {Self::Nothing => true, _ => false}
     }
 }
diff --git a/impl/src/constructor.rs b/impl/src/constructor.rs
index 6d8fb411..284b0643 100644
--- a/impl/src/constructor.rs
+++ b/impl/src/constructor.rs
@@ -29,7 +29,7 @@ pub fn expand(input: &DeriveInput, _: &str) -> TokenStream {
         #[automatically_derived]
         impl #impl_generics #input_type #ty_generics #where_clause {
             #[inline]
-            pub fn new(#(#vars: #original_types),*) -> #input_type #ty_generics {
+            pub const fn new(#(#vars: #original_types),*) -> #input_type #ty_generics {
                 #body
             }
         }
diff --git a/impl/src/is_variant.rs b/impl/src/is_variant.rs
index 2d5e0744..b6c074fc 100644
--- a/impl/src/is_variant.rs
+++ b/impl/src/is_variant.rs
@@ -45,7 +45,7 @@ pub fn expand(input: &DeriveInput, trait_name: &'static str) -> Result<TokenStre
             #[doc = "Returns `true` if this value is of type `"]
             #[doc = #variant_name]
             #[doc = "`. Returns `false` otherwise"]
-            pub fn #fn_name(&self) -> bool {
+            pub const fn #fn_name(&self) -> bool {
                 match self {
                     #enum_name ::#variant_ident #data_pattern => true,
                     _ => false
diff --git a/impl/src/parsing.rs b/impl/src/parsing.rs
index 952b8e73..c465a04d 100644
--- a/impl/src/parsing.rs
+++ b/impl/src/parsing.rs
@@ -412,7 +412,7 @@ fn identifier(input: &str) -> Option<(LeftToParse<'_>, Identifier<'_>)> {
 /// [1]: https://doc.rust-lang.org/stable/std/fmt/index.html#syntax
 fn integer(input: &str) -> Option<(LeftToParse<'_>, usize)> {
     and_then(
-        take_while1(check_char(|c| matches!(c, '0'..='9'))),
+        take_while1(check_char(|c| c.is_ascii_digit())),
         |(i, int)| int.parse().ok().map(|int| (i, int)),
     )(input)
 }
diff --git a/tests/constructor.rs b/tests/constructor.rs
index 4b9ff0d1..8f9be453 100644
--- a/tests/constructor.rs
+++ b/tests/constructor.rs
@@ -5,17 +5,27 @@ use derive_more::Constructor;
 #[derive(Constructor)]
 struct EmptyTuple();
 
+const EMPTY_TUPLE: EmptyTuple = EmptyTuple::new();
+
 #[derive(Constructor)]
 struct EmptyStruct {}
 
+const EMPTY_STRUCT: EmptyStruct = EmptyStruct::new();
+
 #[derive(Constructor)]
 struct EmptyUnit;
 
+const EMPTY_UNIT: EmptyUnit = EmptyUnit::new();
+
 #[derive(Constructor)]
 struct MyInts(i32, i32);
 
+const MY_INTS: MyInts = MyInts::new(1, 2);
+
 #[derive(Constructor)]
 struct Point2D {
     x: i32,
     y: i32,
 }
+
+const POINT_2D: Point2D = Point2D::new(-4, 7);
diff --git a/tests/is_variant.rs b/tests/is_variant.rs
index 07543a8d..d8f41ad3 100644
--- a/tests/is_variant.rs
+++ b/tests/is_variant.rs
@@ -8,18 +8,59 @@ enum Either<TLeft, TRight> {
     Right(TRight),
 }
 
+const _: () = {
+    let either: Either<u8, i16> = Either::Right(7);
+    assert!(either.is_right());
+    assert!(!either.is_left());
+
+    let either: Either<u8, i16> = Either::Left(7);
+    assert!(!either.is_right());
+    assert!(either.is_left());
+};
+
 #[derive(IsVariant)]
 enum Maybe<T> {
     Nothing,
     Just(T),
 }
 
+const _: () = {
+    let maybe: Maybe<u8> = Maybe::Just(7);
+    assert!(maybe.is_just());
+    assert!(!maybe.is_nothing());
+
+    let maybe: Maybe<u8> = Maybe::Nothing;
+    assert!(!maybe.is_just());
+    assert!(maybe.is_nothing());
+};
+
+#[test]
+pub fn test_is_variant() {
+    assert!(Maybe::<()>::Nothing.is_nothing());
+    assert!(!Maybe::<()>::Nothing.is_just());
+}
+
 #[derive(IsVariant)]
 enum Color {
     RGB(u8, u8, u8),
     CMYK { c: u8, m: u8, y: u8, k: u8 },
 }
 
+const _: () = {
+    let color = Color::RGB(0, 0, 0);
+    assert!(color.is_rgb());
+    assert!(!color.is_cmyk());
+
+    let color = Color::CMYK {
+        c: 0,
+        m: 0,
+        y: 0,
+        k: 0,
+    };
+    assert!(!color.is_rgb());
+    assert!(color.is_cmyk());
+};
+
 #[derive(IsVariant)]
 enum Nonsense<'a, T> {
     Ref(&'a T),
@@ -28,6 +69,16 @@ enum Nonsense<'a, T> {
     NoRefIgnored,
 }
 
+const _: () = {
+    let nonsense: Nonsense<u8> = Nonsense::Ref(&7);
+    assert!(nonsense.is_ref());
+    assert!(!nonsense.is_no_ref());
+
+    let nonsense: Nonsense<u8> = Nonsense::NoRef;
+    assert!(!nonsense.is_ref());
+    assert!(nonsense.is_no_ref());
+};
+
 #[derive(IsVariant)]
 enum WithConstraints<T>
 where
@@ -36,6 +87,17 @@ where
     One(T),
     Two,
 }
+
+const _: () = {
+    let wc: WithConstraints<u8> = WithConstraints::One(1);
+    assert!(wc.is_one());
+    assert!(!wc.is_two());
+
+    let wc: WithConstraints<u8> = WithConstraints::Two;
+    assert!(!wc.is_one());
+    assert!(wc.is_two());
+};
+
 #[derive(IsVariant)]
 enum KitchenSink<'a, 'b, T1: Copy, T2: Clone>
 where
@@ -49,8 +111,52 @@ where
     NothingToSeeHere {},
 }
 
-#[test]
-pub fn test_is_variant() {
-    assert!(Maybe::<()>::Nothing.is_nothing());
-    assert!(!Maybe::<()>::Nothing.is_just());
-}
+const _: () = {
+    let ks: KitchenSink<u16, u8> = KitchenSink::Left(&1);
+    assert!(ks.is_left());
+    assert!(!ks.is_right());
+    assert!(!ks.is_own_both());
+    assert!(!ks.is_empty());
+    assert!(!ks.is_never_mind());
+    assert!(!ks.is_nothing_to_see_here());
+
+    let ks: KitchenSink<u16, u8> = KitchenSink::Right(&1);
+    assert!(!ks.is_left());
+    assert!(ks.is_right());
+    assert!(!ks.is_own_both());
+    assert!(!ks.is_empty());
+    assert!(!ks.is_never_mind());
+    assert!(!ks.is_nothing_to_see_here());
+
+    let ks: KitchenSink<u16, u8> = KitchenSink::OwnBoth { left: 1, right: 2 };
+    assert!(!ks.is_left());
+    assert!(!ks.is_right());
+    assert!(ks.is_own_both());
+    assert!(!ks.is_empty());
+    assert!(!ks.is_never_mind());
+    assert!(!ks.is_nothing_to_see_here());
+
+    let ks: KitchenSink<u16, u8> = KitchenSink::Empty;
+    assert!(!ks.is_left());
+    assert!(!ks.is_right());
+    assert!(!ks.is_own_both());
+    assert!(ks.is_empty());
+    assert!(!ks.is_never_mind());
+    assert!(!ks.is_nothing_to_see_here());
+
+    let ks: KitchenSink<u16, u8> = KitchenSink::NeverMind();
+    assert!(!ks.is_left());
+    assert!(!ks.is_right());
+    assert!(!ks.is_own_both());
+    assert!(!ks.is_empty());
+    assert!(ks.is_never_mind());
+    assert!(!ks.is_nothing_to_see_here());
+
+    let ks: KitchenSink<u16, u8> = KitchenSink::NothingToSeeHere {};
+    assert!(!ks.is_left());
+    assert!(!ks.is_right());
+    assert!(!ks.is_own_both());
+    assert!(!ks.is_empty());
+    assert!(!ks.is_never_mind());
+    assert!(ks.is_nothing_to_see_here());
+};