diff --git a/src/asn1_types/any.rs b/src/asn1_types/any.rs
index 8b01e41..330b02a 100644
--- a/src/asn1_types/any.rs
+++ b/src/asn1_types/any.rs
@@ -1,6 +1,7 @@
 use crate::ber::*;
 use crate::*;
 use alloc::borrow::Cow;
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 use core::convert::{TryFrom, TryInto};
 
@@ -103,10 +104,10 @@ impl<'a> Any<'a> {
         let (rem, any) = Any::from_ber(bytes).map_err(Err::convert)?;
         any.tag()
             .assert_eq(Tag(tag))
-            .map_err(|e| nom::Err::Error(e.into()))?;
+            .map_err(|e| Err::Error(e.into()))?;
         any.class()
             .assert_eq(class)
-            .map_err(|e| nom::Err::Error(e.into()))?;
+            .map_err(|e| Err::Error(e.into()))?;
         let (_, res) = op(any.data)?;
         Ok((rem, res))
     }
@@ -127,10 +128,10 @@ impl<'a> Any<'a> {
         let (rem, any) = Any::from_der(bytes).map_err(Err::convert)?;
         any.tag()
             .assert_eq(Tag(tag))
-            .map_err(|e| nom::Err::Error(e.into()))?;
+            .map_err(|e| Err::Error(e.into()))?;
         any.class()
             .assert_eq(class)
-            .map_err(|e| nom::Err::Error(e.into()))?;
+            .map_err(|e| Err::Error(e.into()))?;
         let (_, res) = op(any.data)?;
         Ok((rem, res))
     }
diff --git a/src/asn1_types/generalizedtime.rs b/src/asn1_types/generalizedtime.rs
index 6e039d8..42a5629 100644
--- a/src/asn1_types/generalizedtime.rs
+++ b/src/asn1_types/generalizedtime.rs
@@ -1,6 +1,6 @@
-use crate::datetime::decode_decimal;
 use crate::*;
 use alloc::format;
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 use core::convert::TryFrom;
 use core::fmt;
diff --git a/src/asn1_types/object_descriptor.rs b/src/asn1_types/object_descriptor.rs
index db78870..8f60e7c 100644
--- a/src/asn1_types/object_descriptor.rs
+++ b/src/asn1_types/object_descriptor.rs
@@ -1,5 +1,6 @@
 use crate::{asn1_string, TestValidCharset};
 use crate::{Error, Result};
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 
 // X.680 section 44.3
diff --git a/src/asn1_types/oid.rs b/src/asn1_types/oid.rs
index a0747e6..aa8bc43 100644
--- a/src/asn1_types/oid.rs
+++ b/src/asn1_types/oid.rs
@@ -2,14 +2,13 @@ use crate::*;
 use alloc::borrow::Cow;
 #[cfg(not(feature = "std"))]
 use alloc::format;
+#[cfg(not(feature = "std"))]
 use alloc::string::{String, ToString};
+#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 use core::{
     convert::TryFrom, fmt, iter::FusedIterator, marker::PhantomData, ops::Shl, str::FromStr,
 };
-
-#[cfg(feature = "bigint")]
-use num_bigint::BigUint;
 use num_traits::Num;
 
 /// An error for OID parsing functions.
diff --git a/src/asn1_types/real.rs b/src/asn1_types/real.rs
index 91f64e2..0abcc34 100644
--- a/src/asn1_types/real.rs
+++ b/src/asn1_types/real.rs
@@ -1,7 +1,6 @@
 use crate::*;
 use alloc::format;
 use core::convert::TryFrom;
-use nom::Needed;
 
 mod f32;
 mod f64;
diff --git a/src/asn1_types/sequence.rs b/src/asn1_types/sequence.rs
index 61ab888..ebacf41 100644
--- a/src/asn1_types/sequence.rs
+++ b/src/asn1_types/sequence.rs
@@ -1,5 +1,6 @@
 use crate::*;
 use alloc::borrow::Cow;
+#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 use core::convert::TryFrom;
 
@@ -179,7 +180,7 @@ impl<'a> Sequence<'a> {
     {
         match self.content {
             Cow::Borrowed(b) => f(b),
-            _ => Err(nom::Err::Error(Error::LifetimeError.into())),
+            _ => Err(Err::Error(Error::LifetimeError.into())),
         }
     }
 
diff --git a/src/asn1_types/sequence/sequence_of.rs b/src/asn1_types/sequence/sequence_of.rs
index d8238da..d0e5afa 100644
--- a/src/asn1_types/sequence/sequence_of.rs
+++ b/src/asn1_types/sequence/sequence_of.rs
@@ -1,4 +1,5 @@
 use crate::*;
+#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 use core::convert::TryFrom;
 use core::fmt::{Debug, Display};
@@ -142,10 +143,10 @@ where
                     .map_err(Err::convert)?;
                 any.header
                     .assert_tag(Self::TAG)
-                    .map_err(|e| nom::Err::Error(e.into()))?;
+                    .map_err(|e| Err::Error(e.into()))?;
                 let items = SequenceIterator::<T, DerParser, E>::new(any.data)
                     .collect::<Result<Vec<T>, E>>()
-                    .map_err(nom::Err::Error)?;
+                    .map_err(Err::Error)?;
                 Ok((rem, SequenceOf::new(items)))
             },
             bytes,
diff --git a/src/asn1_types/sequence/vec.rs b/src/asn1_types/sequence/vec.rs
index ca4869c..a5daea1 100644
--- a/src/asn1_types/sequence/vec.rs
+++ b/src/asn1_types/sequence/vec.rs
@@ -1,4 +1,5 @@
 use crate::*;
+#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 use core::convert::TryFrom;
 use core::fmt::Debug;
@@ -121,10 +122,10 @@ where
                     .map_err(Err::convert)?;
                 any.header
                     .assert_tag(Self::TAG)
-                    .map_err(|e| nom::Err::Error(e.into()))?;
+                    .map_err(|e| Err::Error(e.into()))?;
                 let v = SequenceIterator::<T, DerParser, E>::new(any.data)
                     .collect::<Result<Vec<T>, E>>()
-                    .map_err(nom::Err::Error)?;
+                    .map_err(Err::Error)?;
                 Ok((rem, v))
             },
             bytes,
diff --git a/src/asn1_types/set.rs b/src/asn1_types/set.rs
index fb21399..06503c0 100644
--- a/src/asn1_types/set.rs
+++ b/src/asn1_types/set.rs
@@ -1,5 +1,6 @@
 use crate::*;
 use alloc::borrow::Cow;
+#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 use core::convert::TryFrom;
 
@@ -180,7 +181,7 @@ impl<'a> Set<'a> {
     {
         match self.content {
             Cow::Borrowed(b) => f(b),
-            _ => Err(nom::Err::Error(Error::LifetimeError.into())),
+            _ => Err(Err::Error(Error::LifetimeError.into())),
         }
     }
 
diff --git a/src/asn1_types/set/btreeset.rs b/src/asn1_types/set/btreeset.rs
index f00df88..f852aba 100644
--- a/src/asn1_types/set/btreeset.rs
+++ b/src/asn1_types/set/btreeset.rs
@@ -62,13 +62,13 @@ where
                     .map_err(Err::convert)?;
                 any.tag()
                     .assert_eq(Self::TAG)
-                    .map_err(|e| nom::Err::Error(e.into()))?;
+                    .map_err(|e| Err::Error(e.into()))?;
                 any.header
                     .assert_constructed()
-                    .map_err(|e| nom::Err::Error(e.into()))?;
+                    .map_err(|e| Err::Error(e.into()))?;
                 let items = SetIterator::<T, DerParser, E>::new(any.data)
                     .collect::<Result<BTreeSet<T>, E>>()
-                    .map_err(nom::Err::Error)?;
+                    .map_err(Err::Error)?;
                 Ok((rem, items))
             },
             bytes,
diff --git a/src/asn1_types/set/hashset.rs b/src/asn1_types/set/hashset.rs
index 1389ac0..752b941 100644
--- a/src/asn1_types/set/hashset.rs
+++ b/src/asn1_types/set/hashset.rs
@@ -57,13 +57,13 @@ where
                     .map_err(Err::convert)?;
                 any.tag()
                     .assert_eq(Self::TAG)
-                    .map_err(|e| nom::Err::Error(e.into()))?;
+                    .map_err(|e| Err::Error(e.into()))?;
                 any.header
                     .assert_constructed()
-                    .map_err(|e| nom::Err::Error(e.into()))?;
+                    .map_err(|e| Err::Error(e.into()))?;
                 let items = SetIterator::<T, DerParser, E>::new(any.data)
                     .collect::<Result<HashSet<T>, E>>()
-                    .map_err(nom::Err::Error)?;
+                    .map_err(Err::Error)?;
                 Ok((rem, items))
             },
             bytes,
diff --git a/src/asn1_types/set/set_of.rs b/src/asn1_types/set/set_of.rs
index a71eee9..c2846ff 100644
--- a/src/asn1_types/set/set_of.rs
+++ b/src/asn1_types/set/set_of.rs
@@ -1,4 +1,5 @@
 use crate::*;
+#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 use core::convert::TryFrom;
 use core::fmt::{Debug, Display};
@@ -142,10 +143,10 @@ where
                     .map_err(Err::convert)?;
                 any.header
                     .assert_tag(Self::TAG)
-                    .map_err(|e| nom::Err::Error(e.into()))?;
+                    .map_err(|e| Err::Error(e.into()))?;
                 let items = SetIterator::<T, DerParser, E>::new(any.data)
                     .collect::<Result<Vec<T>, E>>()
-                    .map_err(nom::Err::Error)?;
+                    .map_err(Err::Error)?;
                 Ok((rem, SetOf::new(items)))
             },
             bytes,
diff --git a/src/asn1_types/strings/bmpstring.rs b/src/asn1_types/strings/bmpstring.rs
index 5fdaa7a..ff69ba4 100644
--- a/src/asn1_types/strings/bmpstring.rs
+++ b/src/asn1_types/strings/bmpstring.rs
@@ -3,7 +3,9 @@
 
 use crate::*;
 use alloc::borrow::Cow;
+#[cfg(not(feature = "std"))]
 use alloc::string::{String, ToString};
+#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 
 /// ASN.1 `BMPSTRING` type
@@ -42,7 +44,7 @@ impl<'a> From<&'a str> for BmpString<'a> {
 impl From<String> for BmpString<'_> {
     fn from(s: String) -> Self {
         Self {
-            data: alloc::borrow::Cow::Owned(s),
+            data: Cow::Owned(s),
         }
     }
 }
diff --git a/src/asn1_types/strings/generalstring.rs b/src/asn1_types/strings/generalstring.rs
index f455d6a..d176abc 100644
--- a/src/asn1_types/strings/generalstring.rs
+++ b/src/asn1_types/strings/generalstring.rs
@@ -1,5 +1,6 @@
 use crate::{asn1_string, TestValidCharset};
 use crate::{Error, Result};
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 
 asn1_string!(GeneralString);
diff --git a/src/asn1_types/strings/graphicstring.rs b/src/asn1_types/strings/graphicstring.rs
index 3d84040..df4e56a 100644
--- a/src/asn1_types/strings/graphicstring.rs
+++ b/src/asn1_types/strings/graphicstring.rs
@@ -1,5 +1,6 @@
 use crate::{asn1_string, TestValidCharset};
 use crate::{Error, Result};
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 
 asn1_string!(GraphicString);
diff --git a/src/asn1_types/strings/ia5string.rs b/src/asn1_types/strings/ia5string.rs
index 4b37465..0fbf213 100644
--- a/src/asn1_types/strings/ia5string.rs
+++ b/src/asn1_types/strings/ia5string.rs
@@ -1,5 +1,6 @@
 use crate::{asn1_string, TestValidCharset};
 use crate::{Error, Result};
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 
 asn1_string!(Ia5String);
diff --git a/src/asn1_types/strings/numericstring.rs b/src/asn1_types/strings/numericstring.rs
index dbe4ff1..e0462a1 100644
--- a/src/asn1_types/strings/numericstring.rs
+++ b/src/asn1_types/strings/numericstring.rs
@@ -1,5 +1,6 @@
 use crate::{asn1_string, TestValidCharset};
 use crate::{Error, Result};
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 
 asn1_string!(NumericString);
diff --git a/src/asn1_types/strings/printablestring.rs b/src/asn1_types/strings/printablestring.rs
index 5cd51fa..981704a 100644
--- a/src/asn1_types/strings/printablestring.rs
+++ b/src/asn1_types/strings/printablestring.rs
@@ -1,5 +1,6 @@
 use crate::{asn1_string, TestValidCharset};
 use crate::{Error, Result};
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 
 asn1_string!(PrintableString);
diff --git a/src/asn1_types/strings/string.rs b/src/asn1_types/strings/string.rs
index 81f6ba8..f4e0e2e 100644
--- a/src/asn1_types/strings/string.rs
+++ b/src/asn1_types/strings/string.rs
@@ -1,4 +1,5 @@
 use crate::*;
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 use core::convert::TryFrom;
 
diff --git a/src/asn1_types/strings/teletexstring.rs b/src/asn1_types/strings/teletexstring.rs
index 2f58804..590b5e9 100644
--- a/src/asn1_types/strings/teletexstring.rs
+++ b/src/asn1_types/strings/teletexstring.rs
@@ -1,5 +1,6 @@
 use crate::{asn1_string, TestValidCharset};
 use crate::{Error, Result};
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 
 asn1_string!(TeletexString);
diff --git a/src/asn1_types/strings/universalstring.rs b/src/asn1_types/strings/universalstring.rs
index 9006362..ae30612 100644
--- a/src/asn1_types/strings/universalstring.rs
+++ b/src/asn1_types/strings/universalstring.rs
@@ -3,7 +3,9 @@
 
 use crate::*;
 use alloc::borrow::Cow;
+#[cfg(not(feature = "std"))]
 use alloc::string::{String, ToString};
+#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 use core::convert::TryFrom;
 use core::iter::FromIterator;
@@ -43,7 +45,7 @@ impl<'a> From<&'a str> for UniversalString<'a> {
 impl From<String> for UniversalString<'_> {
     fn from(s: String) -> Self {
         Self {
-            data: alloc::borrow::Cow::Owned(s),
+            data: Cow::Owned(s),
         }
     }
 }
diff --git a/src/asn1_types/strings/utf8string.rs b/src/asn1_types/strings/utf8string.rs
index 0bf87d4..78eae12 100644
--- a/src/asn1_types/strings/utf8string.rs
+++ b/src/asn1_types/strings/utf8string.rs
@@ -1,6 +1,7 @@
 use crate::asn1_string;
 use crate::Result;
 use crate::TestValidCharset;
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 
 asn1_string!(Utf8String);
diff --git a/src/asn1_types/strings/videotexstring.rs b/src/asn1_types/strings/videotexstring.rs
index 51c5a46..d9324d6 100644
--- a/src/asn1_types/strings/videotexstring.rs
+++ b/src/asn1_types/strings/videotexstring.rs
@@ -1,5 +1,6 @@
 use crate::{asn1_string, TestValidCharset};
 use crate::{Error, Result};
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 
 asn1_string!(VideotexString);
diff --git a/src/asn1_types/strings/visiblestring.rs b/src/asn1_types/strings/visiblestring.rs
index 2b141dc..cb72348 100644
--- a/src/asn1_types/strings/visiblestring.rs
+++ b/src/asn1_types/strings/visiblestring.rs
@@ -1,5 +1,6 @@
 use crate::{asn1_string, TestValidCharset};
 use crate::{Error, Result};
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 
 asn1_string!(VisibleString);
diff --git a/src/asn1_types/tagged/helpers.rs b/src/asn1_types/tagged/helpers.rs
index dfb1018..5025785 100644
--- a/src/asn1_types/tagged/helpers.rs
+++ b/src/asn1_types/tagged/helpers.rs
@@ -34,7 +34,7 @@ where
     parse_der_container(tag, move |any: Any<'a>| {
         any.header
             .assert_tag(tag)
-            .map_err(|e| nom::Err::convert(e.into()))?;
+            .map_err(|e| Err::convert(e.into()))?;
         f(any.data, any.header)
     })
 }
@@ -51,9 +51,7 @@ where
     let tag = tag.into();
     move |i| {
         let (rem, tagged) = TaggedParser::from_der(i)?;
-        tagged
-            .assert_tag(tag)
-            .map_err(|e| nom::Err::convert(e.into()))?;
+        tagged.assert_tag(tag).map_err(|e| Err::convert(e.into()))?;
         Ok((rem, tagged))
     }
 }
@@ -73,7 +71,7 @@ where
         // verify tag of external header
         any.header
             .assert_tag(tag)
-            .map_err(|e| nom::Err::convert(e.into()))?;
+            .map_err(|e| Err::convert(e.into()))?;
         // build a fake header with the expected tag
         let Any { header, data } = any;
         let header = Header {
@@ -93,10 +91,10 @@ where
     E: ParseError<&'a [u8]> + From<Error>,
 {
     move |i: &[u8]| {
-        let (rem, any) = Any::from_der(i).map_err(nom::Err::convert)?;
+        let (rem, any) = Any::from_der(i).map_err(Err::convert)?;
         any.header
             .assert_tag(tag)
-            .map_err(|e| nom::Err::convert(e.into()))?;
+            .map_err(|e| Err::convert(e.into()))?;
         let (_, output) = f(any)?;
         Ok((rem, output))
     }
diff --git a/src/asn1_types/tagged/implicit.rs b/src/asn1_types/tagged/implicit.rs
index 5594e49..01e6ae5 100644
--- a/src/asn1_types/tagged/implicit.rs
+++ b/src/asn1_types/tagged/implicit.rs
@@ -74,7 +74,7 @@ where
         };
         match T::try_from(any) {
             Ok(inner) => Ok((rem, TaggedValue::implicit(inner))),
-            Err(e) => Err(nom::Err::Error(e)),
+            Err(e) => Err(Err::Error(e)),
         }
     }
 }
@@ -182,7 +182,7 @@ where
                 };
                 Ok((rem, tagged_value))
             }
-            Err(e) => Err(nom::Err::Error(e)),
+            Err(e) => Err(Err::Error(e)),
         }
     }
 }
@@ -217,7 +217,7 @@ where
             },
             data,
         };
-        T::check_constraints(&any).map_err(|e| nom::Err::Error(e.into()))?;
+        T::check_constraints(&any).map_err(|e| Err::Error(e.into()))?;
         match T::try_from(any) {
             Ok(t) => {
                 let tagged_value = TaggedParser {
@@ -228,7 +228,7 @@ where
                 };
                 Ok((rem, tagged_value))
             }
-            Err(e) => Err(nom::Err::Error(e)),
+            Err(e) => Err(Err::Error(e)),
         }
     }
 }
diff --git a/src/asn1_types/tagged/optional.rs b/src/asn1_types/tagged/optional.rs
index 00504df..dcf8307 100644
--- a/src/asn1_types/tagged/optional.rs
+++ b/src/asn1_types/tagged/optional.rs
@@ -1,4 +1,3 @@
-use super::{explicit::TaggedExplicit, implicit::TaggedImplicit};
 use crate::*;
 
 /// Helper object to parse TAGGED OPTIONAL types (explicit or implicit)
diff --git a/src/asn1_types/utctime.rs b/src/asn1_types/utctime.rs
index 8604914..3a67598 100644
--- a/src/asn1_types/utctime.rs
+++ b/src/asn1_types/utctime.rs
@@ -1,4 +1,3 @@
-use crate::datetime::decode_decimal;
 use crate::*;
 use core::convert::TryFrom;
 use core::fmt;
diff --git a/src/datetime.rs b/src/datetime.rs
index f3a48b7..33125ae 100644
--- a/src/datetime.rs
+++ b/src/datetime.rs
@@ -1,5 +1,6 @@
 use crate::{Result, Tag};
 use alloc::format;
+#[cfg(not(feature = "std"))]
 use alloc::string::ToString;
 use core::fmt;
 #[cfg(feature = "datetime")]
diff --git a/src/error.rs b/src/error.rs
index b486f83..8ec409a 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -1,6 +1,7 @@
 use crate::{Class, Tag};
 use alloc::str;
 use alloc::string;
+#[cfg(not(feature = "std"))]
 use alloc::string::String;
 use displaydoc::Display;
 use nom::error::{ErrorKind, FromExternalError, ParseError};
diff --git a/src/header.rs b/src/header.rs
index c2b9aa5..7291cb7 100644
--- a/src/header.rs
+++ b/src/header.rs
@@ -248,17 +248,17 @@ impl<'a> FromBer<'a> for Header<'a> {
             (_, l1) => {
                 // if len is 0xff -> error (8.1.3.5)
                 if l1 == 0b0111_1111 {
-                    return Err(::nom::Err::Error(Error::InvalidLength));
+                    return Err(nom::Err::Error(Error::InvalidLength));
                 }
                 let (i3, llen) = take(l1)(i2)?;
                 match bytes_to_u64(llen) {
                     Ok(l) => {
                         let l =
-                            usize::try_from(l).or(Err(::nom::Err::Error(Error::InvalidLength)))?;
+                            usize::try_from(l).or(Err(nom::Err::Error(Error::InvalidLength)))?;
                         (i3, Length::Definite(l))
                     }
                     Err(_) => {
-                        return Err(::nom::Err::Error(Error::InvalidLength));
+                        return Err(nom::Err::Error(Error::InvalidLength));
                     }
                 }
             }
@@ -284,14 +284,14 @@ impl<'a> FromDer<'a> for Header<'a> {
             }
             (_, 0) => {
                 // Indefinite form is not allowed in DER (10.1)
-                return Err(::nom::Err::Error(Error::DerConstraintFailed(
+                return Err(nom::Err::Error(Error::DerConstraintFailed(
                     DerConstraint::IndefiniteLength,
                 )));
             }
             (_, l1) => {
                 // if len is 0xff -> error (8.1.3.5)
                 if l1 == 0b0111_1111 {
-                    return Err(::nom::Err::Error(Error::InvalidLength));
+                    return Err(nom::Err::Error(Error::InvalidLength));
                 }
                 // DER(9.1) if len is 0 (indefinite form), obj must be constructed
                 der_constraint_fail_if!(
@@ -305,11 +305,11 @@ impl<'a> FromDer<'a> for Header<'a> {
                         // DER: should have been encoded in short form (< 127)
                         // XXX der_constraint_fail_if!(i, l < 127);
                         let l =
-                            usize::try_from(l).or(Err(::nom::Err::Error(Error::InvalidLength)))?;
+                            usize::try_from(l).or(Err(nom::Err::Error(Error::InvalidLength)))?;
                         (i3, Length::Definite(l))
                     }
                     Err(_) => {
-                        return Err(::nom::Err::Error(Error::InvalidLength));
+                        return Err(nom::Err::Error(Error::InvalidLength));
                     }
                 }
             }
diff --git a/src/tag.rs b/src/tag.rs
index 8ce7c57..d610a2f 100644
--- a/src/tag.rs
+++ b/src/tag.rs
@@ -1,4 +1,5 @@
 use crate::{Error, Result};
+#[cfg(not(feature = "std"))]
 use alloc::string::ToString;
 use rusticata_macros::newtype_enum;