diff --git a/src/kernel/src/errno.rs b/src/kernel/src/errno.rs
index 87d404f65..151174301 100644
--- a/src/kernel/src/errno.rs
+++ b/src/kernel/src/errno.rs
@@ -1,6 +1,7 @@
 use std::convert::Infallible;
 use std::error::Error;
 use std::num::NonZeroI32;
+
 // This file contains errno used in a PS4 system. The value of each errno must be the same as the
 // PS4.
 
diff --git a/src/kernel/src/fs/dev/mod.rs b/src/kernel/src/fs/dev/mod.rs
index 9cf72ab1d..d1645b527 100644
--- a/src/kernel/src/fs/dev/mod.rs
+++ b/src/kernel/src/fs/dev/mod.rs
@@ -9,7 +9,6 @@ use crate::errno::{Errno, EEXIST, ENOENT, EOPNOTSUPP};
 use crate::ucred::{Gid, Ucred, Uid};
 use bitflags::bitflags;
 use macros::Errno;
-use std::num::NonZeroI32;
 use std::sync::atomic::{AtomicU32, AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex, RwLock};
 use thiserror::Error;
@@ -310,20 +309,13 @@ impl Devices {
 }
 
 /// Represents an error when [`make_dev()`] is failed.
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum MakeDevError {
     #[error("the device with the same name already exist")]
+    #[errno(EEXIST)]
     AlreadyExist(String),
 }
 
-impl Errno for MakeDevError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::AlreadyExist(_) => EEXIST,
-        }
-    }
-}
-
 pub fn mount(
     conf: &'static FsConfig,
     cred: &Arc<Ucred>,
diff --git a/src/kernel/src/fs/dev/vnode.rs b/src/kernel/src/fs/dev/vnode.rs
index 926c3b3f9..bde4f1e81 100644
--- a/src/kernel/src/fs/dev/vnode.rs
+++ b/src/kernel/src/fs/dev/vnode.rs
@@ -7,7 +7,6 @@ use crate::fs::{
 };
 use crate::process::VThread;
 use macros::Errno;
-use std::num::NonZeroI32;
 use std::sync::Arc;
 use thiserror::Error;
 
@@ -209,36 +208,27 @@ impl crate::fs::VnodeBackend for VnodeBackend {
 }
 
 /// Represents an error when [`lookup()`] is failed.
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 enum LookupError {
     #[error("file is not a directory")]
+    #[errno(ENOTDIR)]
     NotDirectory,
 
     #[error("cannot resolve '..' on the root directory")]
+    #[errno(EIO)]
     DotdotOnRoot,
 
     #[error("access denied")]
     AccessDenied(#[source] Box<dyn Errno>),
 
     #[error("file have no parent")]
+    #[errno(ENOENT)]
     NoParent,
 
     #[error("cannot allocate a vnode")]
     AllocVnodeFailed(#[source] AllocVnodeError),
 }
 
-impl Errno for LookupError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::NotDirectory => ENOTDIR,
-            Self::DotdotOnRoot => EIO,
-            Self::AccessDenied(e) => e.errno(),
-            Self::NoParent => ENOENT,
-            Self::AllocVnodeFailed(e) => e.errno(),
-        }
-    }
-}
-
 /// Represents an error when [`open()`] is failed.
 #[derive(Debug, Error, Errno)]
 enum OpenError {
diff --git a/src/kernel/src/fs/host/vnode.rs b/src/kernel/src/fs/host/vnode.rs
index 6f8b5dadc..8396927a3 100644
--- a/src/kernel/src/fs/host/vnode.rs
+++ b/src/kernel/src/fs/host/vnode.rs
@@ -6,7 +6,6 @@ use crate::process::VThread;
 use crate::ucred::{Gid, Uid};
 use macros::Errno;
 use std::borrow::Cow;
-use std::num::NonZeroI32;
 use std::sync::Arc;
 use thiserror::Error;
 
@@ -128,34 +127,28 @@ enum GetAttrError {
 }
 
 /// Represents an error when [`lookup()`] fails.
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 enum LookupError {
     #[error("current file is not a directory")]
+    #[errno(ENOTDIR)]
     NotDirectory,
 
     #[error("cannot resolve '..' on the root directory")]
+    #[errno(EIO)]
     DotdotOnRoot,
 
     #[error("access denied")]
     AccessDenied(#[source] Box<dyn Errno>),
 
     #[error("name contains unsupported characters")]
+    #[errno(ENOENT)]
     InvalidName,
 
     #[error("couldn't open the specified file")]
+    #[errno(EIO)]
     OpenFailed(#[source] std::io::Error),
 
     #[error("cannot get vnode")]
+    #[errno(EIO)]
     GetVnodeFailed(#[source] GetVnodeError),
 }
-
-impl Errno for LookupError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::NotDirectory => ENOTDIR,
-            Self::DotdotOnRoot | Self::GetVnodeFailed(_) | Self::OpenFailed(_) => EIO,
-            Self::AccessDenied(e) => e.errno(),
-            Self::InvalidName => ENOENT,
-        }
-    }
-}
diff --git a/src/kernel/src/fs/mod.rs b/src/kernel/src/fs/mod.rs
index cf7f18820..bd9803544 100644
--- a/src/kernel/src/fs/mod.rs
+++ b/src/kernel/src/fs/mod.rs
@@ -10,7 +10,7 @@ use macros::vpath;
 use macros::Errno;
 use param::Param;
 use std::fmt::{Display, Formatter};
-use std::num::{NonZeroI32, TryFromIntError};
+use std::num::TryFromIntError;
 use std::path::PathBuf;
 use std::sync::Arc;
 use thiserror::Error;
@@ -1093,15 +1093,18 @@ pub enum FsInitError {
 }
 
 /// Represents an error when FS mounting fails.
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum MountError {
     #[error("fstype is too long")]
+    #[errno(ENAMETOOLONG)]
     FsTooLong,
 
     #[error("fspath is too long")]
+    #[errno(ENAMETOOLONG)]
     PathTooLong,
 
     #[error("fstype is not valid")]
+    #[errno(ENODEV)]
     InvalidFs,
 
     #[error("fspath is not found")]
@@ -1111,95 +1114,51 @@ pub enum MountError {
     MountFailed(#[source] Box<dyn Errno>),
 
     #[error("fspath is already mounted")]
+    #[errno(EBUSY)]
     PathAlreadyMounted,
 
     #[error("cannot get root")]
     GetRootFailed(#[source] Box<dyn Errno>),
 }
 
-impl Errno for MountError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::FsTooLong | Self::PathTooLong => ENAMETOOLONG,
-            Self::InvalidFs => ENODEV,
-            Self::LookupPathFailed(e) => e.errno(),
-            Self::MountFailed(e) => e.errno(),
-            Self::PathAlreadyMounted => EBUSY,
-            Self::GetRootFailed(e) => e.errno(),
-        }
-    }
-}
-
 /// Represents an error when [`Fs::open()`] fails.
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum OpenError {
     #[error("cannot lookup the file")]
     LookupFailed(#[source] LookupError),
 }
 
-impl Errno for OpenError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::LookupFailed(e) => e.errno(),
-        }
-    }
-}
-
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum WriteError {}
 
-impl Errno for WriteError {
-    fn errno(&self) -> NonZeroI32 {
-        todo!()
-    }
-}
-
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum IoctlError {
     #[error("Couldn't get file")]
     FailedToGetFile(#[from] GetFileError),
 
     #[error("Bad file flags {0:?}")]
+    #[errno(EBADF)]
     BadFileFlags(VFileFlags),
 
     #[error(transparent)]
     FileIoctlFailed(#[from] Box<dyn Errno>),
 }
 
-impl Errno for IoctlError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::FailedToGetFile(e) => e.errno(),
-            Self::BadFileFlags(_) => EBADF,
-            Self::FileIoctlFailed(e) => e.errno(),
-        }
-    }
-}
-
 /// Represents an error when [`Fs::lookup()`] fails.
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum LookupError {
     #[error("failed to get mount root")]
     GetRootFailed(#[source] Box<dyn Errno>),
 
     #[error("no such file or directory")]
+    #[errno(ENOENT)]
     NotFound,
 
     #[error("cannot lookup '{1}' from component #{0}")]
     LookupFailed(usize, Box<str>, #[source] Box<dyn Errno>),
 }
 
-impl Errno for LookupError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::GetRootFailed(e) => e.errno(),
-            Self::NotFound => ENOENT,
-            Self::LookupFailed(_, _, e) => e.errno(),
-        }
-    }
-}
-
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum RevokeError {
     #[error("failed to get file attr")]
     GetAttrError(#[source] Box<dyn Errno>),
@@ -1211,17 +1170,8 @@ pub enum RevokeError {
     RevokeFailed(#[source] Box<dyn Errno>),
 }
 
-impl Errno for RevokeError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::GetAttrError(e) => e.errno(),
-            Self::PrivelegeError(e) => e.errno(),
-            Self::RevokeFailed(e) => e.errno(),
-        }
-    }
-}
 /// Represents an error when one of the stat syscalls fails
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum StatError {
     #[error("failed to get file")]
     FailedToGetFile(#[from] GetFileError),
@@ -1230,65 +1180,39 @@ pub enum StatError {
     GetAttrError(#[from] Box<dyn Errno>),
 }
 
-impl Errno for StatError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::FailedToGetFile(e) => e.errno(),
-            Self::GetAttrError(e) => e.errno(),
-        }
-    }
-}
-
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum TruncateError {
     #[error("the provided length is invalid")]
+    #[errno(EINVAL)]
     InvalidLength,
 
     #[error("failed to get file")]
     FailedToLookupFile(#[from] LookupError),
 }
 
-impl Errno for TruncateError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::InvalidLength => EINVAL,
-            Self::FailedToLookupFile(e) => e.errno(),
-        }
-    }
-}
-
 impl From<TruncateLengthError> for TruncateError {
     fn from(_: TruncateLengthError) -> Self {
         Self::InvalidLength
     }
 }
 
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum FileTruncateError {
     #[error("the provided length is invalid")]
+    #[errno(EINVAL)]
     InvalidLength,
 
     #[error("failed to get file")]
     FailedToGetFile(#[from] GetFileError),
 
     #[error("file is not writable")]
+    #[errno(EINVAL)]
     FileNotWritable,
 
     #[error(transparent)]
     TruncateError(#[from] Box<dyn Errno>),
 }
 
-impl Errno for FileTruncateError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::InvalidLength => EINVAL,
-            Self::FailedToGetFile(e) => e.errno(),
-            Self::FileNotWritable => EINVAL,
-            Self::TruncateError(e) => e.errno(),
-        }
-    }
-}
-
 impl From<TruncateLengthError> for FileTruncateError {
     fn from(_: TruncateLengthError) -> Self {
         Self::InvalidLength
diff --git a/src/kernel/src/fs/null/vnode.rs b/src/kernel/src/fs/null/vnode.rs
index 3cc72b117..4e729c7a5 100644
--- a/src/kernel/src/fs/null/vnode.rs
+++ b/src/kernel/src/fs/null/vnode.rs
@@ -3,7 +3,8 @@ use crate::{
     fs::{perm::Access, Mount, MountFlags, OpenFlags, VFile, Vnode, VnodeAttrs, VnodeType},
     process::VThread,
 };
-use std::{num::NonZeroI32, sync::Arc};
+use macros::Errno;
+use std::sync::Arc;
 use thiserror::Error;
 
 #[derive(Debug)]
@@ -87,83 +88,45 @@ impl crate::fs::VnodeBackend for VnodeBackend {
     }
 }
 
-#[derive(Debug, Error)]
+/// See `null_nodeget` on the PS4 for a reference.
+pub(super) fn null_nodeget(
+    mnt: &Arc<Mount>,
+    lower: Arc<Vnode>,
+) -> Result<Arc<Vnode>, NodeGetError> {
+    todo!()
+}
+
+#[derive(Debug, Error, Errno)]
 pub enum AccessError {
     #[error("mounted as readonly")]
+    #[errno(EROFS)]
     Readonly,
 
     #[error("vnode is directory")]
+    #[errno(EISDIR)]
     IsDirectory,
 
     #[error("access from lower vnode failed")]
     AccessFromLowerFailed(#[from] Box<dyn Errno>),
 }
 
-impl Errno for AccessError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::Readonly => EROFS,
-            Self::IsDirectory => EISDIR,
-            Self::AccessFromLowerFailed(e) => e.errno(),
-        }
-    }
-}
-
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum GetAttrError {
     #[error("getattr from lower vnode failed")]
     GetAttrFromLowerFailed(#[from] Box<dyn Errno>),
 }
 
-impl Errno for GetAttrError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::GetAttrFromLowerFailed(e) => e.errno(),
-        }
-    }
-}
-
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum LookupError {
     #[error("lookup from lower vnode failed")]
     LookupFromLowerFailed(#[source] Box<dyn Errno>),
 }
 
-impl Errno for LookupError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::LookupFromLowerFailed(e) => e.errno(),
-        }
-    }
-}
-
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum OpenError {
     #[error("open from lower vnode failed")]
     OpenFromLowerFailed(#[source] Box<dyn Errno>),
 }
 
-impl Errno for OpenError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::OpenFromLowerFailed(e) => e.errno(),
-        }
-    }
-}
-
-/// See `null_nodeget` on the PS4 for a reference.
-pub(super) fn null_nodeget(
-    mnt: &Arc<Mount>,
-    lower: Arc<Vnode>,
-) -> Result<Arc<Vnode>, NodeGetError> {
-    todo!()
-}
-
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub(super) enum NodeGetError {}
-
-impl Errno for NodeGetError {
-    fn errno(&self) -> NonZeroI32 {
-        todo!()
-    }
-}
diff --git a/src/kernel/src/fs/tmp/mod.rs b/src/kernel/src/fs/tmp/mod.rs
index c40d32aa4..b6d7e9462 100644
--- a/src/kernel/src/fs/tmp/mod.rs
+++ b/src/kernel/src/fs/tmp/mod.rs
@@ -2,7 +2,7 @@ use self::node::{AllocNodeError, Node, Nodes};
 use super::{Filesystem, FsConfig, Mount, MountFlags, MountOpts, MountSource, VPathBuf, Vnode};
 use crate::errno::{Errno, EINVAL};
 use crate::ucred::{Ucred, Uid};
-use std::num::NonZeroI32;
+use macros::Errno;
 use std::sync::atomic::AtomicI32;
 use std::sync::Arc;
 use thiserror::Error;
@@ -119,9 +119,10 @@ fn alloc_vnode(mnt: &Arc<Mount>, node: &Arc<Node>) -> Result<Arc<Vnode>, AllocVn
 }
 
 /// Represents an error when [`mount()`] fails.
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 enum MountError {
     #[error("update is not supported")]
+    #[errno(EINVAL)]
     UpdateNotSupported,
 
     #[error("cannot get mount point attributes")]
@@ -131,21 +132,5 @@ enum MountError {
     AllocRootFailed(#[from] AllocNodeError),
 }
 
-impl Errno for MountError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::UpdateNotSupported => EINVAL,
-            Self::GetParentAttrsFailed(e) => e.errno(),
-            Self::AllocRootFailed(e) => e.errno(),
-        }
-    }
-}
-
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 enum AllocVnodeError {}
-
-impl Errno for AllocVnodeError {
-    fn errno(&self) -> NonZeroI32 {
-        todo!()
-    }
-}
diff --git a/src/kernel/src/net/socket.rs b/src/kernel/src/net/socket.rs
index a9558b942..0cc9969ce 100644
--- a/src/kernel/src/net/socket.rs
+++ b/src/kernel/src/net/socket.rs
@@ -5,7 +5,8 @@ use crate::{
     process::VThread,
 };
 use bitflags::bitflags;
-use std::{num::NonZeroI32, sync::Arc};
+use macros::Errno;
+use std::sync::Arc;
 use thiserror::Error;
 
 #[derive(Debug)]
@@ -54,15 +55,6 @@ bitflags! {
     }
 }
 
-#[derive(Debug, Error)]
-pub enum SocketCreateError {}
-
-impl Errno for SocketCreateError {
-    fn errno(&self) -> NonZeroI32 {
-        match *self {}
-    }
-}
-
 impl FileBackend for Socket {
     /// See soo_read on the PS4 for a reference.
     fn read(
@@ -118,25 +110,15 @@ impl FileBackend for Socket {
     }
 }
 
-#[derive(Debug, Error)]
-enum ReceiveError {}
+#[derive(Debug, Error, Errno)]
+pub enum SocketCreateError {}
 
-impl Errno for ReceiveError {
-    fn errno(&self) -> NonZeroI32 {
-        todo!()
-    }
-}
+#[derive(Debug, Error, Errno)]
+enum ReceiveError {}
 
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 enum SendError {
     #[error("Broken pipe")]
+    #[errno(EPIPE)]
     BrokenPipe,
 }
-
-impl Errno for SendError {
-    fn errno(&self) -> NonZeroI32 {
-        match self {
-            Self::BrokenPipe => EPIPE,
-        }
-    }
-}
diff --git a/src/kernel/src/process/thread.rs b/src/kernel/src/process/thread.rs
index 3e3b98050..cbb004a85 100644
--- a/src/kernel/src/process/thread.rs
+++ b/src/kernel/src/process/thread.rs
@@ -6,6 +6,7 @@ use crate::ucred::{Privilege, PrivilegeError, Ucred};
 use bitflags::bitflags;
 use gmtx::{Gutex, GutexGroup, GutexReadGuard, GutexWriteGuard};
 use llt::{OsThread, SpawnError};
+use macros::Errno;
 use std::num::NonZeroI32;
 use std::sync::atomic::Ordering;
 use std::sync::Arc;
@@ -185,11 +186,5 @@ impl Drop for Running {
 
 static VTHREAD: Tls<Arc<VThread>> = Tls::new();
 
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum FileAllocError {}
-
-impl Errno for FileAllocError {
-    fn errno(&self) -> NonZeroI32 {
-        match *self {}
-    }
-}
diff --git a/src/kernel/src/shm/mod.rs b/src/kernel/src/shm/mod.rs
index e23b0825d..4a26e1624 100644
--- a/src/kernel/src/shm/mod.rs
+++ b/src/kernel/src/shm/mod.rs
@@ -1,3 +1,4 @@
+use macros::Errno;
 use thiserror::Error;
 
 use crate::{
@@ -11,7 +12,7 @@ use crate::{
     syscalls::{SysErr, SysIn, SysOut, Syscalls},
     ucred::{Gid, Ucred, Uid},
 };
-use std::{convert::Infallible, num::NonZeroI32, sync::Arc};
+use std::{convert::Infallible, sync::Arc};
 
 pub struct SharedMemoryManager {
     mm: Arc<MemoryManager>,
@@ -159,11 +160,5 @@ impl FileBackend for Shm {
     }
 }
 
-#[derive(Debug, Error)]
+#[derive(Debug, Error, Errno)]
 pub enum TruncateError {}
-
-impl Errno for TruncateError {
-    fn errno(&self) -> NonZeroI32 {
-        match *self {}
-    }
-}
diff --git a/src/macros/src/errno.rs b/src/macros/src/errno.rs
index fb7c79ab1..0da1e31dc 100644
--- a/src/macros/src/errno.rs
+++ b/src/macros/src/errno.rs
@@ -1,6 +1,6 @@
 use proc_macro2::{Ident, TokenStream};
 use quote::quote;
-use syn::{punctuated::Punctuated, Fields, ItemEnum, Meta, Token, Variant};
+use syn::{punctuated::Punctuated, Fields, Index, ItemEnum, Meta, Token, Variant};
 
 pub fn transform(arg: ItemEnum) -> syn::Result<TokenStream> {
     let enum_name = &arg.ident;
@@ -11,17 +11,25 @@ pub fn transform(arg: ItemEnum) -> syn::Result<TokenStream> {
         .map(|variant| process_variant(variant, enum_name))
         .collect::<Result<Vec<_>, _>>()?;
 
-    let res = quote!(
-        impl Errno for #enum_name {
-            fn errno(&self) -> std::num::NonZeroI32 {
-                match self {
-                    #(#arms)*
+    if arms.is_empty() {
+        Ok(quote!(
+            impl Errno for #enum_name {
+                fn errno(&self) -> std::num::NonZeroI32 {
+                    match *self {}
                 }
             }
-        }
-    );
-
-    Ok(res)
+        ))
+    } else {
+        Ok(quote!(
+            impl Errno for #enum_name {
+                fn errno(&self) -> std::num::NonZeroI32 {
+                    match self {
+                        #(#arms)*
+                    }
+                }
+            }
+        ))
+    }
 }
 
 fn process_variant(variant: &Variant, enum_name: &Ident) -> syn::Result<TokenStream> {
@@ -61,8 +69,59 @@ fn process_variant(variant: &Variant, enum_name: &Ident) -> syn::Result<TokenStr
         }
     }
 
-    Err(syn::Error::new_spanned(
-        variant,
-        "variant does not have an errno attribute",
-    ))
+    // If we are here, that means we haven't found any errno attributes
+    // Now we try to check the data to find a field marked with
+    // either #[source] or #[from] (which belong to the thiserror crate)
+
+    match &variant.fields {
+        Fields::Named(_) => todo!("Named fields are not supported yet"),
+        Fields::Unnamed(fields) => {
+            let ref fields = fields.unnamed;
+
+            let mut pos = None;
+
+            fields
+                .iter()
+                .enumerate()
+                .try_for_each(|(i, field)| {
+                    for attr in field.attrs.iter() {
+                        if attr.path().is_ident("source") || attr.path().is_ident("from") {
+                            if let Some(_) = pos.replace(i) {
+                                return Err(syn::Error::new_spanned(
+                                    attr,
+                                    format!(
+                                        "multiple fields marked with either #[source] or #[from] found. \
+                                        Only one field is allowed"
+                                    ),
+                                ))
+                            }
+                        }
+
+                    }
+
+                    Ok(())
+                })?;
+
+            return match pos {
+                Some(pos) => {
+                    let variant_name = &variant.ident;
+                    // The field at index `pos` is the one we are interested in
+
+                    // We have to use this. otherwise the macro would expand to something like
+                    // `{ 0usize: e, .. }` which is accepted, but only temporarily
+                    let index = Index::from(pos);
+
+                    Ok(quote!(#enum_name::#variant_name { #index: e, .. } => e.errno(),))
+                }
+                None => Err(syn::Error::new_spanned(
+                    variant,
+                    "no fields of this variant are marked with either #[source] or #[from]",
+                )),
+            };
+        }
+        Fields::Unit => Err(syn::Error::new_spanned(
+            variant,
+            "no errno attribute found on a variant with no fields",
+        )),
+    }
 }