Split general error into specific

Currently we have a large general error type.

Create specific error types for each function as needed so that a
function returns ever variant available in its return type.

examples/sign_verify_recovery.rs

@@ -4,6 +4,8 @@ extern crate secp256k1;
 use hashes::{sha256, Hash};
 use secp256k1::{ecdsa, Error, Message, PublicKey, Secp256k1, SecretKey, Signing, Verification};
 
+// Notice that we provide a general error type for this crate and conversion
+// functions to it from all the other error types so `?` works as expected.
 fn recover<C: Verification>(
     secp: &Secp256k1<C>,
     msg: &[u8],
@@ -15,7 +17,7 @@ fn recover<C: Verification>(
     let id = ecdsa::RecoveryId::from_i32(recovery_id as i32)?;
     let sig = ecdsa::RecoverableSignature::from_compact(&sig, id)?;
 
-    secp.recover_ecdsa(&msg, &sig)
+    Ok(secp.recover_ecdsa(&msg, &sig)?)
 }
 
 fn sign_recovery<C: Signing>(

src/context.rs

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: CC0-1.0
 
+use core::fmt;
 use core::marker::PhantomData;
 use core::mem::ManuallyDrop;
 use core::ptr::NonNull;
@@ -8,7 +9,7 @@ use core::ptr::NonNull;
 pub use self::alloc_only::*;
 use crate::ffi::types::{c_uint, c_void, AlignedType};
 use crate::ffi::{self, CPtr};
-use crate::{Error, Secp256k1};
+use crate::Secp256k1;
 
 #[cfg(all(feature = "global-context", feature = "std"))]
 /// Module implementing a singleton pattern for a global `Secp256k1` context.
@@ -320,14 +321,33 @@ unsafe impl<'buf> PreallocatedContext<'buf> for AllPreallocated<'buf> {}
 unsafe impl<'buf> PreallocatedContext<'buf> for SignOnlyPreallocated<'buf> {}
 unsafe impl<'buf> PreallocatedContext<'buf> for VerifyOnlyPreallocated<'buf> {}
 
+/// Not enough preallocated memory for the requested buffer size.
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[non_exhaustive]
+#[allow(missing_copy_implementations)] // Don't implement Copy when we use non_exhaustive.
+pub struct NotEnoughMemoryError;
+
+impl fmt::Display for NotEnoughMemoryError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str("not enough preallocated memory for the requested buffer size")
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for NotEnoughMemoryError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+}
+
 impl<'buf, C: Context + PreallocatedContext<'buf>> Secp256k1<C> {
     /// Lets you create a context with a preallocated buffer in a generic manner (sign/verify/all).
-    pub fn preallocated_gen_new(buf: &'buf mut [AlignedType]) -> Result<Secp256k1<C>, Error> {
+    pub fn preallocated_gen_new(
+        buf: &'buf mut [AlignedType],
+    ) -> Result<Secp256k1<C>, NotEnoughMemoryError> {
         #[cfg(target_arch = "wasm32")]
         ffi::types::sanity_checks_for_wasm();
 
         if buf.len() < Self::preallocate_size_gen() {
-            return Err(Error::NotEnoughMemory);
+            return Err(NotEnoughMemoryError);
         }
         // Safe because buf is not null since it is not empty.
         let buf = unsafe { NonNull::new_unchecked(buf.as_mut_c_ptr() as *mut c_void) };
@@ -343,7 +363,7 @@ impl<'buf> Secp256k1<AllPreallocated<'buf>> {
     /// Creates a new Secp256k1 context with all capabilities.
     pub fn preallocated_new(
         buf: &'buf mut [AlignedType],
-    ) -> Result<Secp256k1<AllPreallocated<'buf>>, Error> {
+    ) -> Result<Secp256k1<AllPreallocated<'buf>>, NotEnoughMemoryError> {
         Secp256k1::preallocated_gen_new(buf)
     }
     /// Uses the ffi `secp256k1_context_preallocated_size` to check the memory size needed for a context.
@@ -378,7 +398,7 @@ impl<'buf> Secp256k1<SignOnlyPreallocated<'buf>> {
     /// Creates a new Secp256k1 context that can only be used for signing.
     pub fn preallocated_signing_only(
         buf: &'buf mut [AlignedType],
-    ) -> Result<Secp256k1<SignOnlyPreallocated<'buf>>, Error> {
+    ) -> Result<Secp256k1<SignOnlyPreallocated<'buf>>, NotEnoughMemoryError> {
         Secp256k1::preallocated_gen_new(buf)
     }
 
@@ -402,7 +422,7 @@ impl<'buf> Secp256k1<VerifyOnlyPreallocated<'buf>> {
     /// Creates a new Secp256k1 context that can only be used for verification
     pub fn preallocated_verification_only(
         buf: &'buf mut [AlignedType],
-    ) -> Result<Secp256k1<VerifyOnlyPreallocated<'buf>>, Error> {
+    ) -> Result<Secp256k1<VerifyOnlyPreallocated<'buf>>, NotEnoughMemoryError> {
         Secp256k1::preallocated_gen_new(buf)
     }
 

src/ecdh.rs

@@ -4,13 +4,13 @@
 //!
 
 use core::borrow::Borrow;
-use core::{ptr, str};
+use core::{fmt, ptr, str};
 
 use secp256k1_sys::types::{c_int, c_uchar, c_void};
 
 use crate::ffi::{self, CPtr};
 use crate::key::{PublicKey, SecretKey};
-use crate::{constants, hex, Error};
+use crate::{constants, hex};
 
 // The logic for displaying shared secrets relies on this (see `secret.rs`).
 const SHARED_SECRET_SIZE: usize = constants::SECRET_KEY_SIZE;
@@ -65,25 +65,25 @@ impl SharedSecret {
 
     /// Creates a shared secret from `bytes` slice.
     #[inline]
-    pub fn from_slice(bytes: &[u8]) -> Result<SharedSecret, Error> {
+    pub fn from_slice(bytes: &[u8]) -> Result<SharedSecret, SharedSecretError> {
         match bytes.len() {
             SHARED_SECRET_SIZE => {
                 let mut ret = [0u8; SHARED_SECRET_SIZE];
                 ret[..].copy_from_slice(bytes);
                 Ok(SharedSecret(ret))
             }
-            _ => Err(Error::InvalidSharedSecret),
+            _ => Err(SharedSecretError),
         }
     }
 }
 
 impl str::FromStr for SharedSecret {
-    type Err = Error;
+    type Err = SharedSecretError;
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         let mut res = [0u8; SHARED_SECRET_SIZE];
         match hex::from_hex(s, &mut res) {
             Ok(SHARED_SECRET_SIZE) => Ok(SharedSecret::from_bytes(res)),
-            _ => Err(Error::InvalidSharedSecret),
+            _ => Err(SharedSecretError),
         }
     }
 }
@@ -183,6 +183,21 @@ impl<'de> ::serde::Deserialize<'de> for SharedSecret {
     }
 }
 
+/// Share secret is invalid.
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[non_exhaustive]
+#[allow(missing_copy_implementations)] // Don't implement Copy when we use non_exhaustive.
+pub struct SharedSecretError;
+
+impl fmt::Display for SharedSecretError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.write_str("shared secret is invalid") }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for SharedSecretError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+}
+
 #[cfg(test)]
 #[allow(unused_imports)]
 mod tests {