error: replace num_enum with minimal macro rule.

The `num_enum` dependency brings with it a large number of transitive
dependencies. Some of those transitive deps now have an aggressive MSRV
of 1.64+.

The existing usage of `num_enum` is very minimal: just one derived trait
for the `rustls_result` enum to provide it with a `From` impl for u32
primitive values.

This commit replaces the `num_enum` crate with a small
`u32_enum_builder!` macro rule loosely based on the Rustls
`enum_builder` macro. Since our use case is narrower, I've simplified
the macro and tailored it to the rustls-ffi use-case.

With the new implementation we can also drop the use of `try_from`. In
each case where we're converting from code -> result we're happy for the
default `from` impl's `InvalidParameter` variant to be used when given
an unknown code, making the use of `try_from` unnecessary.

This approach adds one complication related to `cbindgen`: it now has to
be instructed to expand the `rustls-ffi` crate before generating
bindings in order to find the `rustls_result` enum. Doing this requires
tweaking the `cbindgen.toml` to add a `parse.expand` configuration
setting. Notably this also means `cbindgen` now has to be run with a
nightly `rustc`. Since this is a developer only workflow it shouldn't be
too onerous a requirement.

We're now happily building with Rust 1.60 again and can also breathe
easy knowing we have a slimmer dependency profile!

tidy: replace rustls_result::try_from -> from.

Author: cpu

.github/workflows/test.yaml

@@ -77,6 +77,12 @@ jobs:
       - uses: actions/checkout@v2
         with:
           persist-credentials: false
+      - name: Install nightly rust toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          override: true
+          default: true
       - name: Configure CMake
         run: cmake -S . -B build
       - name: Build, debug configuration
@@ -93,6 +99,12 @@ jobs:
       - uses: actions/checkout@v2
         with:
           persist-credentials: false
+      - name: Install nightly rust toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          override: true
+          default: true
       - name: Configure CMake
         run: cmake -S . -B build
       - name: Build, release configuration
@@ -106,6 +118,12 @@ jobs:
       - uses: actions/checkout@v2
         with:
           persist-credentials: false
+      - name: Install nightly rust toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          toolchain: nightly
+          override: true
+          default: true
       - run: touch src/lib.rs
       - run: cbindgen --version
       - run: make src/rustls.h

Cargo.toml

@@ -29,7 +29,6 @@ libc = "0.2"
 sct = "0.7"
 rustls-pemfile = "0.2.1"
 log = "0.4.17"
-num_enum = "0.5.10"
 
 [lib]
 name = "rustls_ffi"

cbindgen.toml

@@ -11,3 +11,7 @@ include = ["rustls_tls_version"]
 
 [defines]
 "feature = read_buf" = "DEFINE_READ_BUF"
+
+[parse.expand]
+crates = ["rustls-ffi"]
+features = ["read_buf"]

src/client.rs

@@ -1,5 +1,5 @@
 use std::borrow::Cow;
-use std::convert::{TryFrom, TryInto};
+use std::convert::TryInto;
 use std::ffi::{CStr, OsStr};
 use std::fs::File;
 use std::io::BufReader;
@@ -263,9 +263,7 @@ impl rustls::client::ServerCertVerifier for Verifier {
             rustls::Error::General("internal error with thread-local storage".to_string())
         })?;
         let result: u32 = unsafe { cb(userdata, &params) };
-        let result: rustls_result =
-            rustls_result::try_from(result).unwrap_or(rustls_result::General);
-        match result {
+        match rustls_result::from(result) {
             rustls_result::Ok => Ok(ServerCertVerified::assertion()),
             r => Err(error::cert_result_to_error(r)),
         }