Support Error in no_std environments (#268)

Resolves #261 

## Synopsis

The `Error` derive can be made to work well for the most part in
`no_std` environments by enabling `#![feature(error_in_core)]`. This
changes the `Error` derive slightly to import `Error` and related
traits from core, when the `std` feature is disabled.

In passing this also fixes actually running the nightly error tests.
They were not actually run anymore because there was no `build.rs` file
in the root of the repo, only in the `impl` package. So the `nightly`
config was not available in tests.

Co-authored-by: tyranron <[email protected]>

.github/workflows/ci.yml

@@ -116,6 +116,10 @@ jobs:
 
   test-features:
     name: test features
+    strategy:
+      fail-fast: false
+      matrix:
+        std: ["std", "no_std"]
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
@@ -128,7 +132,7 @@ jobs:
           go install github.com/pelletier/go-toml/cmd/tomljson@latest
           echo "$HOME/go/bin" >> $GITHUB_PATH
 
-      - run: ci/test_all_features.sh
+      - run: ci/test_all_features.sh ${{ matrix.std }}
 
 
 

CHANGELOG.md

@@ -54,6 +54,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
   should prevent code style linters from attempting to modify the generated
   code.
 - Upgrade to `syn` 2.0.
+- The `Error` derive now works in nightly `no_std` environments when enabling
+  `#![feature(error_in_core)]`.
 
 ### Fixed
 

Cargo.toml

@@ -28,6 +28,9 @@ members = ["impl"]
 [dependencies]
 derive_more-impl = { version = "=0.99.17", path = "impl" }
 
+[build-dependencies]
+rustc_version = { version = "0.4", optional = true }
+
 [dev-dependencies]
 rustversion = "1.0"
 trybuild = "1.0.56"
@@ -50,7 +53,7 @@ debug = ["derive_more-impl/debug"]
 deref = ["derive_more-impl/deref"]
 deref_mut = ["derive_more-impl/deref_mut"]
 display = ["derive_more-impl/display"]
-error = ["derive_more-impl/error", "std"]
+error = ["derive_more-impl/error"]
 from = ["derive_more-impl/from"]
 from_str = ["derive_more-impl/from_str"]
 index = ["derive_more-impl/index"]
@@ -96,7 +99,7 @@ full = [
     "try_unwrap",
 ]
 
-testing-helpers = ["derive_more-impl/testing-helpers"]
+testing-helpers = ["derive_more-impl/testing-helpers", "dep:rustc_version"]
 
 [[test]]
 name = "add_assign"
@@ -151,7 +154,7 @@ required-features = ["display"]
 [[test]]
 name = "error"
 path = "tests/error_tests.rs"
-required-features = ["error", "std"]
+required-features = ["error"]
 
 [[test]]
 name = "from"

build.rs

@@ -0,0 +1,15 @@
+#[cfg(not(feature = "testing-helpers"))]
+fn detect_nightly() {}
+
+#[cfg(feature = "testing-helpers")]
+fn detect_nightly() {
+    use rustc_version::{version_meta, Channel};
+
+    if version_meta().unwrap().channel == Channel::Nightly {
+        println!("cargo:rustc-cfg=nightly");
+    }
+}
+
+fn main() {
+    detect_nightly();
+}

ci/test_all_features.sh

@@ -1,6 +1,12 @@
 #!/usr/bin/env bash
+
+std=''
+if [ "${1:-}" = 'std' ]; then
+    std=',std'
+fi
+
 set -euxo pipefail
 
-for feature in $(tomljson Cargo.toml | jq --raw-output '.features | keys[]' | grep -v 'default\|std\|testing-helpers'); do
-    cargo test -p derive_more --tests --no-default-features --features "$feature,testing-helpers";
+for feature in $(tomljson Cargo.toml | jq --raw-output '.features | keys[]' | grep -v 'default\|std\|full\|testing-helpers'); do
+    cargo +nightly test -p derive_more --tests --no-default-features --features "$feature$std,testing-helpers"
 done

impl/Cargo.toml

@@ -33,7 +33,7 @@ unicode-xid = { version = "0.2.2", optional = true }
 rustc_version = { version = "0.4", optional = true }
 
 [dev-dependencies]
-derive_more = { path = "..", features = ["add", "debug", "error", "from_str", "not", "try_into", "try_unwrap"] }
+derive_more = { path = "..", features = ["add", "debug", "error", "from_str", "not", "std", "try_into", "try_unwrap"] }
 itertools = "0.11.0"
 
 [badges]

impl/doc/error.md

@@ -42,13 +42,25 @@ ignored for one of these methods by using `#[error(not(backtrace))]` or
 `#[error(not(source))]`.
 
 
+### What works in `no_std`?
+
+If you want to use the `Error` derive on `no_std` environments, then you need to
+compile with nightly and enable this feature:
+```ignore
+#![feature(error_in_core)]
+```
+
+Backtraces don't work though, because the `Backtrace` type is only available in
+`std`.
+
+
 
 
 ## Example usage
 
 ```rust
 # #![cfg_attr(nightly, feature(error_generic_member_access, provide_any))]
-// Nightly requires enabling this features:
+// Nightly requires enabling these features:
 // #![feature(error_generic_member_access, provide_any)]
 # #[cfg(not(nightly))] fn main() {}
 # #[cfg(nightly)] fn main() {

impl/src/error.rs

@@ -18,7 +18,7 @@ pub fn expand(
     let state = State::with_attr_params(
         input,
         trait_name,
-        quote! { ::std::error },
+        quote! { ::derive_more::__private::Error },
         trait_name.to_lowercase(),
         allowed_attr_params(),
     )?;
@@ -39,7 +39,7 @@ pub fn expand(
 
     let source = source.map(|source| {
         quote! {
-            fn source(&self) -> Option<&(dyn ::std::error::Error + 'static)> {
+            fn source(&self) -> Option<&(dyn ::derive_more::__private::Error + 'static)> {
                 use ::derive_more::__private::AsDynError;
                 #source
             }
@@ -48,7 +48,7 @@ pub fn expand(
 
     let provide = provide.map(|provide| {
         quote! {
-            fn provide<'_demand>(&'_demand self, demand: &mut ::std::any::Demand<'_demand>) {
+            fn provide<'_demand>(&'_demand self, demand: &mut ::core::any::Demand<'_demand>) {
                 #provide
             }
         }
@@ -62,7 +62,7 @@ pub fn expand(
             &generics,
             quote! {
                 where
-                    #ident #ty_generics: ::std::fmt::Debug + ::std::fmt::Display
+                    #ident #ty_generics: ::core::fmt::Debug + ::core::fmt::Display
             },
         );
     }
@@ -73,7 +73,7 @@ pub fn expand(
             &generics,
             quote! {
                 where
-                    #(#bounds: ::std::fmt::Debug + ::std::fmt::Display + ::std::error::Error + 'static),*
+                    #(#bounds: ::core::fmt::Debug + ::core::fmt::Display + ::derive_more::__private::Error + 'static),*
             },
         );
     }
@@ -82,7 +82,7 @@ pub fn expand(
 
     let render = quote! {
         #[automatically_derived]
-        impl #impl_generics ::std::error::Error for #ident #ty_generics #where_clause {
+        impl #impl_generics ::derive_more::__private::Error for #ident #ty_generics #where_clause {
             #source
             #provide
         }
@@ -207,7 +207,7 @@ impl<'input, 'state> ParsedFields<'input, 'state> {
         let source_provider = self.source.map(|source| {
             let source_expr = &self.data.members[source];
             quote! {
-                ::std::error::Error::provide(&#source_expr, demand);
+                ::derive_more::__private::Error::provide(&#source_expr, demand);
             }
         });
         let backtrace_provider = self
@@ -217,7 +217,7 @@ impl<'input, 'state> ParsedFields<'input, 'state> {
             .then(|| {
                 let backtrace_expr = &self.data.members[backtrace];
                 quote! {
-                    demand.provide_ref::<std::backtrace::Backtrace>(&#backtrace_expr);
+                    demand.provide_ref::<::std::backtrace::Backtrace>(&#backtrace_expr);
                 }
             });
 
@@ -237,7 +237,7 @@ impl<'input, 'state> ParsedFields<'input, 'state> {
                 let pattern = self.data.matcher(&[source], &[quote! { source }]);
                 Some(quote! {
                     #pattern => {
-                        ::std::error::Error::provide(source, demand);
+                        ::derive_more::__private::Error::provide(source, demand);
                     }
                 })
             }
@@ -248,16 +248,16 @@ impl<'input, 'state> ParsedFields<'input, 'state> {
                 );
                 Some(quote! {
                     #pattern => {
-                        demand.provide_ref::<std::backtrace::Backtrace>(backtrace);
-                        ::std::error::Error::provide(source, demand);
+                        demand.provide_ref::<::std::backtrace::Backtrace>(backtrace);
+                        ::derive_more::__private::Error::provide(source, demand);
                     }
                 })
             }
             None => {
                 let pattern = self.data.matcher(&[backtrace], &[quote! { backtrace }]);
                 Some(quote! {
                     #pattern => {
-                        demand.provide_ref::<std::backtrace::Backtrace>(backtrace);
+                        demand.provide_ref::<::std::backtrace::Backtrace>(backtrace);
                     }
                 })
             }

src/lib.rs

@@ -1,4 +1,5 @@
 #![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(all(not(feature = "std"), feature = "error"), feature(error_in_core))]
 // These links overwrite the ones in `README.md`
 // to become proper intra-doc links in Rust docs.
 //! [`From`]: crate::From
@@ -77,6 +78,11 @@ mod vendor;
 #[doc(hidden)]
 #[cfg(feature = "error")]
 pub mod __private {
+    #[cfg(not(feature = "std"))]
+    pub use ::core::error::Error;
+    #[cfg(feature = "std")]
+    pub use ::std::error::Error;
+
     pub use crate::vendor::thiserror::aserror::AsDynError;
 }
 

src/vendor/thiserror/aserror.rs

@@ -1,5 +1,9 @@
+#[cfg(not(feature = "std"))]
+use core::error::Error;
+#[cfg(feature = "std")]
 use std::error::Error;
-use std::panic::UnwindSafe;
+
+use core::panic::UnwindSafe;
 
 pub trait AsDynError<'a>: Sealed {
     fn as_dyn_error(&self) -> &(dyn Error + 'a);

tests/error/derives_for_enums_with_source.rs

@@ -12,6 +12,7 @@ enum TestErr {
         source: SimpleErr,
         field: i32,
     },
+    #[cfg(feature = "std")]
     NamedImplicitBoxedSource {
         source: Box<dyn Error + Send + 'static>,
         field: i32,
@@ -98,6 +99,7 @@ fn named_implicit_source() {
     assert!(err.source().unwrap().is::<SimpleErr>());
 }
 
+#[cfg(feature = "std")]
 #[test]
 fn named_implicit_boxed_source() {
     let err = TestErr::NamedImplicitBoxedSource {

tests/error/mod.rs

@@ -1,3 +1,6 @@
+#[cfg(not(feature = "std"))]
+use core::error::Error;
+#[cfg(feature = "std")]
 use std::error::Error;
 
 use derive_more::Error;
@@ -29,17 +32,17 @@ use derive_more::Error;
 /// ```
 macro_rules! derive_display {
     (@fmt) => {
-        fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+        fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
             write!(f, "")
         }
     };
     ($type:ident) => {
-        impl ::std::fmt::Display for $type {
+        impl ::core::fmt::Display for $type {
             derive_display!(@fmt);
         }
     };
     ($type:ident, $($type_parameters:ident),*) => {
-        impl<$($type_parameters),*> ::std::fmt::Display for $type<$($type_parameters),*> {
+        impl<$($type_parameters),*> ::core::fmt::Display for $type<$($type_parameters),*> {
             derive_display!(@fmt);
         }
     };
@@ -50,7 +53,7 @@ mod derives_for_generic_enums_with_source;
 mod derives_for_generic_structs_with_source;
 mod derives_for_structs_with_source;
 
-#[cfg(nightly)]
+#[cfg(all(feature = "std", nightly))]
 mod nightly;
 
 derive_display!(SimpleErr);

tests/error_tests.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(not(feature = "std"), no_std)]
 #![cfg_attr(nightly, feature(error_generic_member_access, provide_any))]
+#![cfg_attr(not(feature = "std"), feature(error_in_core))]
 
 mod error;