Support Error in no_std environments

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. Fixes #261
JelteF · Jul 1, 2023 · c0bc2fd · c0bc2fd
1 parent 37cae22
commit c0bc2fd
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Showing 11 changed files with 82 additions and 22 deletions.
diff --git a/CHANGELOG.md b/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
 

diff --git a/Cargo.toml b/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"]
@@ -67,7 +70,7 @@ is_variant = ["derive_more-impl/is_variant"]
 unwrap = ["derive_more-impl/unwrap"]
 try_unwrap = ["derive_more-impl/try_unwrap"]
 
-std = []
+std = ["derive_more-impl/std"]
 full = [
     "add_assign",
     "add",
@@ -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"

diff --git a/build.rs b/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();
+}
diff --git a/impl/Cargo.toml b/impl/Cargo.toml
@@ -71,4 +71,6 @@ is_variant = ["dep:convert_case"]
 unwrap = ["dep:convert_case"]
 try_unwrap = ["dep:convert_case"]
 
+std = []
+
 testing-helpers = ["dep:rustc_version"]
diff --git a/impl/doc/error.md b/impl/doc/error.md
@@ -42,13 +42,23 @@ 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:
+```rust
+#![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() {

diff --git a/impl/src/error.rs b/impl/src/error.rs
@@ -7,6 +7,16 @@ use crate::utils::{
     State,
 };
 
+#[cfg(feature = "std")]
+fn error_module() -> TokenStream {
+    quote! { ::std::error }
+}
+
+#[cfg(not(feature = "std"))]
+fn error_module() -> TokenStream {
+    quote! { ::core::error }
+}
+
 pub fn expand(
     input: &syn::DeriveInput,
     trait_name: &'static str,
@@ -15,10 +25,12 @@ pub fn expand(
         ident, generics, ..
     } = input;
 
+    let error_mod = error_module();
+
     let state = State::with_attr_params(
         input,
         trait_name,
-        quote! { ::std::error },
+        error_mod.clone(),
         trait_name.to_lowercase(),
         allowed_attr_params(),
     )?;
@@ -39,7 +51,7 @@ pub fn expand(
 
     let source = source.map(|source| {
         quote! {
-            fn source(&self) -> Option<&(dyn ::std::error::Error + 'static)> {
+            fn source(&self) -> Option<&(dyn #error_mod::Error + 'static)> {
                 use ::derive_more::__private::AsDynError;
                 #source
             }
@@ -48,7 +60,7 @@ 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 +74,7 @@ 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 +85,7 @@ pub fn expand(
             &generics,
             quote! {
                 where
-                    #(#bounds: ::std::fmt::Debug + ::std::fmt::Display + ::std::error::Error + 'static),*
+                    #(#bounds: ::core::fmt::Debug + ::core::fmt::Display + #error_mod::Error + 'static),*
             },
         );
     }
@@ -82,7 +94,7 @@ pub fn expand(
 
     let render = quote! {
         #[automatically_derived]
-        impl #impl_generics ::std::error::Error for #ident #ty_generics #where_clause {
+        impl #impl_generics #error_mod::Error for #ident #ty_generics #where_clause {
             #source
             #provide
         }
@@ -203,11 +215,12 @@ impl<'input, 'state> ParsedFields<'input, 'state> {
 
     fn render_provide_as_struct(&self) -> Option<TokenStream> {
         let backtrace = self.backtrace?;
+        let error_mod = error_module();
 
         let source_provider = self.source.map(|source| {
             let source_expr = &self.data.members[source];
             quote! {
-                ::std::error::Error::provide(&#source_expr, demand);
+                #error_mod::Error::provide(&#source_expr, demand);
             }
         });
         let backtrace_provider = self
@@ -217,7 +230,7 @@ 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);
                 }
             });
 
@@ -231,13 +244,14 @@ impl<'input, 'state> ParsedFields<'input, 'state> {
 
     fn render_provide_as_enum_variant_match_arm(&self) -> Option<TokenStream> {
         let backtrace = self.backtrace?;
+        let error_mod = error_module();
 
         match self.source {
             Some(source) if source == backtrace => {
                 let pattern = self.data.matcher(&[source], &[quote! { source }]);
                 Some(quote! {
                     #pattern => {
-                        ::std::error::Error::provide(source, demand);
+                        #error_mod::Error::provide(source, demand);
                     }
                 })
             }
@@ -248,16 +262,16 @@ 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);
+                        #error_mod::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);
                     }
                 })
             }

diff --git a/src/lib.rs b/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

diff --git a/src/vendor/thiserror/aserror.rs b/src/vendor/thiserror/aserror.rs
@@ -1,5 +1,10 @@
+#[cfg(feature = "std")]
 use std::error::Error;
-use std::panic::UnwindSafe;
+
+#[cfg(not(feature = "std"))]
+use core::error::Error;
+
+use core::panic::UnwindSafe;
 
 pub trait AsDynError<'a>: Sealed {
     fn as_dyn_error(&self) -> &(dyn Error + 'a);

diff --git a/tests/error/derives_for_enums_with_source.rs b/tests/error/derives_for_enums_with_source.rs
@@ -12,6 +12,7 @@ enum TestErr {
         source: SimpleErr,
         field: i32,
     },
+    #[cfg(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(std)]
 #[test]
 fn named_implicit_boxed_source() {
     let err = TestErr::NamedImplicitBoxedSource {

diff --git a/tests/error/mod.rs b/tests/error/mod.rs
@@ -1,5 +1,9 @@
+#[cfg(feature = "std")]
 use std::error::Error;
 
+#[cfg(not(feature = "std"))]
+use core::error::Error;
+
 use derive_more::Error;
 
 /// Derives `std::fmt::Display` for structs/enums.
@@ -29,17 +33,17 @@ 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 +54,7 @@ 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);

diff --git a/tests/error_tests.rs b/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;