Revert "error_in_core is stable"

This reverts commit 61b1c17.

Cargo.toml

@@ -30,3 +30,8 @@ rustdoc-args = ["--generate-link-to-definition"]
 [features]
 std = []
 default = ["std"]
+
+[[test]]
+name = "compiletest"
+# Merely to avoid spraying cfg(error_in_core)
+required-features = ["std"]

impl/src/expand.rs

@@ -36,7 +36,7 @@ fn fallback(input: &DeriveInput, error: syn::Error) -> TokenStream {
         #error
 
         #[allow(unused_qualifications)]
-        impl #impl_generics ::core::error::Error for #ty #ty_generics #where_clause
+        impl #impl_generics ::thiserror::__private::error::Error for #ty #ty_generics #where_clause
         where
             // Work around trivial bounds being unstable.
             // https://github.com/rust-lang/rust/issues/48214
@@ -60,17 +60,19 @@ fn impl_struct(input: Struct) -> TokenStream {
     let source_body = if let Some(transparent_attr) = &input.attrs.transparent {
         let only_field = &input.fields[0];
         if only_field.contains_generic {
-            error_inferred_bounds.insert(only_field.ty, quote!(::core::error::Error));
+            error_inferred_bounds
+                .insert(only_field.ty, quote!(::thiserror::__private::error::Error));
         }
         let member = &only_field.member;
         Some(quote_spanned! {transparent_attr.span=>
-            ::core::error::Error::source(self.#member.as_dyn_error())
+            ::thiserror::__private::error::Error::source(self.#member.as_dyn_error())
         })
     } else if let Some(source_field) = input.source_field() {
         let source = &source_field.member;
         if source_field.contains_generic {
             let ty = unoptional_type(source_field.ty);
-            error_inferred_bounds.insert(ty, quote!(::core::error::Error + 'static));
+            error_inferred_bounds
+                .insert(ty, quote!(::thiserror::__private::error::Error + 'static));
         }
         let asref = if type_is_option(source_field.ty) {
             Some(quote_spanned!(source.member_span()=> .as_ref()?))
@@ -88,7 +90,7 @@ fn impl_struct(input: Struct) -> TokenStream {
     };
     let source_method = source_body.map(|body| {
         quote! {
-            fn source(&self) -> ::core::option::Option<&(dyn ::core::error::Error + 'static)> {
+            fn source(&self) -> ::core::option::Option<&(dyn ::thiserror::__private::error::Error + 'static)> {
                 use thiserror::__private::AsDynError as _;
                 #body
             }
@@ -141,7 +143,7 @@ fn impl_struct(input: Struct) -> TokenStream {
             }
         };
         quote! {
-            fn provide<'_request>(&'_request self, #request: &mut ::core::error::Request<'_request>) {
+            fn provide<'_request>(&'_request self, #request: &mut std::error::Request<'_request>) {
                 #body
             }
         }
@@ -211,7 +213,7 @@ fn impl_struct(input: Struct) -> TokenStream {
 
     quote! {
         #[allow(unused_qualifications)]
-        impl #impl_generics ::core::error::Error for #ty #ty_generics #error_where_clause {
+        impl #impl_generics ::thiserror::__private::error::Error for #ty #ty_generics #error_where_clause {
             #source_method
             #provide_method
         }
@@ -231,11 +233,11 @@ fn impl_enum(input: Enum) -> TokenStream {
             if let Some(transparent_attr) = &variant.attrs.transparent {
                 let only_field = &variant.fields[0];
                 if only_field.contains_generic {
-                    error_inferred_bounds.insert(only_field.ty, quote!(::core::error::Error));
+                    error_inferred_bounds.insert(only_field.ty, quote!(::thiserror::__private::error::Error));
                 }
                 let member = &only_field.member;
                 let source = quote_spanned! {transparent_attr.span=>
-                    ::core::error::Error::source(transparent.as_dyn_error())
+                    ::thiserror::__private::error::Error::source(transparent.as_dyn_error())
                 };
                 quote! {
                     #ty::#ident {#member: transparent} => #source,
@@ -244,7 +246,7 @@ fn impl_enum(input: Enum) -> TokenStream {
                 let source = &source_field.member;
                 if source_field.contains_generic {
                     let ty = unoptional_type(source_field.ty);
-                    error_inferred_bounds.insert(ty, quote!(::core::error::Error + 'static));
+                    error_inferred_bounds.insert(ty, quote!(::thiserror::__private::error::Error + 'static));
                 }
                 let asref = if type_is_option(source_field.ty) {
                     Some(quote_spanned!(source.member_span()=> .as_ref()?))
@@ -265,7 +267,7 @@ fn impl_enum(input: Enum) -> TokenStream {
             }
         });
         Some(quote! {
-            fn source(&self) -> ::core::option::Option<&(dyn ::core::error::Error + 'static)> {
+            fn source(&self) -> ::core::option::Option<&(dyn std::error::Error + 'static)> {
                 use thiserror::__private::AsDynError as _;
                 #[allow(deprecated)]
                 match self {
@@ -370,7 +372,7 @@ fn impl_enum(input: Enum) -> TokenStream {
             }
         });
         Some(quote! {
-            fn provide<'_request>(&'_request self, #request: &mut ::core::error::Request<'_request>) {
+            fn provide<'_request>(&'_request self, #request: &mut std::error::Request<'_request>) {
                 #[allow(deprecated)]
                 match self {
                     #(#arms)*
@@ -467,7 +469,7 @@ fn impl_enum(input: Enum) -> TokenStream {
 
     quote! {
         #[allow(unused_qualifications)]
-        impl #impl_generics ::core::error::Error for #ty #ty_generics #error_where_clause {
+        impl #impl_generics ::thiserror::__private::error::Error for #ty #ty_generics #error_where_clause {
             #source_method
             #provide_method
         }

src/aserror.rs

@@ -1,4 +1,4 @@
-use core::error::Error;
+use crate::__private::error::Error;
 use core::panic::UnwindSafe;
 
 #[doc(hidden)]

src/lib.rs

@@ -266,7 +266,12 @@
     clippy::wildcard_imports
 )]
 #![cfg_attr(error_generic_member_access, feature(error_generic_member_access))]
-#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(
+    not(feature = "std"),
+    no_std,
+    feature(error_in_core),
+    doc(test(attr(feature(error_in_core))))
+)]
 
 #[cfg(all(thiserror_nightly_testing, not(error_generic_member_access)))]
 compile_error!("Build script probe failed to compile.");
@@ -288,4 +293,10 @@ pub mod __private {
     #[cfg(error_generic_member_access)]
     #[doc(hidden)]
     pub use crate::provide::ThiserrorProvide;
+    #[cfg(not(feature = "std"))]
+    #[doc(hidden)]
+    pub use core::error;
+    #[cfg(feature = "std")]
+    #[doc(hidden)]
+    pub use std::error;
 }

src/provide.rs

@@ -1,4 +1,4 @@
-use core::error::{Error, Request};
+use crate::__private::error::{Error, Request};
 
 #[doc(hidden)]
 pub trait ThiserrorProvide: Sealed {

tests/test_backtrace.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(not(feature = "std"), feature(error_in_core))]
 #![cfg_attr(thiserror_nightly_testing, feature(error_generic_member_access))]
 
 use thiserror::Error;

tests/test_deprecated.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(not(feature = "std"), feature(error_in_core))]
 #![deny(deprecated, clippy::all, clippy::pedantic)]
 
 use thiserror::Error;

tests/test_display.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(not(feature = "std"), feature(error_in_core))]
 #![allow(clippy::needless_raw_string_hashes, clippy::uninlined_format_args)]
 
 use core::fmt::{self, Display};

tests/test_error.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(not(feature = "std"), feature(error_in_core))]
 #![allow(dead_code)]
 
 use core::fmt::{self, Display};

tests/test_expr.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(not(feature = "std"), feature(error_in_core))]
 #![allow(clippy::iter_cloned_collect, clippy::uninlined_format_args)]
 
 use core::fmt::Display;

tests/test_from.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(not(feature = "std"), feature(error_in_core))]
 #![allow(clippy::extra_unused_type_parameters)]
 
 use std::io;

tests/test_generics.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(not(feature = "std"), feature(error_in_core))]
 #![allow(clippy::needless_late_init, clippy::uninlined_format_args)]
 
 use core::fmt::{self, Debug, Display};

tests/test_lints.rs

@@ -1,8 +1,10 @@
+#![cfg_attr(not(feature = "std"), feature(error_in_core))]
 #![allow(clippy::mixed_attributes_style)]
 
 use thiserror::Error;
 
-pub use core::error::Error;
+// std or core
+pub use thiserror::__private::error::Error;
 
 #[test]
 fn test_unused_qualifications() {

tests/test_source.rs

@@ -1,3 +1,5 @@
+#![cfg_attr(not(feature = "std"), feature(error_in_core))]
+
 use std::error::Error as StdError;
 use std::io;
 use thiserror::Error;

tests/test_transparent.rs

@@ -1,6 +1,9 @@
+#![cfg_attr(not(feature = "std"), feature(error_in_core))]
+
 use anyhow::anyhow;
-use core::error::Error;
 use thiserror::Error;
+// std or core
+use thiserror::__private::error::Error;
 
 #[cfg(feature = "std")]
 #[test]