Add no_std support

Introduce no_std feature which, when enabled, uses core::error::Error
trait rather than std::error::Error and thus makes it possible to use
the crate in no_std builds.

Importantly though, using core::error module requires a unstable
error_in_core feature thus enabling no_std requires building with
nightly compiler.

Furthermore, enabling `no_std` disables handling of Backtrace fields
and support for displaying paths.  Since those types aren’t available
in no_std environments this shouldn’t be an issue.

Cargo.toml

@@ -26,3 +26,6 @@ members = ["impl"]
 [package.metadata.docs.rs]
 targets = ["x86_64-unknown-linux-gnu"]
 rustdoc-args = ["--generate-link-to-definition"]
+
+[features]
+no_std = []

src/aserror.rs

@@ -1,5 +1,5 @@
 use crate::__private::Error;
-use std::panic::UnwindSafe;
+use core::panic::UnwindSafe;
 
 pub trait AsDynError<'a>: Sealed {
     fn as_dyn_error(&self) -> &(dyn Error + 'a);

src/display.rs

@@ -1,5 +1,4 @@
-use std::fmt::Display;
-use std::path::{Path, PathBuf};
+use core::fmt::Display;
 
 pub trait AsDisplay {
     type Target: Display + ?Sized;
@@ -15,39 +14,44 @@ impl<T: Display> AsDisplay for &T {
     }
 }
 
-impl AsDisplay for Path {
-    type Target = PathDisplay;
+#[cfg(not(feature = "no_std"))]
+mod path {
+    use std::path::{Path, PathBuf};
 
-    #[inline(always)]
-    fn as_display(&self) -> &Self::Target {
-        PathDisplay::new(self)
+    impl super::AsDisplay for Path {
+        type Target = PathDisplay;
+
+        #[inline(always)]
+        fn as_display(&self) -> &Self::Target {
+            PathDisplay::new(self)
+        }
     }
-}
 
-impl AsDisplay for PathBuf {
-    type Target = PathDisplay;
+    impl super::AsDisplay for PathBuf {
+        type Target = PathDisplay;
 
-    #[inline(always)]
-    fn as_display(&self) -> &Self::Target {
-        PathDisplay::new(self.as_path())
+        #[inline(always)]
+        fn as_display(&self) -> &Self::Target {
+            PathDisplay::new(self.as_path())
+        }
     }
-}
 
-#[repr(transparent)]
-pub struct PathDisplay(Path);
+    #[repr(transparent)]
+    pub struct PathDisplay(Path);
 
-impl PathDisplay {
-    #[inline(always)]
-    fn new(path: &Path) -> &Self {
-        // SAFETY: PathDisplay is repr(transparent) so casting pointers between
-        // it and its payload is safe.
-        unsafe { &*(path as *const Path as *const Self) }
+    impl PathDisplay {
+        #[inline(always)]
+        fn new(path: &Path) -> &Self {
+            // SAFETY: PathDisplay is repr(transparent) so casting pointers
+            // between it and its payload is safe.
+            unsafe { &*(path as *const Path as *const Self) }
+        }
     }
-}
 
-impl Display for PathDisplay {
-    #[inline(always)]
-    fn fmt(&self, fmtr: &mut std::fmt::Formatter) -> std::fmt::Result {
-        self.0.display().fmt(fmtr)
+    impl core::fmt::Display for PathDisplay {
+        #[inline(always)]
+        fn fmt(&self, fmtr: &mut core::fmt::Formatter) -> core::fmt::Result {
+            self.0.display().fmt(fmtr)
+        }
     }
 }

src/lib.rs

@@ -237,6 +237,8 @@
     clippy::wildcard_imports,
 )]
 #![cfg_attr(provide_any, feature(provide_any))]
+#![cfg_attr(feature = "no_std", feature(error_in_core))]
+#![cfg_attr(feature = "no_std", no_std)]
 
 mod aserror;
 mod display;
@@ -252,5 +254,8 @@ pub mod __private {
     pub use crate::display::AsDisplay;
     #[cfg(provide_any)]
     pub use crate::provide::ThiserrorProvide;
+    #[cfg(feature = "no_std")]
+    pub use core::error::Error;
+    #[cfg(not(feature = "no_std"))]
     pub use std::error::Error;
 }

src/provide.rs

@@ -1,4 +1,4 @@
-use std::any::{Demand, Provider};
+use core::any::{Demand, Provider};
 
 pub trait ThiserrorProvide: Sealed {
     fn thiserror_provide<'a>(&'a self, demand: &mut Demand<'a>);