Use core::error for NoSuchCaseFoldingScheme

`core::error` was stabilized in Rust 1.81.0. Since #249 increased MSRV
to 1.83.0, update the code to remove all dependencies on `std`.

Because this permits us to remove a Cargo feature, this is a breaking
change, since consumers may be explicitly enabling the `std` feature.

Prepare for v2.0.0 release.

Author: lopopolo

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "focaccia"
-version = "1.6.0" # remember to set `html_root_url` in `src/lib.rs`.
+version = "2.0.0" # remember to set `html_root_url` in `src/lib.rs`.
 authors = ["Ryan Lopopolo <rjl@hyperbo.la>"]
 license = "MIT AND Unicode-3.0"
 edition = "2021"
@@ -26,12 +26,6 @@ include = [
     "/README.md",
 ]
 
-[features]
-default = ["std"]
-# Enable a dependency on the Rust `std` library. This feature implements the
-# `Error` trait on error structs in `focaccia`.
-std = []
-
 [dependencies]
 
 [dev-dependencies]

README.md

@@ -39,7 +39,7 @@ Add this to your `Cargo.toml`:
 
 ```toml
 [dependencies]
-focaccia = "1.6.0"
+focaccia = "2.0.0"
 ```
 
 Then make case insensitive string comparisons like:
@@ -109,19 +109,8 @@ implements case folding as defined in the [Unicode standard][casemap] (see
 
 ## `no_std`
 
-Focaccia is `no_std` compatible with an optional and enabled by default
-dependency on `std`. Focaccia does not link to `alloc` in its `no_std`
-configuration.
-
-## Crate features
-
-All features are enabled by default.
-
-- **std** - Enable linking to the [Rust Standard Library]. Enabling this feature
-  adds [`Error`] implementations to error types in this crate.
-
-[rust standard library]: https://doc.rust-lang.org/stable/std/index.html
-[`error`]: https://doc.rust-lang.org/stable/std/error/trait.Error.html
+Focaccia is `no_std` compatible and only depends on `core`. Focaccia does not
+link to `alloc` in its `no_std` configuration.
 
 ### Minimum Supported Rust Version
 

src/folding/mapping/mod.rs

@@ -85,16 +85,13 @@ impl FusedIterator for Iter {}
 
 #[cfg(test)]
 mod tests {
-    use super::Mapping;
+    use core::fmt::Write as _;
+    use std::string::String;
+
+    use super::{Mapping, Mode};
 
     #[test]
-    #[cfg(feature = "std")]
     fn mode_debug_is_not_empty() {
-        use std::fmt::Write;
-        use std::string::String;
-
-        use super::Mode;
-
         let mut buf = String::new();
         write!(&mut buf, "{:?}", Mode::Full).unwrap();
         assert!(!buf.is_empty());
@@ -105,11 +102,7 @@ mod tests {
     }
 
     #[test]
-    #[cfg(feature = "std")]
     fn mapping_debug_is_not_empty() {
-        use std::fmt::Write;
-        use std::string::String;
-
         let mut buf = String::new();
         write!(&mut buf, "{:?}", Mapping::Empty).unwrap();
         assert!(!buf.is_empty());
@@ -128,11 +121,7 @@ mod tests {
     }
 
     #[test]
-    #[cfg(feature = "std")]
     fn mapping_iter_debug_is_not_empty() {
-        use std::fmt::Write;
-        use std::string::String;
-
         let mut buf = String::new();
         write!(&mut buf, "{:?}", Mapping::Empty.into_iter()).unwrap();
         assert!(!buf.is_empty());

src/lib.rs

@@ -82,17 +82,8 @@
 //!
 //! # `no_std`
 //!
-//! Focaccia is `no_std` compatible. By default, Focaccia builds with its
-//! **std** feature enabled to implement [`Error`].
-//!
-//! When built without the **std** feature, Focaccia does not link to `alloc`.
-//!
-//! # Crate features
-//!
-//! All features are enabled by default.
-//!
-//! - **std** - Enable linking to the [Rust Standard Library]. Enabling this
-//!   feature adds [`Error`] implementations to error types in this crate.
+//! Focaccia is `no_std` compatible and only depends on [`core`]. Focaccia does not
+//! link to `alloc` in its `no_std` configuration.
 //!
 //! # Unicode Version
 //!
@@ -106,13 +97,11 @@
 //! [Unicode case folding]: https://www.w3.org/International/wiki/Case_folding
 //! [`Ordering`]: core::cmp::Ordering
 //! [dotted and dotless I]: https://en.wikipedia.org/wiki/Dotted_and_dotless_I
-//! [Rust Standard Library]: https://doc.rust-lang.org/stable/std/index.html
-//! [`Error`]: https://doc.rust-lang.org/stable/std/error/trait.Error.html
 
 #![no_std]
-#![doc(html_root_url = "https://docs.rs/focaccia/1.6.0")]
+#![doc(html_root_url = "https://docs.rs/focaccia/2.0.0")]
 
-#[cfg(feature = "std")]
+#[cfg(any(test, doctest))]
 extern crate std;
 
 use core::cmp::Ordering;
@@ -364,8 +353,7 @@ impl CaseFold {
 /// Error type for returned when a folding scheme could not be resolved in a
 /// [`TryFrom`] implementation.
 ///
-/// When this crate's `std` feature is enabled, `NoSuchCaseFoldingScheme`
-/// implements [`std::error::Error`].
+/// `NoSuchCaseFoldingScheme` implements [`core::error::Error`].
 ///
 /// # Examples
 ///
@@ -378,8 +366,6 @@ impl CaseFold {
 /// assert_eq!(CaseFold::try_from(Some("lithuanian")), Ok(CaseFold::Lithuanian));
 /// assert_eq!(CaseFold::try_from(Some("xxx")), Err(NoSuchCaseFoldingScheme::new()));
 /// ```
-///
-/// [`std::error::Error`]: https://doc.rust-lang.org/stable/std/error/trait.Error.html
 #[derive(Default, Debug, Clone, Copy, Hash, PartialEq, Eq, PartialOrd, Ord)]
 pub struct NoSuchCaseFoldingScheme {
     _private: (),
@@ -407,8 +393,7 @@ impl fmt::Display for NoSuchCaseFoldingScheme {
     }
 }
 
-#[cfg(feature = "std")]
-impl std::error::Error for NoSuchCaseFoldingScheme {}
+impl core::error::Error for NoSuchCaseFoldingScheme {}
 
 impl TryFrom<Option<&str>> for CaseFold {
     type Error = NoSuchCaseFoldingScheme;
@@ -439,9 +424,7 @@ impl TryFrom<Option<&[u8]>> for CaseFold {
 mod tests {
     use core::cmp::Ordering;
 
-    use crate::CaseFold;
-    #[cfg(feature = "std")]
-    use crate::NoSuchCaseFoldingScheme;
+    use crate::{CaseFold, NoSuchCaseFoldingScheme};
 
     // https://tools.ietf.org/html/draft-josefsson-idn-test-vectors-00#section-4.2
     #[test]
@@ -617,7 +600,6 @@ mod tests {
     }
 
     #[test]
-    #[cfg(feature = "std")]
     fn error_display_is_not_empty() {
         use core::fmt::Write as _;
         use std::string::String;
@@ -628,6 +610,7 @@ mod tests {
         assert!(!buf.is_empty());
     }
 }
+
 // Ensure code blocks in `README.md` compile.
 //
 // This module and macro declaration should be kept at the end of the file, in