Allow disabling std dependency on 1.81+

tests/no-std/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "thiserror_no_std_test"
+version = "0.0.0"
+authors = ["David Tolnay <dtolnay@gmail.com>"]
+edition = "2021"
+publish = false
+
+[lib]
+path = "test.rs"
+
+[dependencies]
+thiserror = { path = "../..", default-features = false }

tests/no-std/test.rs

@@ -0,0 +1,58 @@
+#![no_std]
+
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("Error::E")]
+    E(#[from] SourceError),
+}
+
+#[derive(Error, Debug)]
+#[error("SourceError {field}")]
+pub struct SourceError {
+    pub field: i32,
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{Error, SourceError};
+    use core::error::Error as _;
+    use core::fmt::{self, Write};
+    use core::mem;
+
+    struct Buf<'a>(&'a mut [u8]);
+
+    impl Write for Buf<'_> {
+        fn write_str(&mut self, s: &str) -> fmt::Result {
+            if s.len() <= self.0.len() {
+                let (out, rest) = mem::take(&mut self.0).split_at_mut(s.len());
+                out.copy_from_slice(s.as_bytes());
+                self.0 = rest;
+                Ok(())
+            } else {
+                Err(fmt::Error)
+            }
+        }
+    }
+
+    #[test]
+    fn test() {
+        let source = SourceError { field: -1 };
+        let error = Error::from(source);
+
+        let source = error
+            .source()
+            .unwrap()
+            .downcast_ref::<SourceError>()
+            .unwrap();
+
+        let mut msg = [b'~'; 17];
+        write!(Buf(&mut msg), "{error}").unwrap();
+        assert_eq!(msg, *b"Error::E~~~~~~~~~");
+
+        let mut msg = [b'~'; 17];
+        write!(Buf(&mut msg), "{source}").unwrap();
+        assert_eq!(msg, *b"SourceError -1~~~");
+    }
+}