test_no_std: add strictly no_std test

Tests source, from, error and hygiene, like test_source.

tests/test_no_std.rs

@@ -0,0 +1,56 @@
+// There is no negative required-features to suppress this test
+// for not(feature = "std").
+#![cfg(not(feature = "std"))]
+#![no_std]
+
+use core::error::Error as _;
+use thiserror::Error;
+
+#[derive(Error, Debug, Default)]
+#[error("io")]
+pub struct IoError {}
+
+#[derive(Error, Debug)]
+#[error("implicit source")]
+pub struct ImplicitSource {
+    source: IoError,
+}
+
+#[derive(Error, Debug)]
+#[error("explicit source")]
+pub struct ExplicitSource {
+    source: i32,
+    #[source]
+    io: IoError,
+}
+
+#[test]
+fn test_implicit_source() {
+    let io = IoError::default();
+    let error = ImplicitSource { source: io };
+    error.source().unwrap().downcast_ref::<IoError>().unwrap();
+}
+
+#[test]
+fn test_explicit_source() {
+    let io = IoError::default();
+    let error = ExplicitSource { source: 0, io };
+    error.source().unwrap().downcast_ref::<IoError>().unwrap();
+}
+
+macro_rules! error_from_macro {
+    ($($variants:tt)*) => {
+        #[derive(Error)]
+        #[derive(Debug)]
+        pub enum MacroSource {
+            $($variants)*
+        }
+    }
+}
+
+// Test that we generate impls with the proper hygiene
+#[rustfmt::skip]
+error_from_macro! {
+    #[error("Something")]
+    Variant(#[from] IoError)
+}