Add CI job for no-std build verification and fix Infallible StdError implementation

- Add dedicated no-std CI job that builds for wasm32-unknown-unknown target
- Fix Infallible StdError implementation for no-std environments

The CI job validates:
1. Core no-std functionality (no default features)
2. fancy-no-syscall feature set compatible with no-std environments

Author: huitseeker

.github/workflows/ci.yml

@@ -68,6 +68,21 @@ jobs:
       - name: Check wasm target
         run: cargo check --target wasm32-unknown-unknown --features fancy-no-syscall
 
+  no-std:
+    name: Check no-std build
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Rust
+        uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: stable
+          targets: wasm32-unknown-unknown
+      - name: Check no-std core build
+        run: cargo check --target wasm32-unknown-unknown --no-default-features
+      - name: Check no-std with fancy-no-syscall
+        run: cargo check --target wasm32-unknown-unknown --no-default-features --features fancy-no-syscall
+
   miri:
     name: Miri
     runs-on: ubuntu-latest

src/diagnostic_impls.rs

@@ -8,6 +8,9 @@ use core::{convert::Infallible, fmt::Display};
 
 use crate::{Diagnostic, LabeledSpan, Severity, SourceCode};
 
+#[cfg(not(feature = "std"))]
+impl crate::StdError for Infallible {}
+
 impl Diagnostic for Infallible {
     fn code<'a>(&'a self) -> Option<Box<dyn Display + 'a>> {
         match *self {}

src/eyreish/wrapper.rs

@@ -209,13 +209,16 @@ impl<C> StdError for WithSourceCode<Report, C> {
     }
 }
 
-#[cfg(test)]
+#[cfg(all(test, feature = "std"))]
 mod tests {
-    
+    #[cfg(feature = "fancy")]
+    use std::format;
     use std::{
         boxed::Box,
         string::{String, ToString},
     };
+    #[cfg(feature = "fancy")]
+    use std::{vec, vec::Vec};
     use thiserror::Error;
 
     use crate::{Diagnostic, LabeledSpan, Report, SourceCode, SourceSpan};