allow stderr to be ignored on non-beta compilers

Cargo.toml

@@ -24,6 +24,7 @@ compiler_builtins = { version = '0.1.2', optional = true }
 
 [dev-dependencies]
 trybuild = "1.0"
+rustversion = "1.0"
 serde = "1.0"
 serde_derive = "1.0"
 serde_json = "1.0"

tests/compile-fail/.gitignore

@@ -0,0 +1 @@
+*.stderr

tests/compiletest.rs

@@ -1,5 +1,44 @@
+use std::{
+    fs,
+    ffi::OsStr,
+    io,
+    path::Path,
+};
+
 #[test]
 fn compile_fail() {
+    prepare_stderr_files("tests/compile-fail").unwrap();
+
     let t = trybuild::TestCases::new();
     t.compile_fail("tests/compile-fail/*.rs");
 }
+
+fn prepare_stderr_files(path: impl AsRef<Path>) -> io::Result<()> {
+    for entry in fs::read_dir(path)? {
+        let entry = entry?;
+
+        if entry.path().extension().and_then(OsStr::to_str) == Some("beta") {
+            let renamed = entry.path().with_extension("");
+
+            if renamed.exists() {
+                fs::remove_file(&renamed)?;
+            }
+
+            rename_beta_stderr(entry.path(), renamed)?;
+        }
+    }
+
+    Ok(())
+}
+
+#[rustversion::beta]
+fn rename_beta_stderr(from: impl AsRef<Path>, to: impl AsRef<Path>) -> io::Result<()> {
+    fs::copy(from, to)?;
+
+    Ok(())
+}
+
+#[rustversion::not(beta)]
+fn rename_beta_stderr(_: impl AsRef<Path>, _: impl AsRef<Path>) -> io::Result<()> {
+    Ok(())
+}