diff --git a/Cargo.toml b/Cargo.toml
index 6a64624..e099133 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -30,13 +30,12 @@ optional = true
 default-features = false
 
 [dev-dependencies]
+version_check = "0.9"
 test-crate = { path = "./test-crate" }
 syn = { version = "2", features = ["full"] }
 trybuild = { version = "1.0.99", features = ["diff"] }
 
-[build-dependencies]
-version_check = "0.9"
-
 [features]
 default = ["syn-error"]
-syn-error = ["syn"]
+syn-error = ["dep:syn"]
+nightly = []
diff --git a/build.rs b/build.rs
deleted file mode 100644
index 35bbd6b..0000000
--- a/build.rs
+++ /dev/null
@@ -1,13 +0,0 @@
-fn main() {
-    println!("cargo:rustc-check-cfg=cfg(use_fallback)");
-    println!("cargo:rustc-check-cfg=cfg(skip_ui_tests)");
-    if !version_check::is_feature_flaggable().unwrap_or(false) {
-        println!("cargo:rustc-cfg=use_fallback");
-    }
-
-    if version_check::is_max_version("1.38.0").unwrap_or(false)
-        || !version_check::Channel::read().unwrap().is_stable()
-    {
-        println!("cargo:rustc-cfg=skip_ui_tests");
-    }
-}
diff --git a/proc-macro-error-attr/Cargo.toml b/proc-macro-error-attr/Cargo.toml
index f7fc17e..25fb52a 100644
--- a/proc-macro-error-attr/Cargo.toml
+++ b/proc-macro-error-attr/Cargo.toml
@@ -20,6 +20,3 @@ targets = ["x86_64-unknown-linux-gnu"]
 [dependencies]
 quote = "1"
 proc-macro2 = "1"
-
-[build-dependencies]
-version_check = "0.9"
diff --git a/proc-macro-error-attr/build.rs b/proc-macro-error-attr/build.rs
deleted file mode 100644
index fc547be..0000000
--- a/proc-macro-error-attr/build.rs
+++ /dev/null
@@ -1,6 +0,0 @@
-fn main() {
-    println!("cargo:rustc-check-cfg=cfg(always_assert_unwind)");
-    if version_check::is_max_version("1.36.0").unwrap_or(false) {
-        println!("cargo:rustc-cfg=always_assert_unwind");
-    }
-}
diff --git a/proc-macro-error-attr/src/lib.rs b/proc-macro-error-attr/src/lib.rs
index 89d5ac7..3cc8a25 100644
--- a/proc-macro-error-attr/src/lib.rs
+++ b/proc-macro-error-attr/src/lib.rs
@@ -81,7 +81,6 @@ fn impl_proc_macro_error(attr: TokenStream2, input: TokenStream2) -> Result<Toke
     Ok(res.into())
 }
 
-#[cfg(not(always_assert_unwind))]
 fn gen_body(block: TokenTree, settings: Settings) -> proc_macro2::TokenStream {
     let is_proc_macro_hack = settings.is_set(ProcMacroHack);
     let closure = if settings.is_set(AssertUnwindSafe) {
@@ -93,17 +92,6 @@ fn gen_body(block: TokenTree, settings: Settings) -> proc_macro2::TokenStream {
     quote!( ::proc_macro_error::entry_point(#closure, #is_proc_macro_hack) )
 }
 
-// FIXME:
-// proc_macro::TokenStream does not implement UnwindSafe until 1.37.0.
-// Considering this is the closure's return type the unwind safety check would fail
-// for virtually every closure possible, the check is meaningless.
-#[cfg(always_assert_unwind)]
-fn gen_body(block: TokenTree, settings: Settings) -> proc_macro2::TokenStream {
-    let is_proc_macro_hack = settings.is_set(ProcMacroHack);
-    let closure = quote!(::std::panic::AssertUnwindSafe(|| #block ));
-    quote!( ::proc_macro_error::entry_point(#closure, #is_proc_macro_hack) )
-}
-
 fn detect_proc_macro_hack(attrs: &[Attribute]) -> bool {
     attrs
         .iter()
diff --git a/src/imp/delegate.rs b/src/imp/delegate.rs
index 07def2b..a3e6a80 100644
--- a/src/imp/delegate.rs
+++ b/src/imp/delegate.rs
@@ -38,7 +38,6 @@ pub(crate) fn emit_diagnostic(diag: Diagnostic) {
             IS_DIRTY.with(|c| c.set(true));
             PLevel::Error
         }
-        _ => unreachable!(),
     };
 
     let mut res = PDiag::spanned(span, level, msg);
diff --git a/src/lib.rs b/src/lib.rs
index 6b168a7..c3aae8d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -271,7 +271,7 @@
 //! [`ToTokens`]: https://docs.rs/quote/1.0.3/quote/trait.ToTokens.html
 //!
 
-#![cfg_attr(not(use_fallback), feature(proc_macro_diagnostic))]
+#![cfg_attr(feature = "nightly", feature(proc_macro_diagnostic))]
 #![forbid(unsafe_code)]
 #![allow(clippy::needless_doctest_main)]
 
@@ -295,11 +295,11 @@ mod diagnostic;
 mod macros;
 mod sealed;
 
-#[cfg(use_fallback)]
+#[cfg(not(feature = "nightly"))]
 #[path = "imp/fallback.rs"]
 mod imp;
 
-#[cfg(not(use_fallback))]
+#[cfg(feature = "nightly")]
 #[path = "imp/delegate.rs"]
 mod imp;
 
diff --git a/tests/macro-errors.rs b/tests/macro-errors.rs
index bea9770..1addb7b 100644
--- a/tests/macro-errors.rs
+++ b/tests/macro-errors.rs
@@ -1,6 +1,7 @@
-#[cfg_attr(skip_ui_tests, ignore)]
 #[test]
 fn ui() {
     let t = trybuild::TestCases::new();
-    t.compile_fail("tests/ui/*.rs");
+    if !version_check::Channel::read().is_some_and(|c| c.is_nightly()) {
+        t.compile_fail("tests/ui/*.rs");
+    }
 }