fix new location of assert_matches after last-minute stabilisation revert (rust-lang/rust#154999)

Author: MusicalNinjaDad

build.rs

@@ -6,6 +6,15 @@ fn main() {
     let ac = autocfg::new();
     stable_feature(&ac, "assert_matches");
 
+    autocfg::emit_possibility("assert_matches_in_module");
+    let code = r#"
+        #![feature(assert_matches)]
+        use std::assert_matches::assert_matches;
+    "#;
+    if ac.probe_raw(code).is_ok() {
+        autocfg::emit("assert_matches_in_module");
+    }
+
     stable_feature(&ac, "let_chains");
 
     stable_feature(&ac, "if_let_guard");

tests/test_usage.rs

@@ -4,10 +4,10 @@
 #![feature(try_trait_v2)]
 #![feature(try_trait_v2_residual)]
 
-#[cfg(not(stable_assert_matches))]
+#[cfg(assert_matches_in_module)]
 use std::assert_matches::assert_matches;
 
-#[cfg(stable_assert_matches)]
+#[cfg(not(assert_matches_in_module))]
 use std::assert_matches;
 
 use try_v2::{Try, Try_ConvertResult};