Use an em bikeshed instead of an underscore bikeshed.

stm32-metapac/Cargo.toml

@@ -17,7 +17,7 @@ regex = "1.5.4"
 
 [features]
 rt = ["cortex-m-rt/device"]
-memory_x = []
+memory-x = []
 
 # BEGIN GENERATED FEATURES
 # Generated by gen_features.py. DO NOT EDIT.

stm32-metapac/build.rs

@@ -28,7 +28,7 @@ fn main() {
         chip_name.to_ascii_lowercase()
     );
 
-    #[cfg(feature = "memory_x")]
+    #[cfg(feature = "memory-x")]
     println!(
         "cargo:rustc-link-search={}/src/chips/{}/memory_x/",
         out_dir.display(),