Rollup merge of #78244 - workingjubilee:dogfood-fancy-ranges, r=varkor

Dogfood {exclusive,half-open} ranges in compiler (nfc)

In particular, this allows us to write more explicit matches that
avoid the pitfalls of using a fully general fall-through case, yet
remain fairly ergonomic. Less logic is in guard cases, more is in
the actual exhaustive case analysis.

No functional changes.

compiler/rustc_session/src/filesearch.rs

@@ -153,14 +153,14 @@ fn find_libdir(sysroot: &Path) -> Cow<'static, str> {
     const SECONDARY_LIB_DIR: &str = "lib";
 
     match option_env!("CFG_LIBDIR_RELATIVE") {
-        Some(libdir) if libdir != "lib" => libdir.into(),
-        _ => {
+        None | Some("lib") => {
             if sysroot.join(PRIMARY_LIB_DIR).join(RUST_LIB_DIR).exists() {
                 PRIMARY_LIB_DIR.into()
             } else {
                 SECONDARY_LIB_DIR.into()
             }
         }
+        Some(libdir) => libdir.into(),
     }
 }