scripts: rust: replace length checks with match

Use a match expression with slice patterns instead of length checks and
indexing. The result is more idiomatic, which is a better example for
future Rust code authors.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20250529-idiomatic-match-slice-v2-1-4925ca2f1550@gmail.com
[ Reworded title. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: tamird

scripts/rustdoc_test_gen.rs

@@ -85,24 +85,23 @@ fn find_real_path<'a>(srctree: &Path, valid_paths: &'a mut Vec<PathBuf>, file: &
         }
     }
 
-    assert!(
-        valid_paths.len() > 0,
-        "No path candidates found for `{file}`. This is likely a bug in the build system, or some \
-        files went away while compiling."
-    );
-
-    if valid_paths.len() > 1 {
-        eprintln!("Several path candidates found:");
-        for path in valid_paths {
-            eprintln!("    {path:?}");
+    match valid_paths.as_slice() {
+        [] => panic!(
+            "No path candidates found for `{file}`. This is likely a bug in the build system, or \
+            some files went away while compiling."
+        ),
+        [valid_path] => valid_path.to_str().unwrap(),
+        valid_paths => {
+            eprintln!("Several path candidates found:");
+            for path in valid_paths {
+                eprintln!("    {path:?}");
+            }
+            panic!(
+                "Several path candidates found for `{file}`, please resolve the ambiguity by \
+                renaming a file or folder."
+            );
         }
-        panic!(
-            "Several path candidates found for `{file}`, please resolve the ambiguity by renaming \
-            a file or folder."
-        );
     }
-
-    valid_paths[0].to_str().unwrap()
 }
 
 fn main() {