Streamline collect_crate_types.

- The early return can be right at the top.
- The control flow is simplified with `if let`.
- The `collect` isn't necessary.

Author: nnethercote

compiler/rustc_interface/src/util.rs

@@ -482,27 +482,21 @@ fn categorize_crate_type(s: Symbol) -> Option<CrateType> {
 }
 
 pub fn collect_crate_types(session: &Session, attrs: &[ast::Attribute]) -> Vec<CrateType> {
-    // Unconditionally collect crate types from attributes to make them used
-    let attr_types: Vec<CrateType> = attrs
-        .iter()
-        .filter_map(|a| {
-            if a.has_name(sym::crate_type) {
-                match a.value_str() {
-                    Some(s) => categorize_crate_type(s),
-                    _ => None,
-                }
-            } else {
-                None
-            }
-        })
-        .collect();
-
     // If we're generating a test executable, then ignore all other output
     // styles at all other locations
     if session.opts.test {
         return vec![CrateType::Executable];
     }
 
+    // Unconditionally collect crate types from attributes to make them used
+    let attr_types = attrs.iter().filter_map(|a| {
+        if a.has_name(sym::crate_type) && let Some(s) = a.value_str() {
+            categorize_crate_type(s)
+        } else {
+            None
+        }
+    });
+
     // Only check command line flags if present. If no types are specified by
     // command line, then reuse the empty `base` Vec to hold the types that
     // will be found in crate attributes.