Rollup merge of rust-lang#96029 - IsakNyberg:error-messages-fix, r=Dylan-DPC

Refactor loop into iterator; simplify negation logic.

is_dummy should return when a non-dummy is found, but instead is iterated until completion. With some inspiration from line 323 this was refactored to a single line that returns once a single counterexample is found.

Author: Dylan-DPC

compiler/rustc_error_messages/src/lib.rs

@@ -338,18 +338,12 @@ impl MultiSpan {
 
     /// Returns `true` if any of the primary spans are displayable.
     pub fn has_primary_spans(&self) -> bool {
-        self.primary_spans.iter().any(|sp| !sp.is_dummy())
+        !self.is_dummy()
     }
 
     /// Returns `true` if this contains only a dummy primary span with any hygienic context.
     pub fn is_dummy(&self) -> bool {
-        let mut is_dummy = true;
-        for span in &self.primary_spans {
-            if !span.is_dummy() {
-                is_dummy = false;
-            }
-        }
-        is_dummy
+        self.primary_spans.iter().all(|sp| sp.is_dummy())
     }
 
     /// Replaces all occurrences of one Span with another. Used to move `Span`s in areas that don't