Simplify code for special case of annotation

Author: AnthonyMikh

src/librustc_errors/emitter.rs

@@ -622,18 +622,16 @@ impl EmitterWriter {
         // 3 | |
         // 4 | | }
         //   | |_^ test
-        if line.annotations.len() == 1 {
-            if let Some(ref ann) = line.annotations.get(0) {
-                if let AnnotationType::MultilineStart(depth) = ann.annotation_type {
-                    if source_string.chars().take(ann.start_col).all(|c| c.is_whitespace()) {
-                        let style = if ann.is_primary {
-                            Style::UnderlinePrimary
-                        } else {
-                            Style::UnderlineSecondary
-                        };
-                        buffer.putc(line_offset, width_offset + depth - 1, '/', style);
-                        return vec![(depth, style)];
-                    }
+        if let [ann] = &line.annotations[..] {
+            if let AnnotationType::MultilineStart(depth) = ann.annotation_type {
+                if source_string.chars().take(ann.start_col).all(|c| c.is_whitespace()) {
+                    let style = if ann.is_primary {
+                        Style::UnderlinePrimary
+                    } else {
+                        Style::UnderlineSecondary
+                    };
+                    buffer.putc(line_offset, width_offset + depth - 1, '/', style);
+                    return vec![(depth, style)];
                 }
             }
         }