refactor: reduce allocations in no_mentions

This patch adds the below optimisations:

- Allocate a single formatted string for the list of commits instead of
  a mutable vector which may possibly be resized at runtime.
- Use `String::from` instead of `format!` for the warning string.
- Eliminate the `mentions_in_commits_warn` function which moved the
  vector in the parameter and allocated a new string.

Signed-off-by: Prajwal S N <prajwalnadig21@gmail.com>

Author: snprajwal

src/handlers/check_commits/no_mentions.rs

@@ -1,37 +1,29 @@
 //! Purpose: When opening a PR, or pushing new changes, check for github mentions
 //! in commits and notify the user of our no-mentions in commits policy.
 
-use std::fmt::Write;
-
 use crate::{config::NoMentionsConfig, github::GithubCommit};
 
 pub(super) fn mentions_in_commits(
     _conf: &NoMentionsConfig,
     commits: &[GithubCommit],
 ) -> Option<String> {
-    let mut mentions_commits = Vec::new();
-
-    for commit in commits {
-        if !parser::get_mentions(&commit.commit.message).is_empty() {
-            mentions_commits.push(&*commit.sha);
-        }
-    }
+    let mentions_commits = commits
+        .into_iter()
+        .filter(|c| !parser::get_mentions(&c.commit.message).is_empty())
+        .map(|c| format!("    - {}\n", c.sha))
+        .collect::<String>();
 
     if mentions_commits.is_empty() {
         None
     } else {
-        Some(mentions_in_commits_warn(mentions_commits))
-    }
-}
-
-fn mentions_in_commits_warn(commits: Vec<&str>) -> String {
-    let mut warning = format!("There are username mentions (such as `@user`) in the commit messages of the following commits.\n  *Please remove the mentions to avoid spamming these users.*\n");
-
-    for commit in commits {
-        let _ = writeln!(warning, "    - {commit}");
+        Some(
+            String::from(
+                r"There are username mentions (such as `@user`) in the commit messages of the following commits.
+  *Please remove the mentions to avoid spamming these users.*
+",
+            ) + &mentions_commits,
+        )
     }
-
-    warning
 }
 
 #[test]
@@ -69,10 +61,10 @@ fn test_mentions_in_commits() {
     assert_eq!(
         mentions_in_commits(&NoMentionsConfig {}, &commits),
         Some(
-            r#"There are username mentions (such as `@user`) in the commit messages of the following commits.
+            r"There are username mentions (such as `@user`) in the commit messages of the following commits.
   *Please remove the mentions to avoid spamming these users.*
     - d7daa17bc97df9377640b0d33cbd0bbeed703c3a
-"#.to_string()
+".to_string()
         )
     );
 }