perf: use string preallocations for string concatenation

Author: nik-rev

helix-stdx/src/lib.rs

@@ -3,6 +3,7 @@ pub mod faccess;
 pub mod path;
 pub mod range;
 pub mod rope;
+pub mod str;
 pub mod time;
 
 pub use range::Range;

helix-stdx/src/str.rs

@@ -0,0 +1,18 @@
+/// Concatenates strings together.
+///
+/// `concat!(a, " ", b, " ", c)` is:
+/// - more performant than `format!("{a} {b} {c}")`
+/// - more ergonomic than using `String::with_capacity` followed by a series of `String::push_str`
+#[macro_export]
+macro_rules! concat {
+    ($($value:expr),*) => {{
+        // Rust does not allow using `+` as separator between value
+        // so we must add that at the end of everything. The `0` is necessary
+        // at the end so it does not end with "+ " (which would be invalid syntax)
+        let mut buf = String::with_capacity($($value.len() + )* 0);
+        $(
+            buf.push_str(&$value);
+        )*
+        buf
+    }}
+}

helix-stdx/src/time.rs

@@ -71,5 +71,5 @@ pub fn format_relative_time(timestamp: i64, timezone_offset: i32) -> String {
         "from now"
     };
 
-    format!("{value} {unit} {label}")
+    crate::concat!(value, " ", unit, " ", label)
 }