Improve performance of String::from_str_in

benches/benches.rs

@@ -89,6 +89,13 @@ fn format_realloc(bump: &bumpalo::Bump, n: usize) {
     criterion::black_box(s);
 }
 
+#[cfg(feature = "collections")]
+fn string_from_str_in(bump: &bumpalo::Bump, str: &str) {
+    let str = criterion::black_box(str);
+    let s = bumpalo::collections::string::String::from_str_in(str, bump);
+    criterion::black_box(s);
+}
+
 #[cfg(feature = "collections")]
 fn string_push_str(bump: &bumpalo::Bump, str: &str) {
     let str = criterion::black_box(str);
@@ -210,6 +217,21 @@ fn bench_format_realloc(c: &mut Criterion) {
     }
 }
 
+fn bench_string_from_str_in(c: &mut Criterion) {
+    let len: usize = 16;
+
+    let mut group = c.benchmark_group("alloc");
+    group.throughput(Throughput::Elements(len as u64));
+    group.bench_function("from_str_in", |b| {
+        let mut bump = bumpalo::Bump::with_capacity(len);
+        let str = "x".repeat(len);
+        b.iter(|| {
+            bump.reset();
+            string_from_str_in(&bump, &*str);
+        });
+    });
+}
+
 fn bench_string_push_str(c: &mut Criterion) {
     let len: usize = 16 * 1024; // 16 KiB
 
@@ -236,6 +258,7 @@ criterion_group!(
     bench_try_alloc_try_with,
     bench_try_alloc_try_with_err,
     bench_format_realloc,
+    bench_string_from_str_in,
     bench_string_push_str
 );
 criterion_main!(benches);

src/collections/string.rs

@@ -680,8 +680,19 @@ impl<'bump> String<'bump> {
     /// assert_eq!(s, "hello");
     /// ```
     pub fn from_str_in(s: &str, bump: &'bump Bump) -> String<'bump> {
-        let mut t = String::with_capacity_in(s.len(), bump);
-        t.push_str(s);
+        let len = s.len();
+        let mut t = String::with_capacity_in(len, bump);
+        // SAFETY:
+        // * `src` is valid for reads of `s.len()` bytes by virtue of being an allocated `&str`.
+        // * `dst` is valid for writes of `s.len()` bytes as `String::with_capacity_in(s.len(), bump)`
+        //   above guarantees that.
+        // * Alignment is not relevant as `u8` has no alignment requirements.
+        // * Source and destination ranges cannot overlap as we just reserved the destination
+        //   range from the bump.
+        unsafe { ptr::copy_nonoverlapping(s.as_ptr(), t.vec.as_mut_ptr(), len) };
+        // SAFETY: We reserved sufficent capacity for the string above.
+        // The elements at `0..len` were initialized by `copy_nonoverlapping` above.
+        unsafe { t.vec.set_len(len) };
         t
     }