glib: Only optimize IntoGStr for String when capacity allows

Also add a bench to track the performance of GStr conversions.

Author: jf2048

glib/Cargo.toml

@@ -40,6 +40,7 @@ memchr = "2.5.0"
 tempfile = "3"
 gir-format-check = "^0.1"
 trybuild2 = "1"
+criterion = "0.4.0"
 
 [features]
 default = ["gio"]
@@ -65,3 +66,7 @@ features = ["dox"]
 [[test]]
 name = "subclass_compiletest"
 required-features = ["compiletests"]
+
+[[bench]]
+name = "gstring"
+harness = false

glib/benches/gstring.rs

@@ -0,0 +1,46 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use glib::IntoGStr;
+
+pub fn str_into_gstr(c: &mut Criterion) {
+    c.bench_function("str as IntoGStr", |b| {
+        b.iter(|| {
+            "abcdefg".run_with_gstr(|g| {
+                black_box(g);
+            })
+        })
+    });
+}
+
+pub fn string_into_gstr(c: &mut Criterion) {
+    c.bench_function("String as IntoGStr", |b| {
+        b.iter(|| {
+            let mut s = String::from("abcdefg");
+            s.shrink_to_fit();
+            assert_eq!(s.capacity(), s.len());
+            s.run_with_gstr(|g| {
+                black_box(g);
+            })
+        })
+    });
+}
+
+pub fn string_with_capacity_into_gstr(c: &mut Criterion) {
+    c.bench_function("String::with_capacity as IntoGStr", |b| {
+        b.iter(|| {
+            let mut s = String::with_capacity(100);
+            s.push_str("abcdefg");
+            assert!(s.capacity() > s.len());
+            s.run_with_gstr(|g| {
+                black_box(g);
+            })
+        })
+    });
+}
+
+criterion_group!(
+    benches,
+    str_into_gstr,
+    string_into_gstr,
+    string_with_capacity_into_gstr
+);
+criterion_main!(benches);

glib/src/gstring.rs

@@ -2318,9 +2318,17 @@ impl IntoGStr for &str {
 impl IntoGStr for String {
     #[inline]
     fn run_with_gstr<T, F: FnOnce(&GStr) -> T>(mut self, f: F) -> T {
-        self.push('\0');
-        let gs = unsafe { GStr::from_utf8_with_nul_unchecked(self.as_bytes()) };
-        f(gs)
+        let len = self.len();
+        if len < self.capacity() {
+            self.reserve_exact(1);
+            self.push('\0');
+            let gs = unsafe { GStr::from_utf8_with_nul_unchecked(self.as_bytes()) };
+            f(gs)
+        } else if len < MAX_STACK_ALLOCATION {
+            self.as_str().run_with_gstr(f)
+        } else {
+            f(GString::from(self).as_gstr())
+        }
     }
 }