Replace the test with a bench

As per toml-rs#349 (comment)

test-suite/Cargo.toml

@@ -5,6 +5,13 @@ authors = ["Alex Crichton <alex@alexcrichton.com>"]
 publish = false
 edition = "2018"
 
+[[bench]]
+name = "linear"
+harness = false
+
+[dependencies]
+bencher = "0.1"
+
 [dev-dependencies]
 toml = { path = ".." }
 serde = { version = "1.0", features = ["derive"] }

test-suite/benches/linear.rs

@@ -0,0 +1,35 @@
+// Regressoion test for https://github.com/alexcrichton/toml-rs/issues/342
+
+use bencher::{benchmark_group, benchmark_main, black_box, Bencher};
+use toml::Value;
+
+fn parse(bench: &mut Bencher, entries: usize, f: impl Fn(usize) -> String) {
+    let mut s = String::new();
+    for i in 0..entries {
+        s += &f(i);
+        s += "entry = 42\n"
+    }
+    let s = black_box(s);
+    bench.iter(|| {
+        black_box(s.parse::<Value>().unwrap());
+    })
+}
+
+fn map_10(bench: &mut Bencher) {
+    parse(bench, 10, |i| format!("[header_no_{}]\n", i))
+}
+
+fn map_100(bench: &mut Bencher) {
+    parse(bench, 100, |i| format!("[header_no_{}]\n", i))
+}
+
+fn array_10(bench: &mut Bencher) {
+    parse(bench, 10, |_i| "[[header]]\n".to_owned())
+}
+
+fn array_100(bench: &mut Bencher) {
+    parse(bench, 100, |_i| "[[header]]\n".to_owned())
+}
+
+benchmark_group!(benches, map_10, map_100, array_10, array_100);
+benchmark_main!(benches);