Add benchmark for source file analysis

x86 generic:

```
test analyze_source_file::tests::bench_empty_text     ... bench:          12.88 ns/iter (+/- 0.68)
test analyze_source_file::tests::bench_newlines_long  ... bench:         626.39 ns/iter (+/- 19.02)
test analyze_source_file::tests::bench_newlines_short ... bench:         155.36 ns/iter (+/- 8.00)
```

x86 SSE2:

```
test analyze_source_file::tests::bench_empty_text     ... bench:          14.48 ns/iter (+/- 0.34)
test analyze_source_file::tests::bench_newlines_long  ... bench:         236.33 ns/iter (+/- 5.35)
test analyze_source_file::tests::bench_newlines_short ... bench:         148.13 ns/iter (+/- 3.60)
```

Author: heiher

compiler/rustc_span/src/analyze_source_file/tests.rs

@@ -1,12 +1,15 @@
+extern crate test;
+use test::Bencher;
+
 use super::*;
 
 macro_rules! test {
-    (case: $test_name:ident,
+    (case: $name:ident,
      text: $text:expr,
      lines: $lines:expr,
      multi_byte_chars: $multi_byte_chars:expr,) => {
         #[test]
-        fn $test_name() {
+        fn ${concat(test_, $name)}() {
             let (lines, multi_byte_chars) = analyze_source_file($text);
 
             let expected_lines: Vec<RelativeBytePos> =
@@ -107,3 +110,34 @@ test!(
     lines: vec![0, 7, 27],
     multi_byte_chars: vec![(13, 2), (29, 2)],
 );
+
+macro_rules! bench {
+    (case: $name:ident,
+     text: $text:expr,
+     repeat: $repeat:expr,) => {
+        #[bench]
+        fn ${concat(bench_, $name)}(b: &mut Bencher) {
+            use std::hint::black_box;
+            let text = $text.repeat($repeat);
+            b.iter(|| black_box(analyze_source_file(&text)));
+        }
+    };
+}
+
+bench!(
+    case: empty_text,
+    text: "",
+    repeat: 1,
+);
+
+bench!(
+    case: newlines_short,
+    text: "a\nc",
+    repeat: 50,
+);
+
+bench!(
+    case: newlines_long,
+    text: "012345678\nabcdef012345678\na",
+    repeat: 50,
+);

compiler/rustc_span/src/lib.rs

@@ -24,11 +24,13 @@
 #![feature(cfg_select)]
 #![feature(core_io_borrowed_buf)]
 #![feature(if_let_guard)]
+#![feature(macro_metavar_expr_concat)]
 #![feature(map_try_insert)]
 #![feature(negative_impls)]
 #![feature(read_buf)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
+#![feature(test)]
 // tidy-alphabetical-end
 
 // The code produced by the `Encodable`/`Decodable` derive macros refer to