rollup merge of #23920: steveklabnik/gh23881

Fixes #23881

Author: alexcrichton

src/doc/trpl/SUMMARY.md

@@ -42,5 +42,6 @@
     * [Intrinsics](intrinsics.md)
     * [Lang items](lang-items.md)
     * [Link args](link-args.md)
+    * [Benchmark Tests](benchmark-tests.md)
 * [Conclusion](conclusion.md)
 * [Glossary](glossary.md)

src/doc/trpl/benchmark-tests.md

@@ -0,0 +1,152 @@
+% Benchmark tests
+
+Rust supports benchmark tests, which can test the performance of your
+code. Let's make our `src/lib.rs` look like this (comments elided):
+
+```{rust,ignore}
+#![feature(test)]
+
+extern crate test;
+
+pub fn add_two(a: i32) -> i32 {
+    a + 2
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use test::Bencher;
+
+    #[test]
+    fn it_works() {
+        assert_eq!(4, add_two(2));
+    }
+
+    #[bench]
+    fn bench_add_two(b: &mut Bencher) {
+        b.iter(|| add_two(2));
+    }
+}
+```
+
+Note the `test` feature gate, which enables this unstable feature.
+
+We've imported the `test` crate, which contains our benchmarking support.
+We have a new function as well, with the `bench` attribute. Unlike regular
+tests, which take no arguments, benchmark tests take a `&mut Bencher`. This
+`Bencher` provides an `iter` method, which takes a closure. This closure
+contains the code we'd like to benchmark.
+
+We can run benchmark tests with `cargo bench`:
+
+```bash
+$ cargo bench
+   Compiling adder v0.0.1 (file:///home/steve/tmp/adder)
+     Running target/release/adder-91b3e234d4ed382a
+
+running 2 tests
+test tests::it_works ... ignored
+test tests::bench_add_two ... bench:         1 ns/iter (+/- 0)
+
+test result: ok. 0 passed; 0 failed; 1 ignored; 1 measured
+```
+
+Our non-benchmark test was ignored. You may have noticed that `cargo bench`
+takes a bit longer than `cargo test`. This is because Rust runs our benchmark
+a number of times, and then takes the average. Because we're doing so little
+work in this example, we have a `1 ns/iter (+/- 0)`, but this would show
+the variance if there was one.
+
+Advice on writing benchmarks:
+
+
+* Move setup code outside the `iter` loop; only put the part you want to measure inside
+* Make the code do "the same thing" on each iteration; do not accumulate or change state
+* Make the outer function idempotent too; the benchmark runner is likely to run
+  it many times
+*  Make the inner `iter` loop short and fast so benchmark runs are fast and the
+   calibrator can adjust the run-length at fine resolution
+* Make the code in the `iter` loop do something simple, to assist in pinpointing
+  performance improvements (or regressions)
+
+## Gotcha: optimizations
+
+There's another tricky part to writing benchmarks: benchmarks compiled with
+optimizations activated can be dramatically changed by the optimizer so that
+the benchmark is no longer benchmarking what one expects. For example, the
+compiler might recognize that some calculation has no external effects and
+remove it entirely.
+
+```{rust,ignore}
+#![feature(test)]
+
+extern crate test;
+use test::Bencher;
+
+#[bench]
+fn bench_xor_1000_ints(b: &mut Bencher) {
+    b.iter(|| {
+        (0..1000).fold(0, |old, new| old ^ new);
+    });
+}
+```
+
+gives the following results
+
+```text
+running 1 test
+test bench_xor_1000_ints ... bench:         0 ns/iter (+/- 0)
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
+```
+
+The benchmarking runner offers two ways to avoid this. Either, the closure that
+the `iter` method receives can return an arbitrary value which forces the
+optimizer to consider the result used and ensures it cannot remove the
+computation entirely. This could be done for the example above by adjusting the
+`b.iter` call to
+
+```rust
+# struct X;
+# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
+b.iter(|| {
+    // note lack of `;` (could also use an explicit `return`).
+    (0..1000).fold(0, |old, new| old ^ new)
+});
+```
+
+Or, the other option is to call the generic `test::black_box` function, which
+is an opaque "black box" to the optimizer and so forces it to consider any
+argument as used.
+
+```rust
+#![feature(test)]
+
+extern crate test;
+
+# fn main() {
+# struct X;
+# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
+b.iter(|| {
+    let n = test::black_box(1000);
+
+    (0..n).fold(0, |a, b| a ^ b)
+})
+# }
+```
+
+Neither of these read or modify the value, and are very cheap for small values.
+Larger values can be passed indirectly to reduce overhead (e.g.
+`black_box(&huge_struct)`).
+
+Performing either of the above changes gives the following benchmarking results
+
+```text
+running 1 test
+test bench_xor_1000_ints ... bench:       131 ns/iter (+/- 3)
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
+```
+
+However, the optimizer can still modify a testcase in an undesirable manner
+even when using either of the above.

src/doc/trpl/testing.md

@@ -430,149 +430,3 @@ documentation tests: the `_0` is generated for the module test, and `add_two_0`
 for the function test. These will auto increment with names like `add_two_1` as
 you add more examples.
 
-# Benchmark tests
-
-Rust also supports benchmark tests, which can test the performance of your
-code. Let's make our `src/lib.rs` look like this (comments elided):
-
-```{rust,ignore}
-extern crate test;
-
-pub fn add_two(a: i32) -> i32 {
-    a + 2
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-    use test::Bencher;
-
-    #[test]
-    fn it_works() {
-        assert_eq!(4, add_two(2));
-    }
-
-    #[bench]
-    fn bench_add_two(b: &mut Bencher) {
-        b.iter(|| add_two(2));
-    }
-}
-```
-
-We've imported the `test` crate, which contains our benchmarking support.
-We have a new function as well, with the `bench` attribute. Unlike regular
-tests, which take no arguments, benchmark tests take a `&mut Bencher`. This
-`Bencher` provides an `iter` method, which takes a closure. This closure
-contains the code we'd like to benchmark.
-
-We can run benchmark tests with `cargo bench`:
-
-```bash
-$ cargo bench
-   Compiling adder v0.0.1 (file:///home/steve/tmp/adder)
-     Running target/release/adder-91b3e234d4ed382a
-
-running 2 tests
-test tests::it_works ... ignored
-test tests::bench_add_two ... bench:         1 ns/iter (+/- 0)
-
-test result: ok. 0 passed; 0 failed; 1 ignored; 1 measured
-```
-
-Our non-benchmark test was ignored. You may have noticed that `cargo bench`
-takes a bit longer than `cargo test`. This is because Rust runs our benchmark
-a number of times, and then takes the average. Because we're doing so little
-work in this example, we have a `1 ns/iter (+/- 0)`, but this would show
-the variance if there was one.
-
-Advice on writing benchmarks:
-
-
-* Move setup code outside the `iter` loop; only put the part you want to measure inside
-* Make the code do "the same thing" on each iteration; do not accumulate or change state
-* Make the outer function idempotent too; the benchmark runner is likely to run
-  it many times
-*  Make the inner `iter` loop short and fast so benchmark runs are fast and the
-   calibrator can adjust the run-length at fine resolution
-* Make the code in the `iter` loop do something simple, to assist in pinpointing
-  performance improvements (or regressions)
-
-## Gotcha: optimizations
-
-There's another tricky part to writing benchmarks: benchmarks compiled with
-optimizations activated can be dramatically changed by the optimizer so that
-the benchmark is no longer benchmarking what one expects. For example, the
-compiler might recognize that some calculation has no external effects and
-remove it entirely.
-
-```{rust,ignore}
-extern crate test;
-use test::Bencher;
-
-#[bench]
-fn bench_xor_1000_ints(b: &mut Bencher) {
-    b.iter(|| {
-        (0..1000).fold(0, |old, new| old ^ new);
-    });
-}
-```
-
-gives the following results
-
-```text
-running 1 test
-test bench_xor_1000_ints ... bench:         0 ns/iter (+/- 0)
-
-test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
-```
-
-The benchmarking runner offers two ways to avoid this. Either, the closure that
-the `iter` method receives can return an arbitrary value which forces the
-optimizer to consider the result used and ensures it cannot remove the
-computation entirely. This could be done for the example above by adjusting the
-`b.iter` call to
-
-```rust
-# struct X;
-# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
-b.iter(|| {
-    // note lack of `;` (could also use an explicit `return`).
-    (0..1000).fold(0, |old, new| old ^ new)
-});
-```
-
-Or, the other option is to call the generic `test::black_box` function, which
-is an opaque "black box" to the optimizer and so forces it to consider any
-argument as used.
-
-```rust
-# #![feature(test)]
-
-extern crate test;
-
-# fn main() {
-# struct X;
-# impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
-b.iter(|| {
-    let n = test::black_box(1000);
-
-    (0..n).fold(0, |a, b| a ^ b)
-})
-# }
-```
-
-Neither of these read or modify the value, and are very cheap for small values.
-Larger values can be passed indirectly to reduce overhead (e.g.
-`black_box(&huge_struct)`).
-
-Performing either of the above changes gives the following benchmarking results
-
-```text
-running 1 test
-test bench_xor_1000_ints ... bench:       131 ns/iter (+/- 3)
-
-test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured
-```
-
-However, the optimizer can still modify a testcase in an undesirable manner
-even when using either of the above.