Auto merge of #10920 - blyxyas:speedtest, r=llogiq

Add `SPEEDTEST`

In the `master` branch, we currently don't have any way to test the performance of a single lint in changes.
This PR adds `SPEEDTEST`, the environment variable which lets you do a speed test on a lint / category of tests with various configuration options.

Maybe we should merge this with `lintcheck` 🤔
See the book page for more information.

changelog:none

book/src/development/speedtest.md

@@ -0,0 +1,24 @@
+# Speedtest
+`SPEEDTEST` is the tool we use to measure lint's performance, it works by executing the same test several times.
+
+It's useful for measuring changes to current lints and deciding if the performance changes too much. `SPEEDTEST` is
+accessed by the `SPEEDTEST` (and `SPEEDTEST_*`) environment variables.
+
+## Checking Speedtest
+
+To do a simple speed test of a lint (e.g. `allow_attributes`), use this command.
+
+```sh
+$ SPEEDTEST=ui TESTNAME="allow_attributes" cargo uitest -- --nocapture
+```
+
+This will test all `ui` tests (`SPEEDTEST=ui`) whose names start with `allow_attributes`. By default, `SPEEDTEST` will
+iterate your test 1000 times. But you can change this with `SPEEDTEST_ITERATIONS`.
+
+```sh
+$ SPEEDTEST=toml SPEEDTEST_ITERATIONS=100 TESTNAME="semicolon_block" cargo uitest -- --nocapture
+```
+
+> **WARNING**: Be sure to use `-- --nocapture` at the end of the command to see the average test time. If you don't
+> use `-- --nocapture` (e.g. `SPEEDTEST=ui` `TESTNAME="let_underscore_untyped" cargo uitest -- --nocapture`), this
+> will not show up.

tests/compile-test.rs

@@ -217,12 +217,45 @@ fn main() {
     }
 
     set_var("CLIPPY_DISABLE_DOCS_LINKS", "true");
+    // The SPEEDTEST_* env variables can be used to check Clippy's performance on your PR. It runs the
+    // affected test 1000 times and gets the average.
+    if let Ok(speedtest) = std::env::var("SPEEDTEST") {
+        println!("----------- STARTING SPEEDTEST -----------");
+        let f = match speedtest.as_str() {
+            "ui" => run_ui as fn(),
+            "cargo" => run_ui_cargo as fn(),
+            "toml" => run_ui_toml as fn(),
+            "internal" => run_internal_tests as fn(),
+            "rustfix-coverage-known-exceptions-accuracy" => rustfix_coverage_known_exceptions_accuracy as fn(),
+            "ui-cargo-toml-metadata" => ui_cargo_toml_metadata as fn(),
+
+            _ => panic!("unknown speedtest: {speedtest} || accepted speedtests are: [ui, cargo, toml, internal]"),
+        };
+
+        let iterations;
+        if let Ok(iterations_str) = std::env::var("SPEEDTEST_ITERATIONS") {
+            iterations = iterations_str
+                .parse::<u64>()
+                .unwrap_or_else(|_| panic!("Couldn't parse `{iterations_str}`, please use a valid u64"));
+        } else {
+            iterations = 1000;
+        }
+
+        let mut sum = 0;
+        for _ in 0..iterations {
+            let start = std::time::Instant::now();
+            f();
+            sum += start.elapsed().as_millis();
+        }
+        println!("average {} time: {} millis.", speedtest.to_uppercase(), sum / 1000);
+    } else {
         run_ui();
         run_ui_toml();
         run_ui_cargo();
         run_internal_tests();
         rustfix_coverage_known_exceptions_accuracy();
         ui_cargo_toml_metadata();
+    }
 }
 
 const RUSTFIX_COVERAGE_KNOWN_EXCEPTIONS: &[&str] = &[