Do not deny warnings in "fast" try builds

Cargo.lock

@@ -2567,7 +2567,6 @@ dependencies = [
  "humansize",
  "humantime",
  "log",
- "serde",
  "serde_json",
  "sysinfo",
  "tabled",

src/doc/rustc-dev-guide/src/tests/ci.md

@@ -135,12 +135,16 @@ There are several use-cases for try builds:
 - Run a specific CI job (e.g. Windows tests) on a PR, to quickly test if it
   passes the test suite executed by that job.
 
-You can select which CI jobs will
-be executed in the try build by adding lines containing `try-job:
-<job pattern>` to the PR description. All such specified jobs will be executed
-in the try build once the `@bors try` command is used on the PR. If no try
-jobs are specified in this way, the jobs defined in the `try` section of
-[`jobs.yml`] will be executed by default.
+By default, if you send a comment with `@bors try`, the jobs defined in the `try` section of
+[`jobs.yml`] will be executed. We call this mode a "fast try build". Such a try build
+will not execute any tests, and it will allow compilation warnings. It is useful when you want to
+get an optimized toolchain as fast as possible, for a crater run or performance benchmarks,
+even if it might not be working fully correctly.
+
+If you want to run a custom CI job in a try build and make sure that it passes all tests and does
+not produce any compilation warnings, you can select CI jobs to be executed by adding lines
+containing `try-job: <job pattern>` to the PR description. All such specified jobs will be executed
+in the try build once the `@bors try` command is used on the PR.
 
 Each pattern can either be an exact name of a job or a glob pattern that matches multiple jobs,
 for example `*msvc*` or `*-alt`. You can start at most 20 jobs in a single try build. When using

src/tools/opt-dist/Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "opt-dist"
 version = "0.1.0"
-edition = "2021"
+edition = "2024"
 
 [dependencies]
 build_helper = { path = "../../build_helper" }
@@ -15,7 +15,6 @@ fs_extra = "1"
 camino = "1"
 tar = "0.4"
 xz = { version = "0.1", package = "xz2" }
-serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 glob = "0.3"
 tempfile = "3.5"

src/tools/opt-dist/src/environment.rs

@@ -26,6 +26,7 @@ pub struct Environment {
     use_bolt: bool,
     shared_llvm: bool,
     run_tests: bool,
+    fast_try_build: bool,
 }
 
 impl Environment {
@@ -106,6 +107,10 @@ impl Environment {
     pub fn run_tests(&self) -> bool {
         self.run_tests
     }
+
+    pub fn is_fast_try_build(&self) -> bool {
+        self.fast_try_build
+    }
 }
 
 /// What is the extension of binary executables on this platform?

src/tools/opt-dist/src/exec.rs

@@ -113,13 +113,16 @@ impl Bootstrap {
             "library/std",
         ])
         .env("RUST_BACKTRACE", "full");
+        let cmd = add_shared_x_flags(env, cmd);
+
         Self { cmd, metrics_path }
     }
 
     pub fn dist(env: &Environment, dist_args: &[String]) -> Self {
         let metrics_path = env.build_root().join("build").join("metrics.json");
-        let cmd = cmd(&dist_args.iter().map(|arg| arg.as_str()).collect::<Vec<_>>())
-            .env("RUST_BACKTRACE", "full");
+        let args = dist_args.iter().map(|arg| arg.as_str()).collect::<Vec<_>>();
+        let cmd = cmd(&args).env("RUST_BACKTRACE", "full");
+        let cmd = add_shared_x_flags(env, cmd);
         Self { cmd, metrics_path }
     }
 
@@ -184,3 +187,7 @@ impl Bootstrap {
         Ok(())
     }
 }
+
+fn add_shared_x_flags(env: &Environment, cmd: CmdBuilder) -> CmdBuilder {
+    if env.is_fast_try_build() { cmd.arg("--set").arg("rust.deny-warnings=false") } else { cmd }
+}

src/tools/opt-dist/src/main.rs

@@ -95,7 +95,7 @@ enum EnvironmentCmd {
         #[arg(long)]
         benchmark_cargo_config: Vec<String>,
 
-        /// Perform tests after final build if it's not a try build
+        /// Perform tests after final build if it's not a fast try build
         #[arg(long)]
         run_tests: bool,
     },
@@ -111,11 +111,14 @@ enum EnvironmentCmd {
     },
 }
 
-fn is_try_build() -> bool {
+/// For a fast try build, we want to only build the bare minimum of components to get a
+/// working toolchain, and not run any tests.
+fn is_fast_try_build() -> bool {
     std::env::var("DIST_TRY_BUILD").unwrap_or_else(|_| "0".to_string()) != "0"
 }
 
 fn create_environment(args: Args) -> anyhow::Result<(Environment, Vec<String>)> {
+    let is_fast_try_build = is_fast_try_build();
     let (env, args) = match args.env {
         EnvironmentCmd::Local {
             target_triple,
@@ -144,6 +147,7 @@ fn create_environment(args: Args) -> anyhow::Result<(Environment, Vec<String>)>
                 .skipped_tests(skipped_tests)
                 .benchmark_cargo_config(benchmark_cargo_config)
                 .run_tests(run_tests)
+                .fast_try_build(is_fast_try_build)
                 .build()?;
 
             (env, shared.build_args)
@@ -167,6 +171,7 @@ fn create_environment(args: Args) -> anyhow::Result<(Environment, Vec<String>)>
                 .use_bolt(!is_aarch64)
                 .skipped_tests(vec![])
                 .run_tests(true)
+                .fast_try_build(is_fast_try_build)
                 .build()?;
 
             (env, shared.build_args)
@@ -187,6 +192,7 @@ fn create_environment(args: Args) -> anyhow::Result<(Environment, Vec<String>)>
                 .use_bolt(false)
                 .skipped_tests(vec![])
                 .run_tests(true)
+                .fast_try_build(is_fast_try_build)
                 .build()?;
 
             (env, shared.build_args)
@@ -350,9 +356,8 @@ fn execute_pipeline(
 
     // After dist has finished, run a subset of the test suite on the optimized artifacts to discover
     // possible regressions.
-    // The tests are not executed for try builds, which can be in various broken states, so we don't
-    // want to gatekeep them with tests.
-    if !is_try_build() && env.run_tests() {
+    // The tests are not executed for fast try builds, which can be broken and might not pass them.
+    if !is_fast_try_build() && env.run_tests() {
         timer.section("Run tests", |_| run_tests(env))?;
     }
 
@@ -361,7 +366,10 @@ fn execute_pipeline(
 
 fn main() -> anyhow::Result<()> {
     // Make sure that we get backtraces for easier debugging in CI
-    std::env::set_var("RUST_BACKTRACE", "1");
+    unsafe {
+        // SAFETY: we are the only thread running at this point
+        std::env::set_var("RUST_BACKTRACE", "1");
+    }
 
     env_logger::builder()
         .filter_level(LevelFilter::Info)
@@ -393,9 +401,9 @@ fn main() -> anyhow::Result<()> {
 
     let (env, mut build_args) = create_environment(args).context("Cannot create environment")?;
 
-    // Skip components that are not needed for try builds to speed them up
-    if is_try_build() {
-        log::info!("Skipping building of unimportant components for a try build");
+    // Skip components that are not needed for fast try builds to speed them up
+    if is_fast_try_build() {
+        log::info!("Skipping building of unimportant components for a fast try build");
         for target in [
             "rust-docs",
             "rustc-docs",