test-float-parse: Be more accurate about optimization warning

Currently we warn if `debug_assertions` is set, but this isn't always
accurate because debug assertions can be on at higher optimization
levels. Forward some information from the build script and print it to
tell a better story, and update the warning to act on opt-level and
profile rather than `debug_assertions`.

Author: tgross35

Cargo.toml

@@ -77,7 +77,8 @@ strip = true
 debug = 0
 strip = true
 
-# Bigint libraries are slow without optimization, speed up testing
+# Bigint libraries are slow without optimization, speed up testing. Note that this doesn't affect
+# its dependencies, it should still be run in release mode where possible.
 [profile.dev.package.test-float-parse]
 opt-level = 3
 
@@ -92,4 +93,3 @@ codegen-units = 1
 # If you want to use a crate with local modifications, you can set a path or git dependency here.
 # For git dependencies, also add your source to ALLOWED_SOURCES in src/tools/tidy/src/extdeps.rs.
 #[patch.crates-io]
-

src/tools/test-float-parse/build.rs

@@ -0,0 +1,9 @@
+use std::env;
+
+fn main() {
+    // Forward the opt level so we can warn if the tests are going to be slow.
+    let opt = env::var("OPT_LEVEL").expect("OPT_LEVEL unset");
+    let profile = env::var("PROFILE").expect("PROFILE unset");
+    println!("cargo::rustc-env=OPT_LEVEL={opt}");
+    println!("cargo::rustc-env=PROFILE={profile}");
+}

src/tools/test-float-parse/src/main.rs

@@ -39,12 +39,24 @@ enum ArgMode {
 }
 
 fn main() -> ExitCode {
-    if cfg!(debug_assertions) {
+    let opt = env!("OPT_LEVEL");
+    let profile = env!("PROFILE");
+
+    println!("Crate config:");
+    println!("    opt level: {opt}");
+    println!("    profile: {profile}");
+    println!("    debug assertions: {}", cfg!(debug_assertions));
+
+    if !matches!(opt, "2" | "3") || profile != "release" {
+        // Warn for `profile != release` because optimizations being on for this crate (always,
+        // via the workspace Cargo.toml)  may not mean they are on for dependencies.
         println!(
-            "WARNING: running in debug mode. Release mode is recommended to reduce test duration."
+            "WARNING: test-float-parse may be running unoptimized or with unoptimized \
+            dependencies. Release mode is recommended to reduce test duration."
         );
         std::thread::sleep(Duration::from_secs(2));
     }
+    println!("Running with debug assertions={}", cfg!(debug_assertions));
 
     let args: Vec<_> = std::env::args().skip(1).collect();
     if args.iter().any(|arg| arg == "--help" || arg == "-h") {