[cargo-zerocopy] Pass --cfg __ZEROCOPY_INTERNAL_USE_ONLY_DEV_MODE (#2960)

This allows zerocopy code to detect when it's being compiled during
development or in CI. In this commit, we use this to disable compiling
the `doctests` module in production builds.

gherrit-pr-id: G2bd9f86557e00ed04dc9587f4d5f4b0dbb6513ad

Author: joshlf

build.rs

@@ -91,6 +91,7 @@ fn main() {
         println!(
             "cargo:rustc-check-cfg=cfg(__ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS)"
         );
+        println!("cargo:rustc-check-cfg=cfg(__ZEROCOPY_INTERNAL_USE_ONLY_DEV_MODE)");
         println!("cargo:rustc-check-cfg=cfg(coverage_nightly)");
     }
 

src/doctests.rs

@@ -8,7 +8,7 @@
 // those terms.
 
 #![cfg(feature = "derive")] // Required for derives on `SliceDst`
-#![allow(dead_code)]
+#![allow(dead_code, missing_docs, missing_debug_implementations, missing_copy_implementations)]
 
 //! Our UI test framework, built on the `trybuild` crate, does not support
 //! testing for post-monomorphization errors. Instead, we use doctests, which
@@ -18,7 +18,6 @@ use crate::*;
 
 #[derive(KnownLayout, FromBytes, IntoBytes, Immutable)]
 #[repr(C)]
-#[allow(missing_debug_implementations, missing_copy_implementations)]
 pub struct SliceDst<T, U> {
     pub t: T,
     pub u: [U],

src/lib.rs

@@ -338,7 +338,7 @@ pub mod byte_slice;
 pub mod byteorder;
 mod deprecated;
 
-#[doc(hidden)]
+#[cfg(__ZEROCOPY_INTERNAL_USE_ONLY_DEV_MODE)]
 pub mod doctests;
 
 // This module is `pub` so that zerocopy's error types and error handling

tools/cargo-zerocopy/src/main.rs

@@ -164,17 +164,21 @@ fn install_toolchain_or_exit(versions: &Versions, name: &str) -> Result<(), Erro
     Ok(())
 }
 
-fn get_rustflags(name: &str) -> &'static str {
+fn get_rustflags(name: &str) -> String {
     // See #1792 for context on zerocopy_derive_union_into_bytes.
+    let mut flags =
+        "--cfg zerocopy_derive_union_into_bytes --cfg __ZEROCOPY_INTERNAL_USE_ONLY_DEV_MODE"
+            .to_string();
+
     if name == "nightly" {
-        "--cfg __ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS --cfg zerocopy_derive_union_into_bytes "
-    } else {
-        "--cfg zerocopy_derive_union_into_bytes "
+        flags += " --cfg __ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS";
     }
+
+    flags
 }
 
 fn get_toolchain_rustflags(name: &str) -> String {
-    format!("--cfg __ZEROCOPY_TOOLCHAIN=\"{}\" ", name)
+    format!("--cfg __ZEROCOPY_TOOLCHAIN=\"{}\"", name)
 }
 
 fn rustup<'a>(args: impl IntoIterator<Item = &'a str>, env: Option<(&str, &str)>) -> Command {
@@ -228,7 +232,7 @@ fn delegate_cargo() -> Result<(), Error> {
                     .unwrap_or_default();
 
                 let rustflags = format!(
-                    "{}{}{}",
+                    "{} {} {}",
                     get_rustflags(name),
                     get_toolchain_rustflags(name),
                     env_rustflags,