windows: always use RtlGenRandom on pre-1.78 Rust versions (#610)

This PR adds detection of Rust compiler version in the build script
while compiling the crate for Windows targets.

If the version is smaller than 1.78, then we switch to the `windows7`
backend for all Windows targets. This is necessary because on those
older versions we can not disambiguate between win7 and win10 based
on the target triplet.

Author: newpavlov

CHANGELOG.md

@@ -4,6 +4,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.3.2] - UNRELEASED
+
+### Changed
+- Always use `RtlGenRandom` on Windows targets when compiling with pre-1.78 Rust [#610]
+
+[#610]: https://github.com/rust-random/getrandom/pull/610
+
 ## [0.3.1] - 2025-01-28
 
 ### Fixed
@@ -526,6 +533,8 @@ Publish initial implementation.
 ## [0.0.0] - 2019-01-19
 Publish an empty template library.
 
+[0.3.2]: https://github.com/rust-random/getrandom/compare/v0.3.0...v0.3.2
+[0.3.1]: https://github.com/rust-random/getrandom/compare/v0.3.0...v0.3.1
 [0.3.0]: https://github.com/rust-random/getrandom/compare/v0.2.15...v0.3.0
 [0.2.15]: https://github.com/rust-random/getrandom/compare/v0.2.14...v0.2.15
 [0.2.14]: https://github.com/rust-random/getrandom/compare/v0.2.13...v0.2.14

Cargo.toml

@@ -67,7 +67,7 @@ libc = { version = "0.2.154", default-features = false }
 wasi = { version = "0.14", default-features = false }
 
 # windows7
-[target.'cfg(all(windows, not(target_vendor = "win7")))'.dependencies]
+[target.'cfg(all(windows, not(target_vendor = "win7"), not(getrandom_windows_legacy)))'.dependencies]
 windows-targets = "0.53"
 
 # wasm_js
@@ -83,6 +83,7 @@ level = "warn"
 check-cfg = [
   'cfg(getrandom_backend, values("custom", "rdrand", "rndr", "linux_getrandom", "wasm_js"))',
   'cfg(getrandom_msan)',
+  'cfg(getrandom_windows_legacy)',
   'cfg(getrandom_test_linux_fallback)',
   'cfg(getrandom_test_linux_without_fallback)',
   'cfg(getrandom_test_netbsd_fallback)',

README.md

@@ -43,7 +43,7 @@ fn get_random_u128() -> Result<u128, getrandom::Error> {
 | Target             | Target Triple      | Implementation
 | ------------------ | ------------------ | --------------
 | Linux, Android     | `*‑linux‑*`        | [`getrandom`][1] system call if available, otherwise [`/dev/urandom`][2] after successfully polling `/dev/random`
-| Windows 10+        | `*‑windows‑*`      | [`ProcessPrng`]
+| Windows 10+        | `*‑windows‑*`      | [`ProcessPrng`] on Rust 1.78+, [`RtlGenRandom`] otherwise
 | Windows 7, 8       | `*-win7‑windows‑*` | [`RtlGenRandom`]
 | macOS              | `*‑apple‑darwin`   | [`getentropy`][3]
 | iOS, tvOS, watchOS | `*‑apple‑{ios,tvos,watchos}` | [`CCRandomGenerateBytes`]

build.rs

@@ -1,9 +1,57 @@
-// Automatically detect cfg(sanitize = "memory") even if cfg(sanitize) isn't
-// supported. Build scripts get cfg() info, even if the cfg is unstable.
+use std::{env, ffi::OsString, process::Command};
+
+/// Tries to get the minor version of the Rust compiler in use.
+/// If it fails for any reason, returns `None`.
+///
+/// Based on the `rustc_version` crate.
+fn rustc_minor_version() -> Option<u64> {
+    let rustc = env::var_os("RUSTC").unwrap_or_else(|| OsString::from("rustc"));
+    let mut cmd = if let Some(wrapper) = env::var_os("RUSTC_WRAPPER").filter(|w| !w.is_empty()) {
+        let mut cmd = Command::new(wrapper);
+        cmd.arg(rustc);
+        cmd
+    } else {
+        Command::new(rustc)
+    };
+
+    let out = cmd.arg("-vV").output().ok()?;
+
+    if !out.status.success() {
+        return None;
+    }
+
+    let stdout = std::str::from_utf8(&out.stdout).ok()?;
+
+    // Assumes that the first line contains "rustc 1.xx.0-channel (abcdef 2025-01-01)"
+    // where "xx" is the minor version which we want to extract
+    let mut lines = stdout.lines();
+    let first_line = lines.next()?;
+    let minor_ver_str = first_line.split(".").nth(1)?;
+    minor_ver_str.parse().ok()
+}
+
 fn main() {
+    // Automatically detect cfg(sanitize = "memory") even if cfg(sanitize) isn't
+    // supported. Build scripts get cfg() info, even if the cfg is unstable.
     println!("cargo:rerun-if-changed=build.rs");
     let santizers = std::env::var("CARGO_CFG_SANITIZE").unwrap_or_default();
     if santizers.contains("memory") {
         println!("cargo:rustc-cfg=getrandom_msan");
     }
+
+    // Use `RtlGenRandom` on older compiler versions since win7 targets
+    // were introduced only in Rust 1.78
+    let target_family = env::var_os("CARGO_CFG_TARGET_FAMILY").and_then(|f| f.into_string().ok());
+    if target_family.as_deref() == Some("windows") {
+        /// Minor version of the Rust compiler in which win7 targets were inroduced
+        const WIN7_INTRODUCED_MINOR_VER: u64 = 78;
+
+        match rustc_minor_version() {
+            Some(minor_ver) if minor_ver < WIN7_INTRODUCED_MINOR_VER => {
+                println!("cargo:rustc-cfg=getrandom_windows_legacy");
+            }
+            None => println!("cargo:warning=Couldn't detect minor version of the Rust compiler"),
+            _ => {}
+        }
+    }
 }

src/backends.rs

@@ -145,7 +145,7 @@ cfg_if! {
     } else if #[cfg(target_os = "solid_asp3")] {
         mod solid;
         pub use solid::*;
-    } else if #[cfg(all(windows, target_vendor = "win7"))] {
+    } else if #[cfg(all(windows, any(target_vendor = "win7", getrandom_windows_legacy)))] {
         mod windows7;
         pub use windows7::*;
     } else if #[cfg(windows)] {