windows: always use RtlGenRandom on pre-1.78 Rust versions

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)] {