Make ESP-IDF support optional

.github/workflows/tests.yml

@@ -380,6 +380,20 @@ jobs:
           RUSTFLAGS: -Dwarnings --cfg getrandom_backend="rdrand"
         run: cargo build -Z build-std=core --target=${{ matrix.target }}
 
+  build-esp-idf:
+    name: ESP-IDF Build
+    runs-on: ubuntu-22.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@nightly # Required to build libcore
+        with:
+          components: rust-src
+      - uses: Swatinem/rust-cache@v2
+      - nv:
+          RUSTFLAGS: -Dwarnings --cfg getrandom_backend="esp_idf"
+        run: cargo build -Z build-std=core --target=riscv32imc-esp-espidf
+
+
   build-no-atomics:
     name: No Atomics Build
     runs-on: ubuntu-22.04

.github/workflows/workspace.yml

@@ -34,6 +34,8 @@ jobs:
     - name: iOS (apple-other.rs)
       run: cargo clippy -Zbuild-std=core --target x86_64-apple-ios
     - name: ESP-IDF (espidf.rs)
+      env:
+        RUSTFLAGS: -Dwarnings --cfg getrandom_backend="esp_idf"
       run: cargo clippy -Zbuild-std=core --target riscv32imc-esp-espidf
     - name: Fuchsia (fuchsia.rs)
       run: cargo clippy -Zbuild-std=core --target x86_64-unknown-fuchsia

Cargo.toml

@@ -47,7 +47,7 @@ std = []
 [lints.rust.unexpected_cfgs]
 level = "warn"
 check-cfg = [
-  'cfg(getrandom_backend, values("custom", "rdrand", "linux_getrandom", "wasm_js"))',
+  'cfg(getrandom_backend, values("custom", "rdrand", "linux_getrandom", "wasm_js", "esp_idf"))',
   'cfg(getrandom_browser_test)',
 ]
 

src/lib.rs

@@ -170,6 +170,8 @@
 //! - `wasm_js`: use Web or Node.js APIs. Can be enabled only for OS-less
 //!   (i.e. `target_os = "unknown"`) WASM targets.
 //! - `rdrand`: use RDRAND instruction. Can be enabled only for x86 and x86-64 targets.
+//! - `esp_idf`: use [`esp_fill_random`]. Note that without a proper hardware configuration
+//!   it may return poor quality entropy. Can be enabled only for ESP-IDF trgets.
 //! - `custom`: use "custom" backend defined by an extern function.
 //!   See the "custom implementations" section for more information.
 //!
@@ -266,6 +268,10 @@ cfg_if! {
         )))]
         compile_error!("`wasm_js` backend can be enabled only on OS-less WASM targets!");
         #[path = "js.rs"] mod imp;
+    } else if #[cfg(getrandom_backend = "esp_idf")] {
+        #[cfg(not(target_os = "espidf"))]
+        compile_error!("`esp_idf` backend can be enabled only for ESP-IDF targets!");
+        #[path = "espidf.rs"] mod imp;
     } else if #[cfg(any(
         target_os = "haiku",
         target_os = "redox",
@@ -361,8 +367,6 @@ cfg_if! {
         #[path = "vxworks.rs"] mod imp;
     } else if #[cfg(target_os = "solid_asp3")] {
         #[path = "solid.rs"] mod imp;
-    } else if #[cfg(target_os = "espidf")] {
-        #[path = "espidf.rs"] mod imp;
     } else if #[cfg(all(windows, target_vendor = "win7"))] {
         #[path = "windows7.rs"] mod imp;
     } else if #[cfg(windows)] {