Fix build error with targets that do not support atomic CAS operations

Some no-std targets (e.g., ARMv6-M) do not support atomic CAS
operations and cannot use `Arc`, etc. This patch detects those targets
using the `TARGET` environment variables provided by cargo for the
build script, and a list of targets that do not support atomic CAS
operations.

.github/workflows/ci.yml

@@ -26,9 +26,7 @@ jobs:
     strategy:
       matrix:
         target:
-          # TODO: Currently, valuable cannot build with targets that do not have atomic CAS.
-          #       We should port https://github.com/rust-lang/futures-rs/pull/2400.
-          # - thumbv6m-none-eabi
+          - thumbv6m-none-eabi
           - thumbv7m-none-eabi
     runs-on: ubuntu-latest
     steps:
@@ -59,6 +57,19 @@ jobs:
   #       run: cargo install cargo-hack
   #     - run: cargo hack build --workspace --feature-powerset --no-dev-deps
 
+  # When this job failed, run ci/no_atomic_cas.sh and commit result changes.
+  # TODO(taiki-e): Ideally, this should be automated using a bot that creates
+  #                PR when failed, but there is no bandwidth to implement it
+  #                right now...
+  codegen:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install Rust
+        run: rustup update nightly && rustup default nightly
+      - run: ci/no_atomic_cas.sh
+      - run: git diff --exit-code
+
   fmt:
     runs-on: ubuntu-latest
     steps:

ci/no_atomic_cas.sh

@@ -0,0 +1,28 @@
+#!/bin/bash
+
+# Update the list of targets that do not support atomic CAS operations.
+#
+# Usage:
+#    ./ci/no_atomic_cas.sh
+
+set -euo pipefail
+IFS=$'\n\t'
+
+cd "$(cd "$(dirname "$0")" && pwd)"/..
+
+file="valuable/no_atomic_cas.rs"
+
+{
+    echo "// This file is @generated by $(basename "$0")."
+    echo "// It is not intended for manual editing."
+    echo ""
+    echo "const NO_ATOMIC_CAS_TARGETS: &[&str] = &["
+} >"$file"
+
+for target in $(rustc --print target-list); do
+    res=$(rustc --print target-spec-json -Z unstable-options --target "$target" \
+        | jq -r "select(.\"atomic-cas\" == false)")
+    [[ -z "$res" ]] || echo "    \"$target\"," >>"$file"
+done
+
+echo "];" >>"$file"

valuable/build.rs

@@ -0,0 +1,41 @@
+#![warn(rust_2018_idioms, single_use_lifetimes)]
+
+use std::env;
+
+include!("no_atomic_cas.rs");
+
+// The rustc-cfg listed below are considered public API, but it is *unstable*
+// and outside of the normal semver guarantees:
+//
+// - `valuable_no_atomic_cas`
+//      Assume the target does *not* support atomic CAS operations.
+//      This is usually detected automatically by the build script, but you may
+//      need to enable it manually when building for custom targets or using
+//      non-cargo build systems that don't run the build script.
+//
+// With the exceptions mentioned above, the rustc-cfg strings below are
+// *not* public API. Please let us know by opening a GitHub issue if your build
+// environment requires some way to enable these cfgs other than by executing
+// our build script.
+fn main() {
+    let target = match env::var("TARGET") {
+        Ok(target) => target,
+        Err(e) => {
+            println!(
+                "cargo:warning=valuable: unable to get TARGET environment variable: {}",
+                e
+            );
+            return;
+        }
+    };
+
+    // Note that this is `no_*`, not `has_*`. This allows treating
+    // `cfg(target_has_atomic = "ptr")` as true when the build script doesn't
+    // run. This is needed for compatibility with non-cargo build systems that
+    // don't run the build script.
+    if NO_ATOMIC_CAS_TARGETS.contains(&&*target) {
+        println!("cargo:rustc-cfg=valuable_no_atomic_cas");
+    }
+
+    println!("cargo:rerun-if-changed=no_atomic_cas.rs");
+}

valuable/no_atomic_cas.rs

@@ -0,0 +1,11 @@
+// This file is @generated by no_atomic_cas.sh.
+// It is not intended for manual editing.
+
+const NO_ATOMIC_CAS_TARGETS: &[&str] = &[
+    "avr-unknown-gnu-atmega328",
+    "msp430-none-elf",
+    "riscv32i-unknown-none-elf",
+    "riscv32imc-unknown-none-elf",
+    "thumbv4t-none-eabi",
+    "thumbv6m-none-eabi",
+];

valuable/src/enumerable.rs

@@ -213,6 +213,7 @@ impl<E: ?Sized + Enumerable> Enumerable for alloc::rc::Rc<E> {
     }
 }
 
+#[cfg(not(valuable_no_atomic_cas))]
 #[cfg(feature = "alloc")]
 impl<E: ?Sized + Enumerable> Enumerable for alloc::sync::Arc<E> {
     fn definition(&self) -> EnumDef<'_> {

valuable/src/listable.rs

@@ -26,6 +26,7 @@ impl<L: ?Sized + Listable> Listable for alloc::rc::Rc<L> {
     }
 }
 
+#[cfg(not(valuable_no_atomic_cas))]
 #[cfg(feature = "alloc")]
 impl<L: ?Sized + Listable> Listable for alloc::sync::Arc<L> {
     fn size_hint(&self) -> (usize, Option<usize>) {

valuable/src/mappable.rs

@@ -26,6 +26,7 @@ impl<M: ?Sized + Mappable> Mappable for alloc::rc::Rc<M> {
     }
 }
 
+#[cfg(not(valuable_no_atomic_cas))]
 #[cfg(feature = "alloc")]
 impl<M: ?Sized + Mappable> Mappable for alloc::sync::Arc<M> {
     fn size_hint(&self) -> (usize, Option<usize>) {

valuable/src/structable.rs

@@ -108,6 +108,7 @@ impl<S: ?Sized + Structable> Structable for alloc::rc::Rc<S> {
     }
 }
 
+#[cfg(not(valuable_no_atomic_cas))]
 #[cfg(feature = "alloc")]
 impl<S: ?Sized + Structable> Structable for alloc::sync::Arc<S> {
     fn definition(&self) -> StructDef<'_> {

valuable/src/valuable.rs

@@ -69,6 +69,7 @@ impl<V: ?Sized + Valuable> Valuable for alloc::rc::Rc<V> {
     }
 }
 
+#[cfg(not(valuable_no_atomic_cas))]
 #[cfg(feature = "alloc")]
 impl<V: ?Sized + Valuable> Valuable for alloc::sync::Arc<V> {
     fn as_value(&self) -> Value<'_> {