Add build script to detect Provider support

build.rs

@@ -0,0 +1,66 @@
+use std::env;
+use std::fs;
+use std::path::Path;
+use std::process::{Command, ExitStatus, Stdio};
+use std::str;
+
+// This code exercises the surface area that we expect of the Provider API. If
+// the current toolchain is able to compile it, then thiserror is able to use
+// providers for backtrace support.
+const PROBE: &str = r#"
+    #![feature(provide_any)]
+
+    use std::any::{Demand, Provider};
+
+    fn _f<'a, P: Provider>(p: &'a P, demand: &mut Demand<'a>) {
+        p.provide(demand);
+    }
+"#;
+
+fn main() {
+    match compile_probe() {
+        Some(status) if status.success() => println!("cargo:rustc-cfg=provide_any"),
+        _ => {}
+    }
+}
+
+fn compile_probe() -> Option<ExitStatus> {
+    let rustc = env::var_os("RUSTC")?;
+    let out_dir = env::var_os("OUT_DIR")?;
+    let probefile = Path::new(&out_dir).join("probe.rs");
+    fs::write(&probefile, PROBE).ok()?;
+
+    // Make sure to pick up Cargo rustc configuration.
+    let mut cmd = if let Some(wrapper) = env::var_os("RUSTC_WRAPPER") {
+        let mut cmd = Command::new(wrapper);
+        // The wrapper's first argument is supposed to be the path to rustc.
+        cmd.arg(rustc);
+        cmd
+    } else {
+        Command::new(rustc)
+    };
+
+    cmd.stderr(Stdio::null())
+        .arg("--edition=2018")
+        .arg("--crate-name=thiserror_build")
+        .arg("--crate-type=lib")
+        .arg("--emit=metadata")
+        .arg("--out-dir")
+        .arg(out_dir)
+        .arg(probefile);
+
+    if let Some(target) = env::var_os("TARGET") {
+        cmd.arg("--target").arg(target);
+    }
+
+    // If Cargo wants to set RUSTFLAGS, use that.
+    if let Ok(rustflags) = env::var("CARGO_ENCODED_RUSTFLAGS") {
+        if !rustflags.is_empty() {
+            for arg in rustflags.split('\x1f') {
+                cmd.arg(arg);
+            }
+        }
+    }
+
+    cmd.status().ok()
+}