Vendor dependencies

Let's see how I like this workflow.

vendor/anyhow/build.rs

@@ -0,0 +1,131 @@
+#![allow(clippy::option_if_let_else)]
+
+use std::env;
+use std::fs;
+use std::path::Path;
+use std::process::{Command, ExitStatus, Stdio};
+use std::str;
+
+#[cfg(all(feature = "backtrace", not(feature = "std")))]
+compile_error! {
+    "`backtrace` feature without `std` feature is not supported"
+}
+
+// This code exercises the surface area that we expect of the std Backtrace
+// type. If the current toolchain is able to compile it, we go ahead and use
+// backtrace in anyhow.
+const PROBE: &str = r#"
+    #![feature(error_generic_member_access, provide_any)]
+
+    use std::any::{Demand, Provider};
+    use std::backtrace::{Backtrace, BacktraceStatus};
+    use std::error::Error;
+    use std::fmt::{self, Display};
+
+    #[derive(Debug)]
+    struct E {
+        backtrace: Backtrace,
+    }
+
+    impl Display for E {
+        fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
+            unimplemented!()
+        }
+    }
+
+    impl Error for E {
+        fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
+            demand.provide_ref(&self.backtrace);
+        }
+    }
+
+    struct P;
+
+    impl Provider for P {
+        fn provide<'a>(&'a self, _demand: &mut Demand<'a>) {}
+    }
+
+    const _: fn() = || {
+        let backtrace: Backtrace = Backtrace::capture();
+        let status: BacktraceStatus = backtrace.status();
+        match status {
+            BacktraceStatus::Captured | BacktraceStatus::Disabled | _ => {}
+        }
+    };
+
+    const _: fn(&dyn Error) -> Option<&Backtrace> = |err| err.request_ref::<Backtrace>();
+"#;
+
+fn main() {
+    if cfg!(feature = "std") {
+        match compile_probe() {
+            Some(status) if status.success() => println!("cargo:rustc-cfg=backtrace"),
+            _ => {}
+        }
+    }
+
+    let rustc = match rustc_minor_version() {
+        Some(rustc) => rustc,
+        None => return,
+    };
+
+    if rustc < 51 {
+        println!("cargo:rustc-cfg=anyhow_no_ptr_addr_of");
+    }
+
+    if rustc < 52 {
+        println!("cargo:rustc-cfg=anyhow_no_fmt_arguments_as_str");
+    }
+}
+
+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=anyhow_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()
+}
+
+fn rustc_minor_version() -> Option<u32> {
+    let rustc = env::var_os("RUSTC")?;
+    let output = Command::new(rustc).arg("--version").output().ok()?;
+    let version = str::from_utf8(&output.stdout).ok()?;
+    let mut pieces = version.split('.');
+    if pieces.next() != Some("rustc 1") {
+        return None;
+    }
+    pieces.next()?.parse().ok()
+}