Add Cargo post-build scripts

text/0000-cargo-post-build-scripts.md

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+- Feature Name: post_build
+- Start Date: 2016-10-26
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+This will add the ability for an extra build script to be run *after* the build completes.
+
+This is a similar concept current build script concept but rather than processing files beforehand,
+this script would perform tasks _after_ the crate has completed compilation.
+
+# Motivation
+[motivation]: #motivation
+
+This feature will give Rust programmers more flexibility when compiling their crates.
+
+Pre-build scripts as we have today are very useful for performing tasks before the
+crate is compiled, such as for compiling C/C++ sources or generating bindings. It
+is not currently possible to do anything _after_ Cargo finishes.
+
+## Use cases
+
+On most platforms, once linking is done there is no longer any work to do. This
+differs on some systems, such as AVR (which requires linked ELF binaries to be
+converted into raw binary blobs).
+
+Currently this would require something like a Makefile which internally
+calls `cargo`, which prohibits the crate being published to `crates.io`.
+
+In another case, we could use post-build scripts to generate mountable disk
+images for an operating system kernel written in Rust. Again, this is something
+that would require an external build system today.
+
+# Detailed design
+[design]: #detailed-design
+
+The crate manifest `[package]` table will support another textual field `post-build`.
+
+A normal manifest would look something like this:
+
+```toml
+[package]
+name = "foobar"
+
+build = "pre-build.rs"
+post-build = "post-build.rs"
+```
+
+Once `cargo build` finishes compiling a crate, it will check if there is a `post-build` script
+configured, and it will compile and run it similarly to the current build script setup.
+
+A post-build script looks like this:
+
+```rust
+use std::process::Command;
+
+fn main() {
+  let out_dir = env::var("OUT_DIR").unwrap();
+
+    Command::new("avr-objcopy")
+        .arg(&format!("-i elf32 -o binary {}/myobject {}/mybinary", out_dir, out_dir))
+        .spawn();
+}
+```
+
+If the build script returns an error code, Cargo should report compilation as failed.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+* More complexity in the Cargo source
+* A corner case that won't be used very often
+
+# Alternatives
+[alternatives]: #alternatives
+
+* Use a Makefile-like build system which calls into Cargo
+* Expect users to do post-processing manually
+
+If Cargo will not support something given crate authors this ability, people will
+be forced to stray-away from the crates ecosystem and use something more custom.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+* What should "crate compilation finished" mean in this regard?
+* Should `cargo doc` run the post-build script?