cargo:rustc-link-search becomes invalid when you move a Cargo project

[japaric]

STR

# create a dependency that uses cargo:rustc-link-search
$ cargo new --lib foo && cd $_
$ edit build.rs && cat $_

use std::{env, fs};
use std::path::PathBuf;

fn main() {
    let src_dir = PathBuf::from(env::var_os("CARGO_MANIFEST_DIR").unwrap());
    let out_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());

    fs::copy(src_dir.join("link.x"), out_dir.join("link.x")).unwrap();
    println!("cargo:rustc-link-search={}", out_dir.display());
    println!("cargo:rerun-if-changed=build.rs");
    println!("cargo:rerun-if-changed=link.x");
}

# create an empty linker script
$ touch link.x

# create an application that depends on `foo`
$ cd ..
$ cargo new --bin hello && cd $_
$ cargo add foo --path ../../foo

# this won't link because the linker script is empty. that's OK; that's not the issue
$ cargo rustc -- -C link-arg=-Tlink.x
error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" (..) "-L" "/home/japaric/tmp/a/hello/target/debug/build/foo-4cfaa457c325b81c/out" (..)

# note the -L path in the linker error; the linker script *is* in that folder
# so the linker finds the linker script
$ find -name link.x
/home/japaric/tmp/a/hello/target/debug/build/foo-4cfaa457c325b81c/out/link.x

# now move the Cargo project
$ cd ../..
$ mkdir b && cd $_
$ mv ../a/hello .
$ cd hello

# re-run the `cargo rustc` command
# the linker error is different; the linker can't find `link.x` this time
$ cargo rustc -- -C link-arg=-Tlink.x
error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" (..) "-L" "/home/japaric/tmp/a/hello/target/debug/build/foo-4cfaa457c325b81c/out" (..)
  = note: /usr/bin/ld: cannot open linker script file link.x: No such file or directory

Note the linker invocation; the foo search path is the same as before. By moving the hello crate the search path generated by the foo dependency becomes invalidated.

Meta

$ cargo -V
cargo 0.20.0-nightly (cf17c9f71 2017-05-09)

Possible solution: Re-run all build scripts when the Cargo project is moved.

cc @alexcrichton @dpc
Originally reported in rust-embedded/cortex-m#31

[alexcrichton]

I seem to recall we had a bug saying we shouldn't do anything when a Cargo project is moved, and now here's a bug saying we should do things when a Cargo project is moved. There's probably some way to distinguish this which we need to look into.

[carols10cents]

This is the related issue that says we shouldn't need to recompile when the project dir is moved: #3273