nothing is rebuilt when the directory is modified

[ExpHP]

Summary: Changes to the included directory are not tracked by rustc, and nothing is rebuilt unless an actual rust source file is changed (or the user must add a build.rs that recursively walks the directory to print cargo:rerun-if-changed= directives). This is in contrast to what was written in the closing comment on #25 back in 2017 June.

Script:

Bash script

#!/usr/bin/env bash

rm -rf rerun
mkdir rerun
cd rerun

cat >Cargo.toml <<EOF
[package]
name = "rerun"
version = "0.1.0"
authors = ["Michael Lamparski <diagonaldevice@gmail.com>"]

[[bin]]
name = "rerun"
path = "main.rs"

[dependencies]
include_dir = "0.2.1"
EOF

cat >main.rs <<EOF
#[macro_use]
extern crate include_dir;
use include_dir::{Dir};

const DIR: Dir = include_dir!("the-dir");

fn main() {
    show_dir(&DIR);
}

fn show_dir(dir: &Dir) {
    for file in dir.files() {
        println!("{}", file.path().display());
    }
    for dir in dir.dirs() {
        show_dir(dir);
    }
}
EOF

mkdir the-dir
touch the-dir/a
mkdir the-dir/b
touch the-dir/b/c
touch the-dir/b/d

cargo build

echo "--------------"

cargo run

echo "--------------"

touch the-dir/b/e # new file
cargo run

echo "--------------"

echo x > the-dir/b/d # modified file
cargo run

echo "--------------"

touch the-dir/c   # new file at toplevel
cargo run

echo "--------------"

touch the-dir/a   # remove one of the original files
cargo run

echo "--------------"

---

Output:

$ ./repro  
    Updating registry `https://github.com/rust-lang/crates.io-index`
   Compiling proc-macro2 v0.4.13                                                                                                                             
        ...
   Compiling rerun v0.1.0 (file:///home/lampam/cpp/throwaway/rerun/rerun)
    Finished dev [unoptimized + debuginfo] target(s) in 19.41s
--------------
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s                                                                                                
     Running `target/debug/rerun`
a
b/d
b/c
--------------
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s                                                                                                
     Running `target/debug/rerun`
a
b/d
b/c
--------------
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s                                                                                                
     Running `target/debug/rerun`
a
b/d
b/c
--------------
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s                                                                                                
     Running `target/debug/rerun`
a
b/d
b/c
--------------
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s                                                                                                
     Running `target/debug/rerun`
a
b/d
b/c
--------------

---

Expected Output:

$ ./repro  
    Updating registry `https://github.com/rust-lang/crates.io-index`
   Compiling proc-macro2 v0.4.13                                                                                                                             
        ...
   Compiling rerun v0.1.0 (file:///home/lampam/cpp/throwaway/rerun/rerun)
    Finished dev [unoptimized + debuginfo] target(s) in 19.41s
--------------
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s                                                                                                
     Running `target/debug/rerun`
a
b/d
b/c
--------------
   Compiling rerun v0.1.0 (file:///home/lampam/cpp/throwaway/rerun/rerun)
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s                                                                                                
     Running `target/debug/rerun`
a
b/d
b/c
b/e
--------------
   Compiling rerun v0.1.0 (file:///home/lampam/cpp/throwaway/rerun/rerun)
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s                                                                                                
     Running `target/debug/rerun`
a
b/d
b/c
b/e
--------------
   Compiling rerun v0.1.0 (file:///home/lampam/cpp/throwaway/rerun/rerun)
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s                                                                                                
     Running `target/debug/rerun`
a
c
b/d
b/c
b/e
--------------
   Compiling rerun v0.1.0 (file:///home/lampam/cpp/throwaway/rerun/rerun)
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s                                                                                                
     Running `target/debug/rerun`
c
b/d
b/c
b/e
--------------

[Michael-F-Bryan]

Hmm... I'm not sure how we'd solve this. As far as I'm aware, a proc macro can't emit the usual "rerun if changed" messages so there isn't really a mechanism for re-executing the macro without that file being recompiled by rustc.

Ideally the include_dir!() call would emit all the necessary rerun messages so the user doesn't need to write their own build script. Would it be worth me proposing an RFC to enable the "rerun if changed" messages from a proc macro?

[ExpHP]

Hmm, I am uncertain that the cargo flags are the right fit for this since proc macros are expanded by rustc, but it surely is a problem worth solving. It would be good to start a thread on internals.rust-lang.org to start exploring the design space.

[Michael-F-Bryan]

I've created a thread on the internals forum, let's see what other people think. If possible, it'd be cool to emit a cargo:rerun-if-changed=... to fix this issue, or have the ability to emit warnings when including really large files (for some definition of "really large").