cargo build does not rebuild if a source file was modified during a build

[kamalmarhubi]

I haven't checked, but I would guess this is a timestamp issue: the input is modified before the output artifact is created, even though the modified input was not used to build the artifact. This would happen any time after rustc is parsing.

[kamalmarhubi]

As an example with the current master of cargo (8fc3fd8):

cargo$ git checkout 8fc3fd8
cargo$ cargo clean
cargo$ cargo build &
[Cargo output snipped]
[Wait until it a few seconds after 'Compiling cargo v0.9.0 (file://path/to/checkout/cargo)']
cargo$ patch src/cargo/lib.rs <<EOF
82a83
>     println!("a-new-line");
EOF
cargo$ wait $! && cargo build
[1]+  Done                    cargo build
cargo$ target/debug/cargo | grep a-new-line
[no output]
cargo$ touch src/cargo/lib.rs && cargo build
   Compiling cargo v0.9.0 (file:///home/kamal/clones/cargo)
cargo$ target/debug/cargo | grep a-new-line
a-new-line

[alexcrichton]

I personally view this as a feature rather than a bug, but it shouldn't be too too hard to fix this.

[kamalmarhubi]

Yea I figured this could be a bit philosophical! My personal view is that a build should be a pure function of the build inputs, and that skipping rebuilds is an optimization that is allowed providing it maintains the purity. This is a violation of that. :-)

[kamalmarhubi]

(Though the fix to the greater issue would likely require changing from timestamps to hashes to determine if inputs have changed.)

[alexcrichton]

Right now the mtime we compare against is the foo.d file, not the output file. It looks like rustc just generates foo.d far before the output in the compilation process.

This should just be a change to this line

[srinivasreddy]

Anyone working on this?

[alexcrichton]

I don't believe so, no, feel free to take it!

[alexcrichton]

@Jimmio92 that sounds distinct from this issue, feel free to open a separate one!

[Jimmio92]

@alexcrichton The issue I mentioned in the comment has been resolved in the nightly build 20160511. Sorry for the totally unhelpful comment. I was on stable.

[alexcrichton]

Aha, even better! Glad it worked out :)