Cargo recompiles unchanged re-saved files

[mattrossman]

Problem
Re-saving a file but not changing its contents causes cargo to recompile the project.

This doesn't follow what's written in the Rust Programming Language book:

Cargo figured out that the files hadn’t changed, so it just ran the binary. If you had modified your source code, Cargo would have rebuilt the project before running

Steps

1. Create a new project and build it

cargo new hello
cd hello
cargo build

2. Open main.rs and re-save it, e.g. with vim src/main.rs and :wq
3. Run another cargo build

Cargo will show a "Compiling" phase before "Finished" as if the file had changed.

Possible Solution(s)
Not sure what the current implementation is but perhaps it's considering the timestamp at which files were modified?

Notes

Output of cargo version:

cargo 1.45.1 (f242df6ed 2020-07-22)

I am on WSL running Ubuntu 20.04.

[Eh2406]

Yes we use mtime the, adding a way to look at contents is tracked in #6529

[ehuss]

Closing as this is tracked in #6529, and fortunately someone is working on a solution at #8623.