cargo build --features "X" doesn't update binary mtime when list of features change

[schultetwin1]

Problem

Cargo doesn't appear to be updating the mtime on the binary in the ./target/debug directory when grabbing that file from the cache. This is breaking my using cargo inside a Makefile because the Makefile believes the built binary is older than it actually is.

Steps

1. Create a repo with the following source

   # Cargo.toml
   [package]
   name = "minrepro"
   version = "0.1.0"
   edition = "2021"
   
   [dependencies]
   
   [features]
   feature_a = []
   feature_b = []

   // src/main.rs
   fn main() {
       #[cfg(feature = "feature_a")]
       println!("Hello, Feature A!");
       #[cfg(feature = "feature_b")]
       println!("Hello, Feature B!");
   
       println!("Hello, world!");
   }
   

   # Makefile to show the problem
   FEATURES :=
   
   ifeq ($(FEATURE),A)
   FEATURES += "feature_a"
   endif
   
   ifeq ($(FEATURE),B)
   FEATURES += "feature_b"
   endif
   
   .PHONY: all FORCE
   
   all: bin
   	./bin
   
   bin: target/debug/minrepro
   	cp target/debug/minrepro bin
   
   target/debug/minrepro: FORCE
   	cargo build --features="$(FEATURES)"
   

2. Check the current time

   bash$ date
   Fri Mar 14 10:55:00 PM PDT 202
   

3. Build the crate with feature a

   cargo build --features feature_a
     Compiling minrepro v0.1.0 (/home/matsch/dev/minrepro)
     Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s
   

4. stat the built binary

   bash$ stat ./target/debug/minrepro
   stat ./target/debug/minrepro
   File: ./target/debug/minrepro
   Size: 3946232   	Blocks: 7712       IO Block: 4096   regular file
   Device: 253,1	Inode: 58468456    Links: 2
   Access: (0755/-rwxr-xr-x)  Uid: (1155901/  matsch)   Gid: (89939/primarygroup)
   Access: 2025-03-14 22:55:51.930800853 -0700
   Modify: 2025-03-14 22:55:51.930800853 -0700
   Change: 2025-03-14 22:55:51.938800874 -0700
   Birth: 2025-03-14 22:55:51.874800711 -0700
   

5. Build the crate with feature b

   cargo build --features feature_b
     Compiling minrepro v0.1.0 (/home/matsch/dev/minrepro)
     Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s
   

6. stat the built binary again

   stat ./target/debug/minrepro
   File: ./target/debug/minrepro
   Size: 3946216   	Blocks: 7712       IO Block: 4096   regular file
   Device: 253,1	Inode: 58470933    Links: 2
   Access: (0755/-rwxr-xr-x)  Uid: (1155901/  matsch)   Gid: (89939/primarygroup)
   Access: 2025-03-14 23:01:12.287611878 -0700
   Modify: 2025-03-14 23:01:12.287611878 -0700
   Change: 2025-03-14 23:01:12.295611898 -0700
   Birth: 2025-03-14 23:01:12.231611736 -0700
   

7. Build the crate with feature a again

   cargo build --features feature_a
     Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s
   

8. stat the built binary again

   stat ./target/debug/minrepro
   File: ./target/debug/minrepro
   Size: 3946232   	Blocks: 7712       IO Block: 4096   regular file
   Device: 253,1	Inode: 58468456    Links: 2
   Access: (0755/-rwxr-xr-x)  Uid: (1155901/  matsch)   Gid: (89939/primarygroup)
   Access: 2025-03-14 22:55:51.930800853 -0700
   Modify: 2025-03-14 22:55:51.930800853 -0700
   Change: 2025-03-14 23:02:43.063840777 -0700
   Birth: 2025-03-14 22:55:51.874800711 -0700
   

Notice that the "Modify" time is now older than step 5, even though the file on disk has been updated since step "5"

Using the Makefile, I can produce the same issue:

# The first time it works
bash$ FEATURE=B make
cargo build --features=""feature_b""
   Compiling minrepro v0.1.0 (/home/matsch/dev/minrepro)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.13s
cp target/debug/minrepro bin
./bin
Hello, Feature B!
Hello, world!

# Same with feature A
bash$ FEATURE=A make
cargo build --features=""feature_a""
   Compiling minrepro v0.1.0 (/home/matsch/dev/minrepro)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.09s
cp target/debug/minrepro bin
./bin
Hello, Feature A!
Hello, world!

# But now we go back to feature B and it still outputs feature A
# This is because make has determined the `cp` is not needed because the mtime of `./target/debug/minrepro` is older than `bin`
bash$ FEATURE=B make
cargo build --features=""feature_b""
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s
./bin
Hello, Feature A!
Hello, world!

Possible Solution(s)

No response

Notes

No response

Version

cargo --version --verbose
cargo 1.85.0 (d73d2caf9 2024-12-31)
release: 1.85.0
commit-hash: d73d2caf9e41a39daf2a8d6ce60ec80bf354d2a7
commit-date: 2024-12-31
host: x86_64-unknown-linux-gnu
libgit2: 1.8.1 (sys:0.19.0 vendored)
libcurl: 8.9.0-DEV (sys:0.4.74+curl-8.9.0 vendored ssl:OpenSSL/1.1.1w)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: Debian n/a (rodete) [64-bit]

[schultetwin1]

It looks like this is occurring because cargo hardlinks the file in ./target/debug to the cached binary (code)

[weihanglo]

Thanks for the detailed report and the reproducible steps!

This sounds like a variant of #8649. There are some solutions provided in #8649 (comment).

Apart from that, Cargo has an unstable feature -Zchecksum-freshness using file checksum replacing mtime. Depending on mtime might be a bit more unreliable when that is stabilized.

[schultetwin1]

This sounds like a variant of #8649.

That looks right to me as well! Sorry I missed this originally.

There are some solutions provided in #8649 (comment).

Thanks! My current solution is to change the Makefile to cp the output only if the destination and source sha1sum are different. I also like this idea provided by @ehuss, "Cargo could be changed (maybe with an option?) to always update the mtime of the outputs when it runs" but I'd be concerned this would break something else I'm not thinking about.

Depending on mtime might be a bit more unreliable when that is stabilized

Good to know, thanks for the heads up!