Cargo build recompiles when using rustc -- -Z no-trans

[antoyo]

Hello.

When I use cargo rustc -- -Z -no-trans, the following cargo build will recompile even thought I did not changed the source code.

Here is the commands that I ran:

$ cargo build
   Compiling test v0.1.0 (file:///path/to/project)
    Finished debug [unoptimized + debuginfo] target(s) in 0.40 secs
$ cargo rustc -- -Z no-trans
   Compiling test v0.1.0 (file:///path/to/project)
    Finished debug [unoptimized + debuginfo] target(s) in 0.17 secs
$ cargo build
   Compiling test v0.1.0 (file:///path/to/project)
    Finished debug [unoptimized + debuginfo] target(s) in 0.40 secs

The second cargo build should not recompile the project.

Thanks to fix this issue.

[alexcrichton]

Thanks for the report! Right now this is expected, however, due to how Cargo tracks cargo rustc and flags and such. Cargo has no knowledge of rustc flags, so it doesn't understand that -Z no-trans doesn't actually affect compilation, so the second cargo build to Cargo has a different set of flags being passed to the compiler, so the target is recompiled.

[ghost]

There's another side-effect to using -Zno-trans(which I'm using within vim when file gets saved).

Steps to repro:

1. change only the contents of build.rs file (or just touch build.rs ? not tested)
2. run cargo rustc -- -Zno-trans // EDIT2: Same thing happens for cargo check
   (no errors, presumably)
3. run cargo build
   (nothing gets built because of 2.!)

What happened?
Step 2. has updated build.rs output (eg. this file target/debug/build/recompile_self-feb93e46b6e91788/output),
so when I try step 3. it will NOT build anything.

Workaround is step 2.5. touch build.rs

What's a better way?

EDIT: If I change step 1. to touch src/main.rs however, step 3. will re-build, which is good.
Btw, the OP case doesn't seem to be true anymore, currently.

[ehuss]

Closing as no longer applicable.
-Z no-trans was removed a while ago.
The issue @xftroxgpx mentions is fixed by #6720.