Compiled crates are not reused across dev/release even if the configuration is the same

[Vlad-Shcherbina]

Steps to reproduce

1. Create a crate with the following Cargo.toml:

   [package]
   name = "cargo_cache_is_frustrating"
   version = "0.1.0"
   edition = "2018"
   
   [dependencies]
   bitflags = "1.2"
   
   [profile.dev]
   opt-level = 3
   debug = false
   rpath = false
   lto = false
   debug-assertions = false
   codegen-units = 16
   panic = 'unwind'
   incremental = false
   overflow-checks = false
   
   [profile.release]  # exactly the same
   opt-level = 3
   debug = false
   rpath = false
   lto = false
   debug-assertions = false
   codegen-units = 16
   panic = 'unwind'
   incremental = false
   overflow-checks = false

2. cargo build
3. cargo build --release

Actual result

> cargo build
   Compiling bitflags v1.2.1
   Compiling cargo_cache_is_frustrating v0.1.0 (...\cargo_cache_is_frustrating)
    Finished dev [optimized] target(s) in 1.92s
> cargo build --release
   Compiling bitflags v1.2.1
   Compiling cargo_cache_is_frustrating v0.1.0 (...\cargo_cache_is_frustrating)
    Finished release [optimized] target(s) in 4.33s

Expected result

> cargo build
   Compiling bitflags v1.2.1
   Compiling cargo_cache_is_frustrating v0.1.0 (...\cargo_cache_is_frustrating)
    Finished dev [optimized] target(s) in 1.92s
> cargo build --release
    Finished release [optimized] target(s) in 0.00s

I checked cargo --build-plan outputs and they're identical
except they use paths in target/debug and target/release and

 {
     "invocations": [
         ...
         {
             "package_name": "bitflags",
             ...
             "env": {
                 ...
-                "PROFILE": "debug",
+                "PROFILE": "release",

Why do I care

In this case maybe I don't. It's just a small repro. However, this also happens with dependency profile overrides, and there it entirely defeats one of their main purposes.

Version info

> cargo version --verbose
cargo 1.41.0-nightly (8280633db 2019-11-11)
release: 1.41.0
commit-hash: 8280633db680dec5bfe1de25156d1a1d53e6d190
commit-date: 2019-11-11

[alexcrichton]

I believe this would be fixed by #6668, but it's a very tricky change.

[ehuss]

Yea, #6668 would address this. Since this is on our radar (and there is a meta tracking issue at #7150), I'm going to close this. Thanks for the report, though!