per-package-target looks in project root for memory.x rather then crate specific directories

[FrankvdStam]

Unsure if bug or asking for a feature.

Problem

When using the new per-package-target feature in nightly builds, any linker scripts that you might need in a specific crate will not be found when using a workspace. The linker script is expected in root.

Given the following project structure and manifest files:

root (directory)
    cargo.toml

                [workspace]
        members = ["cli", "hardware"]

    cli (directory)
        cargo.toml
            [package]
            name = "cli"
            version = "0.1.0"
            authors = ["redacted"]
            edition = "2018"

        main.rs
            code omitted, not important

    hardware (directory)
        cargo.toml
            cargo-features = ["per-package-target"]

            [package]
            name = "hardware"
            version = "0.1.0"
            authors = ["redacted"]
            edition = "2018"
            forced-target = "thumbv7em-none-eabihf"

            [dependencies]
            cortex-m = "0.7.2"
            cortex-m-rt = "0.6.13"
            cortex-m-semihosting = "0.3.7"
            alloc-cortex-m = "0.4.1"
            stm32h7xx-hal = {version = "0.9.0", features = ["stm32h7a3","rt"]}
            embedded-hal = "0.2.5"

        memory.x
            script ommitted, contents not important

        .cargo (directory)
            config.toml
                [target.thumbv7em-none-eabihf]
                runner = 'arm-none-eabi-gdb'
                rustflags = [
                    # LLD (shipped with the Rust toolchain) is used as the default linker
                    "-C", "link-arg=-Tlink.x",
                ]

                [build]
                target = "thumbv7em-none-eabihf" # Cortex-M4F and Cortex-M7F (with FPU)

cargo build will eventually fail during linking:

note: rust-lld: error: C:\projects\embedded\stm6502\target\thumbv7em-none-eabihf\debug\build\cortex-m-rt-3e6b4c679f86541e\out\link.x:23: cannot find linker script memory.x
          >>> INCLUDE memory.x
          >>>         ^

Even though the script is right there in the crate's subdirectory.

After copying memory.x to the root directory, cargo build runs without problems (except for all those warnings I should really look at)

I would expect the script (memory.x) in the crate directory itself to be picked up. The way that this is setup, I wouldn't be able to target multiple microcontrollers requiring a different memory.x script from the same workspace. I can't specify the name either; "link-arg=-Tlink.x" just looks for memory.x and that's it. In order to target different microcontrollers, you'd have to swap the file in the root directory during build or some crazy shenanigans like that.

Steps

1. Create above project structure or clone this minimal reproduction repository that I set up for this issue (names of directories slightly different): https://github.com/FrankvdStam/PerPackageTargetIssue
2. Run cargo build, note linker error
3. Move memory.x to project root
4. Run cargo build, note linker error disappearing

Possible Solution(s)

Maybe the best would not be to just make it pick up memory.x from the specific crate, but allow users to explicitly tell cargo where to find memory.x in Cargo.toml - the first time I saw memory.x I was confused how/where it was used/what it was, an explicit setting might help users find the documentation for it on their own. Then again I have no idea as to the complexities of implementing any of this.

Notes

Output of cargo version:

cargo 1.54.0-nightly (e931e4796 2021-05-24)

Output of rustup show:

Default host: x86_64-pc-windows-gnu
rustup home:  C:\Users\Frank\.rustup

installed toolchains
--------------------

stable-x86_64-pc-windows-gnu
stable-x86_64-pc-windows-msvc
nightly-x86_64-pc-windows-gnu
nightly-x86_64-pc-windows-msvc (default)

installed targets for active toolchain
--------------------------------------

thumbv6m-none-eabi
thumbv7em-none-eabihf
thumbv7m-none-eabi
x86_64-pc-windows-msvc

active toolchain
----------------

nightly-x86_64-pc-windows-msvc (default)
rustc 1.54.0-nightly (dbe459ded 2021-06-02)

[mattiekat]

My understanding is it is a missing feature that other have been asking for in different forms. Found #7004 first which is simmilar.

I am also running into the same issue (more or less). I am trying to change the rustflags to link against opengl and a few other libs which has to happen at the end in my case because I am also working with cxx but I don't want the rustflags to get modified for all crates in my workspace. At this time the subdirectories' configs in the workspace are ignored and only the main config is used.

The current workaround seems to be running cargo from within the crate's workspace which some people have been creating makefiles for.

Also just found this which seems it might be a solution (but it does require a build script): https://github.com/rust-lang/cargo/pull/8441

[Nikita240]

+1 encountered the same issue setting up a workspace that targets different microcontrollers