LDC: Apply dflags affecting symbol visibility to all deps

[kinke]

On Windows.

[github-actions]

✅ PR OK, no changes in deprecations or warnings

Total deprecations: 0

Total warnings: 0

Build statistics:

 statistics (-before, +after)
 executable size=5318488 bin/dub
-rough build time=61s
+rough build time=62s

Full build output

DUB version 1.35.1, built on Jan  6 2024
LDC - the LLVM D compiler (1.36.0):
  based on DMD v2.106.1 and LLVM 17.0.6
  built with LDC - the LLVM D compiler (1.36.0)
  Default target: x86_64-unknown-linux-gnu
  Host CPU: znver3
  http://dlang.org - http://wiki.dlang.org/LDC


  Registered Targets:
    aarch64     - AArch64 (little endian)
    aarch64_32  - AArch64 (little endian ILP32)
    aarch64_be  - AArch64 (big endian)
    amdgcn      - AMD GCN GPUs
    arm         - ARM
    arm64       - ARM64 (little endian)
    arm64_32    - ARM64 (little endian ILP32)
    armeb       - ARM (big endian)
    avr         - Atmel AVR Microcontroller
    bpf         - BPF (host endian)
    bpfeb       - BPF (big endian)
    bpfel       - BPF (little endian)
    hexagon     - Hexagon
    lanai       - Lanai
    loongarch32 - 32-bit LoongArch
    loongarch64 - 64-bit LoongArch
    mips        - MIPS (32-bit big endian)
    mips64      - MIPS (64-bit big endian)
    mips64el    - MIPS (64-bit little endian)
    mipsel      - MIPS (32-bit little endian)
    msp430      - MSP430 [experimental]
    nvptx       - NVIDIA PTX 32-bit
    nvptx64     - NVIDIA PTX 64-bit
    ppc32       - PowerPC 32
    ppc32le     - PowerPC 32 LE
    ppc64       - PowerPC 64
    ppc64le     - PowerPC 64 LE
    r600        - AMD GPUs HD2XXX-HD6XXX
    riscv32     - 32-bit RISC-V
    riscv64     - 64-bit RISC-V
    sparc       - Sparc
    sparcel     - Sparc LE
    sparcv9     - Sparc V9
    spirv32     - SPIR-V 32-bit
    spirv64     - SPIR-V 64-bit
    systemz     - SystemZ
    thumb       - Thumb
    thumbeb     - Thumb (big endian)
    ve          - VE
    wasm32      - WebAssembly 32-bit
    wasm64      - WebAssembly 64-bit
    x86         - 32-bit X86: Pentium-Pro and above
    x86-64      - 64-bit X86: EM64T and AMD64
    xcore       - XCore
   Upgrading project in /home/runner/work/dub/dub/
    Starting Performing "release" build using /opt/hostedtoolcache/dc/ldc2-1.36.0/x64/ldc2-1.36.0-linux-x86_64/bin/ldc2 for x86_64.
    Building dub 1.36.0-beta.1+commit.41.g2f121b3b: building configuration [application]
     Linking dub
STAT:statistics (-before, +after)
STAT:executable size=5318488 bin/dub
STAT:rough build time=62s

[kinke]

Example scenarios:

• If an .exe is linked against druntime/Phobos DLLs (e.g., to enable loading other D DLLs, all sharing central druntime/Phobos), all static-lib dub dependencies need to be compiled with -dllimport=defaultLibsOnly too [the default with -link-defaultlib-shared].
• To make large DLLs with few selective exported symbols work after LDC: Use -fvisibility=public -dllimport=all when compiling a Windows DLL #2412, the implicit flags need to be overridden - for the DLL itself and all its static-lib dependencies. This can currently only be accomplished by setting the DFLAGS environment variable. This PR enables overriding these dflags either in the DLL root project/config directly, or in some dependency shared by multiple DLLs/executables. E.g., at Symmetry, we have an .exe with lots of plugin DLLs. All plugins and the .exe have a shared dependency, so adding dflags "-fvisibility=hidden" "-dllimport=defaultLibsOnly" platform="windows-ldc" once in the dub.sdlof that shared dependency suffices to compile everything with those required flags.

TODO:

• Tests
• Decide whether this should be Windows only, or whether -fvisibility should be handled identically on Posix platforms, enabling to use hidden visibility for all static-lib deps of some .so/dylib too.

[kinke]

FYI @rikkimax @p0nce

[p0nce]

Would be super nice to be able to get rid of DFLAGS tweaking! Thanks.

Our flags are:
"dflags-windows-ldc": ["-mscrtlib=libcmt","-fvisibility=hidden", "-link-defaultlib-shared=false"], (in adddition to the DFLAGS override as current workaround)

Does -mscrtlib need to be passed through too?

[kinke]

Does -mscrtlib need to be passed through too?

Nope, that flag really only affects linking, not compilation. And I think LDC defaults to static libcmt with -link-defaultlib-shared=false, for DLLs too [if an MSVC installation is found].

[rikkimax]

My one concern (apart from code review), is that this would be better suited to be handled from a pretty significant refactor in the form of placing directives to have the relationship to other packages of just this, goes up, goes down. But since this would be the default, its still a good PR in theory.

[WebFreak001]

no longer draft? in that case also adjust title pls

[kinke]

Rebased, changelog entry added, and not passing stuff down to shared-library deps.

While I fully agree that there would ideally be a generic way of controlling the flags propagation, this automatism solves a number of use cases which can currently only be handled by setting the DFLAGS env variable. As DMD seems to be adopting the relevant LDC switches (-fvisibility and -dllimport), the approach could eventually be generalized to DMD too. GDC has -fvisibility already, so we could include it already (but the main use cases are on Windows).