feat: full support for features and profiles

[perazz]

This PR implements comprehensive Features and Profiles support, enabling conditional compilation, platform-specific configurations, and dependency feature propagation.

Overview

This implementation adds two major capabilities to FPM:

1. Features: Named sets of conditional build configurations that can be selectively enabled.
2. Profiles: Predefined combinations of features for common build scenarios

Key Features

✅ Feature System

• Complete control: Features are complete representations of an fpm manifest, allowing full customization
• Platform/compiler targeting: Features can be scoped to specific platforms or compilers:

[features]
my_feat.flag = ["-O3"] # Always applied
my_feat.gfortran.flag = ["-123"] # gfortran only, all OSes
my_feat.windows.flag = ["-DWIN32"] # all compilers, Windows-only
my_feat.macos.ifx.flag = ["..."] #
my_feat.ifx.macos.flag = ["..."] # specific compiler/OS combinations

• Conditional compilation: Enable/disable code paths with preprocessor macros.
• Compiler flags: Add feature-specific compilation flags
• Dependencies: Conditionally include dependencies based on active features

[features]
with_mpi.dependencies.mpi = ["*"]
with_mpi.preprocess.cpp.macros = ["HAVE_MPI"]

• Dependency feature propagation: Pass features to dependencies

[dependencies]
# Request fortran-regex with optional `mpi` feature enabled
fortran-regex = { git="https://github.com/perazz/fortran-regex", tag="1.1.2", features = ["mpi"]}

✅ Profile System

• Predefined configurations: Named combinations of features (e.g., development = ["debug","with_blas"])
• Default profiles: Built-in debug and release profiles (backward compatible; can be overridden)
• CLI integration: --profile <name> and --features <list> command line options

[features]
parallel_cpu.dependencies.mpi = "*"
parallel_cpu.dependencies.openmp = "*"
gpu_support.macos.flags = ["-framework OpenCL"]
gpu_support.unix.link = ["OpenCL"]
[profiles]
parallel = ["parallel_cpu", "gpu_support"]

✅ Advanced Capabilities

• Nested targeting: feature.os or feature.compiler or feature.compiler.os or feature.os.compiler syntax for fine-grained control
• Dependency features: Specify which features to enable in dependencies
• Preprocessing integration: CPP macro definitions and conditional compilation
• Validation: Comprehensive error checking for invalid feature/profile combinations

Usage Examples

Basic Feature Definition

[features]
debug.flags = "-g"
debug.preprocess.cpp.macros = "DEBUG"

release.flags = "-O3"
release.preprocess.cpp.macros = "RELEASE"

#  Platform/Compiler Specific Features

[features]
debug.gfortran.flags = "-Wall -fcheck=bounds"
debug.linux.preprocess.cpp.macros = "LINUX_BUILD"

#Dependency Features

[features]
mpi_support.dependencies.mpi_lib = { version = "*", features = ["parallel"] }

# Profiles

[profiles]
development = ["debug"]
production = ["release", "openmp"]

CLI Usage

Enable specific features

  fpm run --features debug,openmp

Use a profile

  fpm run --profile development

Build with dependency features

fpm build --features mpi_support

Backward Compatibility

• ✅ Fully backward compatible: Existing fpm.toml files work unchanged
• ✅ Default behavior preserved: No features => define default debug and release features and profiles
• ✅ Optional sections: [features] and [profiles] sections are completely optional

Testing

• Core implementation + comprehensive tests
• Integration tests: Multi-platform CI testing via ci/test_features.sh
• Unit tests: Feature parsing, validation, and merging
• Example packages: Real-world usage demonstrations
• CLI tests: Command-line interface validation

This implementation provides a solid foundation for conditional compilation and advanced build configurations in FPM while maintaining full backward compatibility with existing projects.

[davidpfister]

@perazz, thanks for this PR. This should make the manifest cross-compiler and cross-plateform. Since I am working on both Linux and Windows, this feature has been desired for a long time 👏.
Just one idea that came to my mind (and I am not sure this should be the role of fpm): gfortran lacks the definitions of _WIN32 and _WIN64 while compilers like ifort/ifx implement these macros by default (I believe GCC too). Would it be interesting to have fpm implement those when missing?

[perazz]

I have thought about this although have not deployed that yet. Currently, a "collection" i.e. a series of platform/compiler versions, only applies to features but not to the main manifest. One way to accomplish that could be to enable compiler/OS variants also in the main fpm.toml. I believe that would be totally backward compatible:

[gfortran.windows]
preprocess.cpp.macros = ["_WIN32","_WIN64"]

[davidpfister]

That would be really cool. I look forward to it.
That said, if you really want to make the manifest cross-platform, then the platform architecture should also be in the toml section. That would mean something like

[gfortran.windows.x64]
preprocess.cpp.macros = ["_WIN32","_WIN64"]
[gfortran.windows.x86]
preprocess.cpp.macros = ["_WIN32"]

because I do not want to define _WIN64 when I target a 32bits build.

[perazz]

Makes total sense and I think it deserves a separate PR as the current one is already huge enough.