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11 — Features: what a package declares, and what a consumer pays for

Every heterogeneous example in this repository consumes a feature (features = ["rules-cuda"] on a dependency edge). This one declares them.

cd examples/11-features/greeter
mcpp run                      # HELLO, WORLD!   metrics: off
mcpp run --features metrics   # HELLO, WORLD!   metrics: on
mcpp test                     # the dev-dependency reaches the test

Three shapes of feature, in one manifest

[features]
default = ["shout"]

shout = {}                              # a compile macro and nothing else

[features.metrics]                      # ...and a source
sources = ["src/metrics.cpp"]

[feature-deps.metrics]                  # ...and a dependency
counters = { path = "../counters" }
shape what it adds reached in the source as
shout = {} MCPP_FEATURE_SHOUT #ifdef MCPP_FEATURE_SHOUT
[features.metrics] sources one file in the source set the file is compiled or it is not
[feature-deps.metrics] a package in the graph import counters; inside that file

Two more keys are here because a library has them and no other example does:

[dev-dependencies]                      # reaches tests, never the artifact
counters = { path = "../counters" }

[profile.release]
opt_level = 3

The criterion

Not "the default build still works" — that passes when the optional dependency is resolved and simply unused. The criterion is that the default build's resolution does not name the optional package:

mcpp build                    counters appears 0 times in the build output
mcpp build --features metrics counters appears 2 times

Measured on 2026.9.8.1. That is the property [feature-deps] exists for: a consumer that does not ask for the backend does not download it, does not compile it, and does not link it.

A build program sees the resolved feature set

if (mcpp::has_feature("metrics"))
    mcpp::warning("the metrics feature is active; `counters` is in the graph");

A rule or a generator acts on the feature set without the project stating the decision a second time. This is the mechanism the accelerator lanes use: a rule package declares its toolkit under the feature that selects it, so a build that does not name the feature installs nothing.

Current limitations

A default feature is turned off in the manifest, not on the command line. There is no --no-default-features. mcpp run --features metrics yields default ∪ {metrics}; to build without shout, [features] default is edited. With default = [] this example prints Hello, world. instead of HELLO, WORLD!.