An interesting bit of Android's Rust integration: android.googlesource.com/platform/devel. It converts Cargo packages into Blueprint modules integrated into the normal build system. Blueprint is Android's declarative build system implemented on top of ninja. Fits into overall incremental build.
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Question, does this (and/or "Blueprint") create a path to writing Android software without using Gradle?
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The Android Open Source Project builds everything including apps with ninja. The modern approach is the declarative Blueprint system. The legacy approach is the template-based Android.mk build system which was the basis for Blueprint. It's far nicer than gradle...
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Here's an example of a Rust crate (futures) converted to Blueprint:
android.googlesource.com/platform/exter
Here's a manually written example for a C executable and static library:
android.googlesource.com/platform/exter
This is what it looks like for an app:
android.googlesource.com/platform/packa
I think it's great.
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This is an example of an app using the legacy Android.mk-based system:
github.com/GrapheneOS/pla
It's essentially the same thing, but everything gets expanded into an enormous Makefile. Legacy system isn't implemented via make anymore but rather Kati and integrates with the rest.
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Kati is a reimplementation of GNU make into ninja. AOSP incorporates it in a way that's interoperable with the new declarative Blueprint build system. The old approach was essentially a poor man's declarative build system. The usage is very similar to how they use Blueprint.
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I can't stand gradle. It's the opposite of the approach taken by Android itself. AOSP wants everything to be declarative and a major reason for moving away from make is to enforce it. Helps a lot with avoiding bugs including rock solid reproducible builds and incremental builds.
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Ant is (or can be used) declaratively. I was horrified when I saw Gradle pushed so hard. Definite step backward.
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What makes Android.bp so nice is that it's a declarative build system made specifically for the Android Open Source Project so nearly everything you would ever want to do is very simple/clean and native code is first class using the same build system in an interoperable way.
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The directory structure, etc. is also cleaner.
android.googlesource.com/platform/packa is a trivial example.
android.googlesource.com/platform/packa is one signed with the platform key (SIM Toolkit) to get access to special permissions.
android.googlesource.com/platform/packa is the system file picker UI / Files app.
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Every external dependency pulled into AOSP gets an Android.bp build system written for it. Since Rust has Cargo, they get to auto-convert the upstream build metadata into the appropriate Android.bp build metadata. Usually, someone has to manually convert autotools/cmake to it.