Create an avr-unknown-unknown target

[Patryk27]

Proposal

I'm proposing to remove the existing avr-unknown-gnu-atmega328 target and replace it with a generic avr-unknown-unknown variant. This variant can be then specialized for a specific CPU (well, microcontroller) using the -C target-cpu flag, like so:

# .cargo/config.toml

[build]
target = "avr-unknown-unknown"
rustflags = ["-C", "target-cpu=atmega328p"]

[unstable]
build-std = ["core"]

Using the generic variant is the better approach in my opinion, because that's how LLVM already works.

In particular, at the moment rustc exposes only the atmega328 target, but following the current logic we'd have to create ~three hundred targets more - for nothing, since -C target-cpu (aka -mcpu in LLVM) achieves the same thing.

To avoid confusing people, I'm proposing to remove the current avr-unknown-gnu-atmega328 target - it shouldn't be widely used in practice, because virtually all Rust+AVR tutorials and examples rely on target.jsons (see e.g. examples in avr-hal). If backwards compatibility for tier 3 targets is important though, we can think about providing a dedicated error message for people trying to use the older target.

Implementation

This MCP is mostly implemented: rust-lang/rust#131651; I've created the MCP post factum, following @tgross35's idea.

[rustbot]

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