From the rust avr gitter channel;
This mostly worked but avr-hal isn't hosted on crates.io and is imported via git. This would be fine but I wasn't familiar with how workspaces and git imported crates worked.
hal-avr defines a Cago.toml entry that looks like this
[workspace] members = [ "avr-hal-generic", ... "chips/attiny85-hal", ... "boards/trinket", ]
This defines a series of "workspaces" in cargo terminology. A workspace is a package in a package and allows you to split up a project into a series of interdependent crates. In typical use of Crates.io these all just show up as separate crates.
When you define a dependency in your own Cargo.toml like
[dependencies.attiny85-hal] git = "https://github.com/Rahix/avr-hal" rev = "a22f954"
It tells Cargo to find the "attiny85-hal" crate somewhere in the linked repository including any workspaces. So in this case since we want to use this with our ATtiny85 board we are importing specifically the attiny85-hal crate from the avr project.
I ran into some issues getting my very simple test compiling (just a loop in main). The avr-hal project provides an entry macro that sets up your main function to be properly loaded for running by the chip. In this case while I could use attiny85_hal, no macro entry was found. This ended up having to do with "features". This isn't a feature of cargo that I'm familiar with and it took me a bit of fumbling to get it working. Eventually I added this to my Cargo.toml.
[features] default = ["rt"] rt = ["attiny85-hal/rt"]
rt is a Cargo feature that includes a runtime. It appears to be derived from the scd2rust crate which provides automated code generation based on SVD specs.