update toolchain to 2025-02-22

[tones111]

This change bumps the toolchain to a recent nightly that incorporates LLVM codegen improvements and avr-none target.

Much thanks to @Patryk27 for all your hard work

closes #537

[tones111]

I'd like to phone a friend (@Patryk27)

When I build locally I get an error like
/usr/bin/avr-ld: avr architecture of input file `/tmp/rustcyPlIoS/symbols.o' is incompatible with avr:51 output
which led me here and suggests a -mavr51 flag needs to be passed to the linker.

I'm not sure where symbols.o comes from. Maybe we need to (somehow) pass extra flags when building the core library?

It appears the avr-gcc on the CI system doesn't understand that flag

my versions

$ avr-gcc --version
avr-gcc (GCC) 14.2.0
$ avr-ld --version
GNU ld (GNU Binutils) 2.43

[Patryk27]

[...] suggests a -mavr51 flag needs to be passed to the linker.

Yes, this is supposed to be done automatically by rustc:

https://github.com/rust-lang/rust/blob/bb2cc59a2172d6e35c89b409a4e6b5058d9039d7/compiler/rustc_codegen_ssa/src/back/linker.rs#L580

I'll take a look later at what's going on here!

[Patryk27]

shouldn't we have something like --target=avr-none -C target-cpu={{matrix.something}} in here?

[Rahix]

Yeah, with avr-none being upstream now, we should start making use of it everywhere. I do wonder, however, what to do with the other target spec changes we currently do:

avr-hal/avr-specs/sync-from-upstream.py

Lines 57 to 64 in d1782c6

	COMMON = {
	# needed because we currently rely on avr-libc
	"no-default-libraries": False,
	# 8-bit operations on AVR are atomic
	# LLVM also supports 16-bit atomics by disabling interrupts
	# see also https://github.com/rust-lang/rust/pull/114495
	"max-atomic-width": 16,
	}

avr-hal/avr-specs/sync-from-upstream.py

Line 103 in d1782c6

pre_link_args.append("-Wl,--as-needed,--print-memory-usage")

[Patryk27]

max-atomic-width is already set to 16:

https://github.com/rust-lang/rust/blob/ad27045c31a9f37ad7d44ca2a403de52d1a896d3/compiler/rustc_target/src/spec/targets/avr_none.rs#L25

I'm not sure on no-default-libraries, though - gotta look at it.

As for the extra link arguments, they can be always passed via RUSTFLAGS (e.g. through .cargo/config) or even through a build.rs script (cargo:rustc-link-arg).

[tones111]

I tried adding -C default-linker-libraries to Cargo.toml for one of the problematic cpus and while it removed the -nodefaultlibs argument from the avr-gcc invocation the linker error remains.