Don't initialize memory for late resources

[japaric]

Late resources are initialized at runtime so their memory doesn't need to be initialized before main / init. However, today, late resources are placed in the .data section so not only they get initialized (using memcpy) but they also use up FLASH memory (because everything in .data has some initial value stored in FLASH).

To fix this we'll have to create a new section, let's call it .uninit, in cortex-m-rt and then place the static variables that will be used with late resources in that section. The change in this repo is minimal; here's the patch (macros/src/trans.rs):

                 None => quote! {
                     // Resource initialized in `init`
-                    static mut #_name: #krate::UntaggedOption<#ty> = #krate::UntaggedOption { none: () };
+                    #[link_section = ".uninit"]
+                    static mut #_name: #krate::UntaggedOption<#ty> =
+                        #krate::UntaggedOption { none: () };
                 },

But this change requires updating cortex-m-rt, specially if rust-embedded/cortex-m-rt#43 lands. It's probably best to wait until rust-embedded/cortex-m-rt#43 lands.

[jonas-schievink]

There is a rustc "bug" hidden in here: There's no need to put an UntaggedOption intialized with none into the .data section, rustc/LLVM can put it in .bss in all cases (or .uninit, of course, but that's not a standard section), which would also help in this case.

[jonas-schievink]

The underlying rustc bug is rust-lang/rust#41315, which was fixed by rust-lang/rust#48908, so this should work now

[japaric]

triage: blocked on rust-embedded/cortex-m-rt#116

[korken89]

Is this fixed with having the .uninit section?
If so can we close this?

[japaric]

Yes, this has already been implemented.