support emiting custom sections on wasm targets

[zigazeljko]

There should be a way to emit custom wasm sections from Zig code.

LLVM already has support for this (landed in https://reviews.llvm.org/D45297), using the wasm.custom_sections named metadata:

!0 = !{ !"red", !"foo" }
!1 = !{ !"green", !"bar" }
!2 = !{ !"green", !"qux" }
!wasm.custom_sections = !{ !0, !1, !2 }

The code above compiles into:

.section  .custom_section.red,"",@
.ascii  "foo"

.section  .custom_section.green,"",@
.ascii  "bar"

.section  .custom_section.green,"",@
.ascii  "qux"

From the code above it seems like this should work, but it does not:

export const foo linksection(".custom_section.red") = [_]u8{ 'f', 'o', 'o' };

Should we special-case the linksection(...) syntax on wasm targets to generate the named metadata shown above, or should we use some other syntax for this?

[andrewrk]

Indeed it does look like that code should work. I'm going to call this a bug.

One thing we should double check is why LLVM uses this weird custom thing instead of hooking into their own link section API that they already have. It could just be bad API design, which is not unheard of, but it's also possible there is something more subtle going on here that we should be aware of.