b4d9835f6c
wasm: Explicitly export all symbols with LLD This commit fixes an oddity on the wasm target where LTO can produce working executables but plain old optimizations doesn't. The compiler already knows what set of symbols it would like to export, but LLD only discovers this list transitively through symbol visibilities. LLD may not, however, always find all the symbols that we'd like to export. For example if you depend on an rlib with a `#[no_mangle]` symbol, then if you don't actually use anything from the rlib then the symbol won't appear in the final artifact! It will appear, however, with LTO. This commit attempts to rectify this situation by ensuring that all symbols rustc would otherwise preserve through LTO are also preserved through the linking process with LLD by default.
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc guide.