Static variables are not exported in wasm

[MaikKlein]

Repro https://github.com/EmbarkStudios/missing-symbols rustc 1.40.0 (73528e339 2019-12-16)

// lib.rs
#[no_mangle]
#[used]
pub static FOO: u64 = 4242;

cargo build --release --target wasm32-unknown-unknown, the resulting wasm file doesn't have the symbol FOO.

(module
  (table (;0;) 1 1 funcref)
  (memory (;0;) 16)
  (global (;0;) (mut i32) (i32.const 1048576))
  (global (;1;) i32 (i32.const 1048576))
  (global (;2;) i32 (i32.const 1048576))
  (export "memory" (memory 0))
  (export "__data_end" (global 1))
  (export "__heap_base" (global 2)))

But if we compile with -C link-dead-code, the symbol is correctly exported in the wasm file.

RUSTFLAGS="-C link-dead-code" cargo build --release --target wasm32-unknown-unknown

(module
  (table (;0;) 1 1 funcref)
  (memory (;0;) 17)
  (global (;0;) (mut i32) (i32.const 1048576))
  (global (;1;) i32 (i32.const 1048584))
  (global (;2;) i32 (i32.const 1048584))
  (global (;3;) i32 (i32.const 1048576))
  (export "memory" (memory 0))
  (export "__data_end" (global 1))
  (export "__heap_base" (global 2))
  (export "FOO" (global 3))
  (data (;0;) (i32.const 1048576) "\92\10\00\00\00\00\00\00"))

Expected behavior: Public static variables should get exported by default.

We believe this is a regression from 1.39.0.

rustup override set 1.39.0
cargo build --release --target wasm32-unknown-unknown

Does correctly export static variables.

[Centril]

cc @japaric @alexcrichton

[alexcrichton]

Symbols for wasm are exported here and it looks like no --export flag is passed for FOO, which is the bug. It looks like this may trace back to exported_symbols_provider_local not including statics, but that's just a guess. I don't personally have time to deeply investigate this right now.

[MaikKlein]

After a some investigation:

• link-dead-code disables --gc-section, which means all the symbols are exported. This is why FOO appears.
• If --gc-section is set, symbols have to be exported --export NAME, which is not the case for FOO as @alexcrichton has stated above
• The reason for this is SymbolExportLevel::is_below_threshold is false. FOO is a rust symbol and we export to a c lib.

Statics are explicitly marked as SymbolExportLevel::Rust.

    if is_extern && !std_internal {
        let target = &tcx.sess.target.target.llvm_target;
        // WebAssembly cannot export data symbols, so reduce their export level
        if target.contains("wasm32") || target.contains("emscripten") {
            if let Some(Node::Item(&hir::Item { kind: hir::ItemKind::Static(..), .. })) =
                tcx.hir().get_if_local(sym_def_id)
            {
                return SymbolExportLevel::Rust;
            }
        }

        SymbolExportLevel::C
    } else {
        SymbolExportLevel::Rust
    }

It is unclear to me how this should be handled. Maybe statics should be exported with SymbolExportLevel::C if the static is a number?

I'll make a PR for it, that might be a better place to talk about implementation details.

[alexcrichton]

Oh good find! I'm not sure about emscripten, but wasm has changed in the meantime so that it can export statics, so the wasm case there I believe can be removed.