Fix emscripten linker invocation

Author: bjorn3

Diff for: Cargo.lock

@@ -3686,6 +3686,7 @@ dependencies = [
  "rustc_span",
  "rustc_symbol_mangling",
  "rustc_target",
+ "serde_json",
  "smallvec",
  "snap",
  "tempfile",

Diff for: compiler/rustc_codegen_ssa/Cargo.toml

@@ -16,6 +16,7 @@ jobserver = "0.1.22"
 tempfile = "3.2"
 thorin-dwp = "0.2"
 pathdiff = "0.2.0"
+serde_json = "1.0.59"
 snap = "1"
 smallvec = { version = "1.6.1", features = ["union", "may_dangle"] }
 regex = "1.4"

Diff for: compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -1146,24 +1146,17 @@ impl<'a> Linker for EmLinker<'a> {
     }
 
     fn export_symbols(&mut self, _tmpdir: &Path, _crate_type: CrateType, symbols: &[String]) {
-        use std::fmt::Write;
-
         debug!("EXPORTED SYMBOLS:");
 
-        let mut encoded = "[".to_string();
-        let mut symbols = symbols.iter();
-        if let Some(first_symbol) = symbols.next() {
-            write!(encoded, "{:?}", first_symbol).unwrap();
-        }
-        for symbol in symbols {
-            write!(encoded, ",{:?}", symbol).unwrap();
-        }
-        encoded.push(']');
-        debug!("{}", encoded);
-
         self.cmd.arg("-s");
 
         let mut arg = OsString::from("EXPORTED_FUNCTIONS=");
+        let encoded = serde_json::to_string(
+            &symbols.iter().map(|sym| "_".to_owned() + sym).collect::<Vec<_>>(),
+        )
+        .unwrap();
+        debug!("{}", encoded);
+
         arg.push(encoded);
 
         self.cmd.arg(arg);