Add & document rustc -C export-executable-symbols

MsvcLinker:
- Give export_symbols a debug!() header to match GccLinker
- Make debug!() dumped symbols match actual .def symbols
- Export executable symbols when -C export-executable-symbols

GccLinker:
- Export executable symbols when -C export-executable-symbols

src/doc/rustc/src/codegen-options/index.md

@@ -203,6 +203,13 @@ for determining whether or not it is possible to statically or dynamically
 link with a dependency. For example, `cdylib` crate types may only use static
 linkage.
 
+## export-executable-symbols
+
+This flag causes `rustc` to export symbols from executables, as if they were dynamic libraries.
+
+You might use this to allow the JVM or MSCLR to call back into your executable's
+Rust code from Java/C# when embedding their runtimes into your Rust executable.
+
 ## no-integrated-as
 
 `rustc` normally uses the LLVM internal assembler to create object code. This

src/librustc_codegen_ssa/back/linker.rs

@@ -421,6 +421,7 @@ impl<'a> Linker for GccLinker<'a> {
         // Symbol visibility in object files typically takes care of this.
         if crate_type == CrateType::Executable
             && self.sess.target.target.options.override_export_symbols.is_none()
+            && !self.sess.opts.cg.export_executable_symbols
         {
             return;
         }
@@ -699,10 +700,14 @@ impl<'a> Linker for MsvcLinker<'a> {
     // their symbols exported.
     fn export_symbols(&mut self, tmpdir: &Path, crate_type: CrateType) {
         // Symbol visibility takes care of this typically
-        if crate_type == CrateType::Executable {
+        if crate_type == CrateType::Executable
+            && !self.sess.opts.cg.export_executable_symbols
+        {
             return;
         }
 
+        debug!("EXPORTED SYMBOLS:");
+
         let path = tmpdir.join("lib.def");
         let res: io::Result<()> = try {
             let mut f = BufWriter::new(File::create(&path)?);
@@ -712,7 +717,7 @@ impl<'a> Linker for MsvcLinker<'a> {
             writeln!(f, "LIBRARY")?;
             writeln!(f, "EXPORTS")?;
             for symbol in self.info.exports[&crate_type].iter() {
-                debug!("  _{}", symbol);
+                debug!("  {}", symbol);
                 writeln!(f, "  {}", symbol)?;
             }
         };

src/librustc_session/options.rs

@@ -652,6 +652,8 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
         "use soft float ABI (*eabihf targets only)"),
     prefer_dynamic: bool = (false, parse_bool, [TRACKED],
         "prefer dynamic linking to static linking"),
+    export_executable_symbols: bool = (false, parse_bool, [UNTRACKED],
+        "export symbols from executables, as if they were dynamic libraries"),
     no_integrated_as: bool = (false, parse_bool, [TRACKED],
         "use an external assembler rather than LLVM's integrated one"),
     no_redzone: Option<bool> = (None, parse_opt_bool, [TRACKED],