add codegen flag export symbols from executable

Author: csmoe

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -641,9 +641,12 @@ impl<'a> Linker for GccLinker<'a> {
 
     fn export_symbols(&mut self, tmpdir: &Path, crate_type: CrateType, symbols: &[String]) {
         // Symbol visibility in object files typically takes care of this.
-        if crate_type == CrateType::Executable && self.sess.target.override_export_symbols.is_none()
-        {
-            return;
+        if crate_type == CrateType::Executable {
+            if self.sess.target.override_export_symbols.is_none()
+                && !self.sess.opts.cg.export_executable_symbols
+            {
+                return;
+            }
         }
 
         // We manually create a list of exported symbols to ensure we don't expose any more.
@@ -970,7 +973,9 @@ impl<'a> Linker for MsvcLinker<'a> {
     fn export_symbols(&mut self, tmpdir: &Path, crate_type: CrateType, symbols: &[String]) {
         // Symbol visibility takes care of this typically
         if crate_type == CrateType::Executable {
-            return;
+            if !self.sess.opts.cg.export_executable_symbols {
+                return;
+            }
         }
 
         let path = tmpdir.join("lib.def");

compiler/rustc_interface/src/tests.rs

@@ -574,6 +574,7 @@ fn test_codegen_options_tracking_hash() {
     tracked!(debug_assertions, Some(true));
     tracked!(debuginfo, 0xdeadbeef);
     tracked!(embed_bitcode, false);
+    tracked!(export_executable_symbols, true);
     tracked!(force_frame_pointers, Some(false));
     tracked!(force_unwind_tables, Some(true));
     tracked!(inline_threshold, Some(0xf007ba11));

compiler/rustc_session/src/options.rs

@@ -1089,6 +1089,8 @@ options! {
         "allow the linker to link its default libraries (default: no)"),
     embed_bitcode: bool = (true, parse_bool, [TRACKED],
         "emit bitcode in rlibs (default: yes)"),
+    export_executable_symbols: bool = (false, parse_bool, [TRACKED],
+        "export symbols from executables, as if they were dynamic libraries"),
     extra_filename: String = (String::new(), parse_string, [UNTRACKED],
         "extra data to put in each output filename"),
     force_frame_pointers: Option<bool> = (None, parse_opt_bool, [TRACKED],

src/test/run-make/export-executable-symbols/main.rs

@@ -0,0 +1,10 @@
+// edition:2018
+
+fn main() {
+    foo();
+}
+
+#[no_mangle]
+pub extern "C" fn foo() -> i32 {
+    1 + 1
+}