[rebase]: don't require rust_try exported

Author: alexcrichton

src/libstd/rt/unwind/mod.rs

@@ -129,12 +129,17 @@ pub unsafe fn try<F: FnOnce()>(f: F) -> Result<(), Box<Any + Send>> {
     let mut f = Some(f);
     return inner_try(try_fn::<F>, &mut f as *mut _ as *mut c_void);
 
-    // We use an inner non-generic function here to avoid the requirement that
-    // the `rust_try` symbol needs to be exposed from the standard library. On
-    // MSVC this requires the symbol to be tagged with `dllexport`, but it's
-    // easier to not have conditional `src/rt/rust_try.ll` files and instead
-    // just have this non-generic shim the compiler can take care of exposing
-    // correctly.
+    // If an inner function were not used here, then this generic function `try`
+    // uses the native symbol `rust_try`, for which the code is statically
+    // linked into the standard library. This means that the DLL for the
+    // standard library must have `rust_try` as an exposed symbol that
+    // downstream crates can link against (because monomorphizations of `try` in
+    // downstream crates will have a reference to the `rust_try` symbol).
+    //
+    // On MSVC this requires the symbol `rust_try` to be tagged with
+    // `dllexport`, but it's easier to not have conditional `src/rt/rust_try.ll`
+    // files and instead just have this non-generic shim the compiler can take
+    // care of exposing correctly.
     unsafe fn inner_try(f: extern fn(*mut c_void), data: *mut c_void)
                         -> Result<(), Box<Any + Send>> {
         let prev = PANICKING.with(|s| s.get());