Rollup merge of rust-lang#62615 - wesleywiser:pgo_error, r=nagisa

 Only error about MSVC + PGO + unwind if we're generating code

In rust-lang#61853, we changed the error when using PGO & MSVC toolchain & panic=unwind into a warning. However, in the compiler team meeting on 2019-07-11, we found that not everybody was in favor of that change because of the possibility of broken binaries.

This PR reverts that change so this is again an error. However, to work around an [issue the Firefox team is having](rust-lang#61002 (comment)), we will only emit the error if we're actually supposed to generate a binary. If the `rustc` is invoked with `--print` arguments (which means that no binary will actually be emitted), then we won't emit the error because there is not a possibility of the issue occurring.

cc @EricRahm @nikomatsakis @pnkfelix @Centril

src/librustc/session/mod.rs

@@ -9,7 +9,7 @@ use crate::lint;
 use crate::lint::builtin::BuiltinLintDiagnostics;
 use crate::middle::allocator::AllocatorKind;
 use crate::middle::dependency_format;
-use crate::session::config::{OutputType, SwitchWithOptPath};
+use crate::session::config::{OutputType, PrintRequest, SwitchWithOptPath};
 use crate::session::search_paths::{PathKind, SearchPath};
 use crate::util::nodemap::{FxHashMap, FxHashSet};
 use crate::util::common::{duration_to_secs_str, ErrorReported};
@@ -1303,15 +1303,18 @@ fn validate_commandline_args_with_session_available(sess: &Session) {
     }
 
     // PGO does not work reliably with panic=unwind on Windows. Let's make it
-    // a warning to combine the two for now. It always runs into an assertions
+    // an error to combine the two for now. It always runs into an assertions
     // if LLVM is built with assertions, but without assertions it sometimes
     // does not crash and will probably generate a corrupted binary.
+    // We should only display this error if we're actually going to run PGO.
+    // If we're just supposed to print out some data, don't show the error (#61002).
     if sess.opts.cg.profile_generate.enabled() &&
        sess.target.target.options.is_like_msvc &&
-       sess.panic_strategy() == PanicStrategy::Unwind {
-        sess.warn("Profile-guided optimization does not yet work in conjunction \
-                   with `-Cpanic=unwind` on Windows when targeting MSVC. \
-                   See https://github.com/rust-lang/rust/issues/61002 for details.");
+       sess.panic_strategy() == PanicStrategy::Unwind &&
+       sess.opts.prints.iter().all(|&p| p == PrintRequest::NativeStaticLibs) {
+        sess.err("Profile-guided optimization does not yet work in conjunction \
+                  with `-Cpanic=unwind` on Windows when targeting MSVC. \
+                  See https://github.com/rust-lang/rust/issues/61002 for details.");
     }
 }