Auto merge of #27304 - alexcrichton:revert-picky-dllimport, r=brson

This reverts commit a0efd3a.

This commit caused a lot of unintended breakage for many Cargo builds. The problem is that Cargo compiles build scripts with `-C prefer-dynamic`, so the standard library is always dynamically linked and hence any imports need to be marked with `dllimport`. Dependencies of build scripts, however, were compiled as rlibs and did not have their imports tagged with `dllimport`, so build scripts would fail to link.

While known that this situation would break, it was unknown that it was a common scenario in the wild. As a result I'm just reverting these heuristics for now.

Author: bors

src/librustc_trans/trans/base.rs

@@ -227,7 +227,6 @@ pub fn get_extern_const<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, did: ast::DefId,
     // FIXME(nagisa): perhaps the map of externs could be offloaded to llvm somehow?
     // FIXME(nagisa): investigate whether it can be changed into define_global
     let c = declare::declare_global(ccx, &name[..], ty);
-
     // Thread-local statics in some other crate need to *always* be linked
     // against in a thread-local fashion, so we need to be sure to apply the
     // thread-local attribute locally if it was present remotely. If we
@@ -239,42 +238,7 @@ pub fn get_extern_const<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, did: ast::DefId,
             llvm::set_thread_local(c, true);
         }
     }
-
-    // MSVC is a little ornery about how items are imported across dlls, and for
-    // lots more info on dllimport/dllexport see the large comment in
-    // SharedCrateContext::new. Unfortunately, unlike functions, statics
-    // imported from dlls *must* be tagged with dllimport (if you forget
-    // dllimport on a function then the linker fixes it up with an injected
-    // shim). This means that to link correctly to an upstream Rust dynamic
-    // library we need to make sure its statics are tagged with dllimport.
-    //
-    // Hence, if this translation is using dll storage attributes and the crate
-    // that this const originated from is being imported as a dylib at some
-    // point we tag this with dllimport.
-    //
-    // Note that this is not 100% correct for a variety of reasons:
-    //
-    // 1. If we are producing an rlib and linking to an upstream rlib, we'll
-    //    omit the dllimport. It's a possibility, though, that some later
-    //    downstream compilation will link the same upstream dependency as a
-    //    dylib and use our rlib, causing linker errors because we didn't use
-    //    dllimport.
-    // 2. We may have multiple crate output types. For example if we are
-    //    emitting a statically linked binary as well as a dynamic library we'll
-    //    want to omit dllimport for the binary but we need to have it for the
-    //    dylib.
-    //
-    // For most every day uses, however, this should suffice. During the
-    // bootstrap we're almost always linking upstream to a dylib for some crate
-    // type output, so most imports will be tagged with dllimport (somewhat
-    // appropriately). Otherwise rust dylibs linking against rust dylibs is
-    // pretty rare in Rust so this will likely always be `false` and we'll never
-    // tag with dllimport.
-    //
-    // Note that we can't just blindly tag all constants with dllimport as can
-    // cause linkage errors when we're not actually linking against a dll. For
-    // more info on this see rust-lang/rust#26591.
-    if ccx.use_dll_storage_attrs() && ccx.upstream_dylib_used(did.krate) {
+    if ccx.use_dll_storage_attrs() {
         llvm::SetDLLStorageClass(c, llvm::DLLImportStorageClass);
     }
     ccx.externs().borrow_mut().insert(name.to_string(), c);

src/librustc_trans/trans/context.rs

@@ -11,7 +11,6 @@
 use llvm;
 use llvm::{ContextRef, ModuleRef, ValueRef, BuilderRef};
 use metadata::common::LinkMeta;
-use metadata::cstore;
 use middle::def::ExportMap;
 use middle::traits;
 use trans::adt;
@@ -788,29 +787,6 @@ impl<'b, 'tcx> CrateContext<'b, 'tcx> {
     pub fn use_dll_storage_attrs(&self) -> bool {
         self.shared.use_dll_storage_attrs()
     }
-
-    /// Tests whether the given `krate` (an upstream crate) is ever used as a
-    /// dynamic library for the final linkage of this crate.
-    pub fn upstream_dylib_used(&self, krate: ast::CrateNum) -> bool {
-        let tcx = self.tcx();
-        let formats = tcx.dependency_formats.borrow();
-        tcx.sess.crate_types.borrow().iter().any(|ct| {
-            match formats[ct].get(krate as usize - 1) {
-                // If a crate is explicitly linked dynamically then we're
-                // definitely using it dynamically. If it's not being linked
-                // then currently that means it's being included through another
-                // dynamic library, so we're including it dynamically.
-                Some(&Some(cstore::RequireDynamic)) |
-                Some(&None) => true,
-
-                // Static linkage isn't included dynamically and if there's not
-                // an entry in the array then this crate type isn't actually
-                // doing much linkage so there's nothing dynamic going on.
-                Some(&Some(cstore::RequireStatic)) |
-                None => false,
-            }
-        })
-    }
 }
 
 /// Declare any llvm intrinsics that you might need