Add unstable -Zdefault-hidden-visibility cmdline flag for rustc.

The new flag has been described in the Major Change Proposal at
rust-lang/compiler-team#656

compiler/rustc_codegen_gcc/src/allocator.rs

@@ -90,7 +90,7 @@ fn create_wrapper_function(
         .collect();
     let func = context.new_function(None, FunctionType::Exported, output.unwrap_or(void), &args, from_name, false);
 
-    if tcx.sess.target.options.default_hidden_visibility {
+    if tcx.sess.default_hidden_visibility() {
         #[cfg(feature="master")]
         func.add_attribute(FnAttribute::Visibility(gccjit::Visibility::Hidden));
     }

compiler/rustc_codegen_llvm/src/allocator.rs

@@ -76,7 +76,7 @@ pub(crate) unsafe fn codegen(
     // __rust_alloc_error_handler_should_panic
     let name = OomStrategy::SYMBOL;
     let ll_g = llvm::LLVMRustGetOrInsertGlobal(llmod, name.as_ptr().cast(), name.len(), i8);
-    if tcx.sess.target.default_hidden_visibility {
+    if tcx.sess.default_hidden_visibility() {
         llvm::LLVMRustSetVisibility(ll_g, llvm::Visibility::Hidden);
     }
     let val = tcx.sess.opts.unstable_opts.oom.should_panic();
@@ -85,7 +85,7 @@ pub(crate) unsafe fn codegen(
 
     let name = NO_ALLOC_SHIM_IS_UNSTABLE;
     let ll_g = llvm::LLVMRustGetOrInsertGlobal(llmod, name.as_ptr().cast(), name.len(), i8);
-    if tcx.sess.target.default_hidden_visibility {
+    if tcx.sess.default_hidden_visibility() {
         llvm::LLVMRustSetVisibility(ll_g, llvm::Visibility::Hidden);
     }
     let llval = llvm::LLVMConstInt(i8, 0, False);
@@ -130,7 +130,7 @@ fn create_wrapper_function(
             None
         };
 
-        if tcx.sess.target.default_hidden_visibility {
+        if tcx.sess.default_hidden_visibility() {
             llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
         }
         if tcx.sess.must_emit_unwind_tables() {

compiler/rustc_codegen_llvm/src/declare.rs

@@ -84,7 +84,7 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         fn_type: &'ll Type,
     ) -> &'ll Value {
         // Declare C ABI functions with the visibility used by C by default.
-        let visibility = if self.tcx.sess.target.default_hidden_visibility {
+        let visibility = if self.tcx.sess.default_hidden_visibility() {
             llvm::Visibility::Hidden
         } else {
             llvm::Visibility::Default
@@ -107,7 +107,7 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         unnamed: llvm::UnnamedAddr,
         fn_type: &'ll Type,
     ) -> &'ll Value {
-        let visibility = if self.tcx.sess.target.default_hidden_visibility {
+        let visibility = if self.tcx.sess.default_hidden_visibility() {
             llvm::Visibility::Hidden
         } else {
             llvm::Visibility::Default

compiler/rustc_interface/src/tests.rs

@@ -748,6 +748,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(cross_crate_inline_threshold, InliningThreshold::Always);
     tracked!(debug_info_for_profiling, true);
     tracked!(debug_macros, true);
+    tracked!(default_hidden_visibility, Some(true));
     tracked!(dep_info_omit_d_target, true);
     tracked!(dual_proc_macros, true);
     tracked!(dwarf_version, Some(5));

compiler/rustc_monomorphize/src/partitioning.rs

@@ -883,7 +883,7 @@ fn mono_item_visibility<'tcx>(
 }
 
 fn default_visibility(tcx: TyCtxt<'_>, id: DefId, is_generic: bool) -> Visibility {
-    if !tcx.sess.target.default_hidden_visibility {
+    if !tcx.sess.default_hidden_visibility() {
         return Visibility::Default;
     }
 

compiler/rustc_session/src/options.rs

@@ -1552,6 +1552,8 @@ options! {
         "compress debug info sections (none, zlib, zstd, default: none)"),
     deduplicate_diagnostics: bool = (true, parse_bool, [UNTRACKED],
         "deduplicate identical diagnostics (default: yes)"),
+    default_hidden_visibility: Option<bool> = (None, parse_opt_bool, [TRACKED],
+        "overrides the `default_hidden_visibility` setting of the target"),
     dep_info_omit_d_target: bool = (false, parse_bool, [TRACKED],
         "in dep-info output, omit targets for tracking dependencies of the dep-info files \
         themselves (default: no)"),

compiler/rustc_session/src/session.rs

@@ -961,6 +961,14 @@ impl Session {
             termize::dimensions().map_or(default_column_width, |(w, _)| w)
         }
     }
+
+    /// Whether the default visibility of symbols should be "hidden" rather than "default".
+    pub fn default_hidden_visibility(&self) -> bool {
+        self.opts
+            .unstable_opts
+            .default_hidden_visibility
+            .unwrap_or(self.target.options.default_hidden_visibility)
+    }
 }
 
 // JUSTIFICATION: defn of the suggested wrapper fns

compiler/rustc_target/src/spec/mod.rs

@@ -2094,7 +2094,11 @@ pub struct TargetOptions {
     pub no_builtins: bool,
 
     /// The default visibility for symbols in this target should be "hidden"
-    /// rather than "default"
+    /// rather than "default".
+    ///
+    /// This value typically shouldn't be accessed directly, but through
+    /// the `rustc_session::Session::default_hidden_visibility` method, which
+    /// allows `rustc` users to override this setting using cmdline flags.
     pub default_hidden_visibility: bool,
 
     /// Whether a .debug_gdb_scripts section will be added to the output object file

src/doc/unstable-book/src/compiler-flags/default-hidden-visibility.md

@@ -0,0 +1,12 @@
+# `default-hidden-visibility`
+
+The tracking issue for this feature is: https://github.com/rust-lang/compiler-team/issues/656
+
+------------------------
+
+This flag can be used to override the target's
+[`default_hidden_visibility`](https://doc.rust-lang.org/beta/nightly-rustc/rustc_target/spec/struct.TargetOptions.html#structfield.default_hidden_visibility)
+setting.
+Using `-Zdefault_hidden_visibility=yes` is roughly equivalent to Clang's
+[`-fvisibility=hidden`](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-fvisibility)
+cmdline flag.

tests/codegen/default-hidden-visibility.rs

@@ -0,0 +1,32 @@
+// Verifies that `Session::default_hidden_visibility` is affected when using the related cmdline
+// flag.  This is a regression test for https://github.com/rust-lang/compiler-team/issues/656.  See
+// also https://github.com/rust-lang/rust/issues/73295 and
+// https://github.com/rust-lang/rust/issues/37530.
+
+// revisions:DEFAULT YES NO
+//[YES] compile-flags: -Zdefault-hidden-visibility=yes
+//[NO]  compile-flags: -Zdefault-hidden-visibility=no
+
+// The test scenario is specifically about visibility of symbols exported out of dynamically linked
+// libraries.
+#![crate_type = "dylib"]
+
+// The test scenario needs to use a Rust-public, but non-explicitly-exported symbol
+// (e.g. the test doesn't use `#[no_mangle]`, because currently it implies that
+// the symbol should be exported;  we don't want that - we want to test the *default*
+// export setting instead).
+#[used]
+pub static tested_symbol: [u8; 6] = *b"foobar";
+
+// Exact LLVM IR differs depending on the target triple (e.g. `hidden constant`
+// vs `internal constant` vs `constant`).  Because of this, we only apply the
+// specific test expectations below to one specific target triple.  If needed,
+// additional targets can be covered by adding copies of this test file with
+// a different `only-X` directive.
+//
+//     only-x86_64
+//     only-linux
+
+// DEFAULT: @{{.*}}default_hidden_visibility{{.*}}tested_symbol{{.*}} = constant
+// YES:     @{{.*}}default_hidden_visibility{{.*}}tested_symbol{{.*}} = hidden constant
+// NO:      @{{.*}}default_hidden_visibility{{.*}}tested_symbol{{.*}} = constant