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_interface/src/tests.rs

@@ -749,6 +749,7 @@ 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, None);
     tracked!(dep_info_omit_d_target, true);
     tracked!(dual_proc_macros, true);
     tracked!(dwarf_version, Some(5));

compiler/rustc_session/src/options.rs

@@ -1538,6 +1538,8 @@ 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

@@ -960,6 +960,21 @@ impl Session {
             termize::dimensions().map_or(default_column_width, |(w, _)| w)
         }
     }
+
+    pub fn default_hidden_visibility(&self) -> bool {
+        // FIXME:
+        //
+        // * Combine:
+        //   1) `self.target.default_hidden_visibility` (or maybe
+        //      `self.target.options.default_hidden_visibility`?)
+        //   and
+        //   2) `self.opts.unstable_opts.default_hidden_visibility`
+        //
+        // * Grep the whole source for `default_hidden_visibility` and use
+        //   `session.default_hidden_visibility()` instead of
+        //   `sess.target.default_hidden_visibility`, etc.
+        todo!("DO NOT SUBMIT");
+    }
 }
 
 // JUSTIFICATION: defn of the suggested wrapper fns

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 +1,34 @@
+// Verifies that `TargetOptions::default_hidden_visibility` is set 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:NONE YES
+//[YES] compile-flags: -Zdefault-hidden-visibility=yes
+//
+// FIXME: 3) also cover NO?
+
+// The test scenario is specifically about visibility of symbols in Rust static libraries.
+//
+// This mimics the relevant part from https://github.com/rust-lang/rust/issues/73295 which
+// says:
+//
+// > We build Rust code into these DSOs in the approved way, which is to aggregate a bunch of Rust
+// > libraries (rlibs) into a separate Rust ***staticlib*** for each of the DSOs. (For example,
+// > libbase_rust_deps.a and libservices_rust_deps.a). The final C++ linker links exactly one of
+// > these staticlibs together with the C++ .a and .o in the final construction of the DSO.
+#![crate_type = "staticlib"]
+
+// The test scenario needs to use a public, but non-`#[no_mangle]` Rust symbol.
+//
+// We want to check the visibility of this symbol:
+//
+// FIXME: 1) confirm that this test file actually repros the problem at hand (I am not at all
+// confident that it does)
+// FIXME: 2) fix test expectations below ("internal" below seems unexpected for NONE scenario?  no
+// idea what should be the expectation for YES scenario)
+//
+// NONE: @_ZN25default_hidden_visibility15exported_symbol17hc5deee4a42a30cf5E = internal constant
+// YES: ???????????
+#[used]
+pub static exported_symbol: [u8; 6] = *b"foobar";