Auto merge of #114470 - pnkfelix:dont-export-no-mangle-from-proc-macros-issue-99978, r=bjorn3

Restrict linker version script of proc-macro crates to just its two symbols

Restrict linker version script of proc-macro crates to just the two symbols of each proc-macro crate.

The main known effect of doing this is to stop including `#[no_mangle]` symbols in the linker version script.

Background:

The combination of a proc-macro crate with an import of another crate that itself exports a no_mangle function was broken for a period of time, because:

* In PR #99944 we stopped exporting no_mangle symbols from proc-macro crates; proc-macro crates have a very limited interface and are meant to be treated as a blackbox to everything except rustc itself. However: he constructed linker version script still referred to them, but resolving that discrepancy was left as a FIXME in the code, tagged with issue #99978.
* In PR #108017 we started telling the linker to check (via the`--no-undefined-version` linker invocation flag) that every symbol referenced in the "linker version script" is provided as linker input. So the unresolved discrepancy from #99978 started surfacing as a compile-time error (e.g. #111888).

Fix #111888
Fix #99978.

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -13,6 +13,7 @@ use std::{env, mem, str};
 use rustc_hir::def_id::{CrateNum, LOCAL_CRATE};
 use rustc_metadata::find_native_static_library;
 use rustc_middle::middle::dependency_format::Linkage;
+use rustc_middle::middle::exported_symbols;
 use rustc_middle::middle::exported_symbols::{ExportedSymbol, SymbolExportInfo, SymbolExportKind};
 use rustc_middle::ty::TyCtxt;
 use rustc_session::config::{self, CrateType, DebugInfo, LinkerPluginLto, Lto, OptLevel, Strip};
@@ -659,8 +660,6 @@ impl<'a> Linker for GccLinker<'a> {
             return;
         }
 
-        // FIXME(#99978) hide #[no_mangle] symbols for proc-macros
-
         let is_windows = self.sess.target.is_like_windows;
         let path = tmpdir.join(if is_windows { "list.def" } else { "list" });
 
@@ -1679,8 +1678,15 @@ pub(crate) fn exported_symbols(tcx: TyCtxt<'_>, crate_type: CrateType) -> Vec<St
         return exports.iter().map(ToString::to_string).collect();
     }
 
-    let mut symbols = Vec::new();
+    if let CrateType::ProcMacro = crate_type {
+        exported_symbols_for_proc_macro_crate(tcx)
+    } else {
+        exported_symbols_for_non_proc_macro(tcx, crate_type)
+    }
+}
 
+fn exported_symbols_for_non_proc_macro(tcx: TyCtxt<'_>, crate_type: CrateType) -> Vec<String> {
+    let mut symbols = Vec::new();
     let export_threshold = symbol_export::crates_export_threshold(&[crate_type]);
     for_each_exported_symbols_include_dep(tcx, crate_type, |symbol, info, cnum| {
         if info.level.is_below_threshold(export_threshold) {
@@ -1691,6 +1697,19 @@ pub(crate) fn exported_symbols(tcx: TyCtxt<'_>, crate_type: CrateType) -> Vec<St
     symbols
 }
 
+fn exported_symbols_for_proc_macro_crate(tcx: TyCtxt<'_>) -> Vec<String> {
+    // `exported_symbols` will be empty when !should_codegen.
+    if !tcx.sess.opts.output_types.should_codegen() {
+        return Vec::new();
+    }
+
+    let stable_crate_id = tcx.sess.local_stable_crate_id();
+    let proc_macro_decls_name = tcx.sess.generate_proc_macro_decls_symbol(stable_crate_id);
+    let metadata_symbol_name = exported_symbols::metadata_symbol_name(tcx);
+
+    vec![proc_macro_decls_name, metadata_symbol_name]
+}
+
 pub(crate) fn linked_symbols(
     tcx: TyCtxt<'_>,
     crate_type: CrateType,

tests/ui/proc-macro/auxiliary/exports_no_mangle.rs

@@ -0,0 +1,11 @@
+// force-host
+// no-prefer-dynamic
+#![crate_type="lib"]
+
+// Issue 111888: this crate (1.) is imported by a proc-macro crate and (2.)
+// exports a no_mangle function; that combination of acts was broken for some
+// period of time. See further discussion in the test file that imports this
+// crate.
+
+#[no_mangle]
+pub fn some_no_mangle_function() { }

tests/ui/proc-macro/no-mangle-in-proc-macro-issue-111888.rs

@@ -0,0 +1,22 @@
+// build-pass
+// force-host
+// no-prefer-dynamic
+// aux-build:exports_no_mangle.rs
+#![crate_type = "proc-macro"]
+
+// Issue #111888: this proc-macro crate imports another crate that itself
+// exports a no_mangle function.
+//
+// That combination was broken for a period of time, because:
+//
+// In PR #99944 we *stopped* exporting no_mangle symbols from
+// proc-macro crates. The constructed linker version script still referred
+// to them, but resolving that discrepancy was left as a FIXME in the code.
+//
+// In PR #108017 we started telling the linker to check (via the
+// `--no-undefined-version` linker invocation flag) that every symbol referenced
+// in the "linker version script" is actually present in the linker input. So
+// the unresolved discrepancy from #99944 started surfacing as a compile-time
+// error.
+
+extern crate exports_no_mangle;