WIP: Add weak native linkage modifier

Passing the `-weak-l` / `-weak_framework`, and makes the dynamic linker `dyld` wait with resolving symbols until they're actually used.

This is somewhat niche, but I have at least three different reasons to do this:
-

If we ever want to support rust-lang#121293 in some shape or form, or just generally want `rustc` to have a more global view of the linking, we need to support the `-weak_framework` linker flag.

Supporting `-weak_framework` natively

- Never makes sense together with `as_needed`.

https://github.com/denoland/deno/blob/ab18dac09d9e5b00afee55ae0108f7c98bb2e2d3/.cargo/config.toml#L14-L24

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -1357,7 +1357,7 @@ fn link_sanitizer_runtime(
         let path = find_sanitizer_runtime(sess, &filename);
         let rpath = path.to_str().expect("non-utf8 component in path");
         linker.link_args(&["-rpath", rpath]);
-        linker.link_dylib_by_name(&filename, false, true);
+        linker.link_dylib_by_name(&filename, false, true, false);
     } else if sess.target.is_like_msvc && flavor == LinkerFlavor::Msvc(Lld::No) && name == "asan" {
         // MSVC provides the `/INFERASANLIBS` argument to automatically find the
         // compatible ASAN library.
@@ -1543,6 +1543,7 @@ fn print_native_static_libs(
         .filter_map(|lib| {
             let name = lib.name;
             match lib.kind {
+                NativeLibKind::Dylib { weak: Some(true), .. } => Some(format!("-weak-l{name}")),
                 NativeLibKind::Static { bundle: Some(false), .. }
                 | NativeLibKind::Dylib { .. }
                 | NativeLibKind::Unspecified => {
@@ -1555,9 +1556,13 @@ fn print_native_static_libs(
                         Some(format!("-l{name}"))
                     }
                 }
-                NativeLibKind::Framework { .. } => {
-                    // ld-only syntax, since there are no frameworks in MSVC
-                    Some(format!("-framework {name}"))
+                // ld-only syntax, since there are no frameworks in MSVC.
+                NativeLibKind::Framework { weak, .. } => {
+                    if weak.unwrap_or(false) {
+                        Some(format!("-weak_framework {name}"))
+                    } else {
+                        Some(format!("-framework {name}"))
+                    }
                 }
                 // These are included, no need to print them
                 NativeLibKind::Static { bundle: None | Some(true), .. }
@@ -2638,9 +2643,14 @@ fn add_native_libs_from_crate(
                     }
                 }
             }
-            NativeLibKind::Dylib { as_needed } => {
+            NativeLibKind::Dylib { as_needed, weak } => {
                 if link_dynamic {
-                    cmd.link_dylib_by_name(name, verbatim, as_needed.unwrap_or(true))
+                    cmd.link_dylib_by_name(
+                        name,
+                        verbatim,
+                        as_needed.unwrap_or(true),
+                        weak.unwrap_or(false),
+                    )
                 }
             }
             NativeLibKind::Unspecified => {
@@ -2651,12 +2661,17 @@ fn add_native_libs_from_crate(
                         cmd.link_staticlib_by_name(name, verbatim, false);
                     }
                 } else if link_dynamic {
-                    cmd.link_dylib_by_name(name, verbatim, true);
+                    cmd.link_dylib_by_name(name, verbatim, true, false);
                 }
             }
-            NativeLibKind::Framework { as_needed } => {
+            NativeLibKind::Framework { as_needed, weak } => {
                 if link_dynamic {
-                    cmd.link_framework_by_name(name, verbatim, as_needed.unwrap_or(true))
+                    cmd.link_framework_by_name(
+                        name,
+                        verbatim,
+                        as_needed.unwrap_or(true),
+                        weak.unwrap_or(false),
+                    )
                 }
             }
             NativeLibKind::RawDylib => {
@@ -2963,7 +2978,7 @@ fn add_static_crate(
 
 // Same thing as above, but for dynamic crates instead of static crates.
 fn add_dynamic_crate(cmd: &mut dyn Linker, sess: &Session, cratepath: &Path) {
-    cmd.link_dylib_by_path(&rehome_lib_path(sess, cratepath), true);
+    cmd.link_dylib_by_path(&rehome_lib_path(sess, cratepath), true, false);
 }
 
 fn relevant_lib(sess: &Session, lib: &NativeLib) -> bool {

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -303,13 +303,19 @@ pub(crate) trait Linker {
         crate_type: CrateType,
         out_filename: &Path,
     );
-    fn link_dylib_by_name(&mut self, _name: &str, _verbatim: bool, _as_needed: bool) {
+    fn link_dylib_by_name(&mut self, _name: &str, _verbatim: bool, _as_needed: bool, _weak: bool) {
         bug!("dylib linked with unsupported linker")
     }
-    fn link_dylib_by_path(&mut self, _path: &Path, _as_needed: bool) {
+    fn link_dylib_by_path(&mut self, _path: &Path, _as_needed: bool, _weak: bool) {
         bug!("dylib linked with unsupported linker")
     }
-    fn link_framework_by_name(&mut self, _name: &str, _verbatim: bool, _as_needed: bool) {
+    fn link_framework_by_name(
+        &mut self,
+        _name: &str,
+        _verbatim: bool,
+        _as_needed: bool,
+        _weak: bool,
+    ) {
         bug!("framework linked with unsupported linker")
     }
     fn link_staticlib_by_name(&mut self, name: &str, verbatim: bool, whole_archive: bool);
@@ -569,7 +575,7 @@ impl<'a> Linker for GccLinker<'a> {
         }
     }
 
-    fn link_dylib_by_name(&mut self, name: &str, verbatim: bool, as_needed: bool) {
+    fn link_dylib_by_name(&mut self, name: &str, verbatim: bool, as_needed: bool, weak: bool) {
         if self.sess.target.os == "illumos" && name == "c" {
             // libc will be added via late_link_args on illumos so that it will
             // appear last in the library search order.
@@ -581,26 +587,39 @@ impl<'a> Linker for GccLinker<'a> {
         self.hint_dynamic();
         self.with_as_needed(as_needed, |this| {
             let colon = if verbatim && this.is_gnu { ":" } else { "" };
-            this.link_or_cc_arg(format!("-l{colon}{name}"));
+            if weak {
+                this.link_or_cc_arg(format!("-weak-l{colon}{name}"));
+            } else {
+                this.link_or_cc_arg(format!("-l{colon}{name}"));
+            }
         });
     }
 
-    fn link_dylib_by_path(&mut self, path: &Path, as_needed: bool) {
+    fn link_dylib_by_path(&mut self, path: &Path, as_needed: bool, weak: bool) {
         self.hint_dynamic();
         self.with_as_needed(as_needed, |this| {
-            this.link_or_cc_arg(path);
+            if weak {
+                this.link_or_cc_arg("-weak_library");
+                this.link_or_cc_arg(path);
+            } else {
+                this.link_or_cc_arg(path);
+            }
         })
     }
 
-    fn link_framework_by_name(&mut self, name: &str, _verbatim: bool, as_needed: bool) {
+    fn link_framework_by_name(&mut self, name: &str, _verbatim: bool, as_needed: bool, weak: bool) {
         self.hint_dynamic();
         if !as_needed {
             // FIXME(81490): ld64 as of macOS 11 supports the -needed_framework
             // flag but we have no way to detect that here.
             // self.link_or_cc_arg("-needed_framework").link_or_cc_arg(name);
             self.sess.dcx().emit_warn(errors::Ld64UnimplementedModifier);
         }
-        self.link_or_cc_args(&["-framework", name]);
+        if weak {
+            self.link_or_cc_args(&["-weak_framework", name]);
+        } else {
+            self.link_or_cc_args(&["-framework", name]);
+        }
     }
 
     fn link_staticlib_by_name(&mut self, name: &str, verbatim: bool, whole_archive: bool) {
@@ -923,7 +942,7 @@ impl<'a> Linker for MsvcLinker<'a> {
         }
     }
 
-    fn link_dylib_by_name(&mut self, name: &str, verbatim: bool, _as_needed: bool) {
+    fn link_dylib_by_name(&mut self, name: &str, verbatim: bool, _as_needed: bool, _weak: bool) {
         // On MSVC-like targets rustc supports import libraries using alternative naming
         // scheme (`libfoo.a`) unsupported by linker, search for such libraries manually.
         if let Some(path) = try_find_native_dynamic_library(self.sess, name, verbatim) {
@@ -933,7 +952,7 @@ impl<'a> Linker for MsvcLinker<'a> {
         }
     }
 
-    fn link_dylib_by_path(&mut self, path: &Path, _as_needed: bool) {
+    fn link_dylib_by_path(&mut self, path: &Path, _as_needed: bool, _weak: bool) {
         // When producing a dll, MSVC linker may not emit an implib file if the dll doesn't export
         // any symbols, so we skip linking if the implib file is not present.
         let implib_path = path.with_extension("dll.lib");
@@ -1171,12 +1190,12 @@ impl<'a> Linker for EmLinker<'a> {
     ) {
     }
 
-    fn link_dylib_by_name(&mut self, name: &str, _verbatim: bool, _as_needed: bool) {
+    fn link_dylib_by_name(&mut self, name: &str, _verbatim: bool, _as_needed: bool, _weak: bool) {
         // Emscripten always links statically
         self.link_or_cc_args(&["-l", name]);
     }
 
-    fn link_dylib_by_path(&mut self, path: &Path, _as_needed: bool) {
+    fn link_dylib_by_path(&mut self, path: &Path, _as_needed: bool, _weak: bool) {
         self.link_or_cc_arg(path);
     }
 
@@ -1338,11 +1357,11 @@ impl<'a> Linker for WasmLd<'a> {
         }
     }
 
-    fn link_dylib_by_name(&mut self, name: &str, _verbatim: bool, _as_needed: bool) {
+    fn link_dylib_by_name(&mut self, name: &str, _verbatim: bool, _as_needed: bool, _weak: bool) {
         self.link_or_cc_args(&["-l", name]);
     }
 
-    fn link_dylib_by_path(&mut self, path: &Path, _as_needed: bool) {
+    fn link_dylib_by_path(&mut self, path: &Path, _as_needed: bool, _weak: bool) {
         self.link_or_cc_arg(path);
     }
 
@@ -1643,12 +1662,12 @@ impl<'a> Linker for AixLinker<'a> {
         }
     }
 
-    fn link_dylib_by_name(&mut self, name: &str, _verbatim: bool, _as_needed: bool) {
+    fn link_dylib_by_name(&mut self, name: &str, _verbatim: bool, _as_needed: bool, _weak: bool) {
         self.hint_dynamic();
         self.link_or_cc_arg(format!("-l{name}"));
     }
 
-    fn link_dylib_by_path(&mut self, path: &Path, _as_needed: bool) {
+    fn link_dylib_by_path(&mut self, path: &Path, _as_needed: bool, _weak: bool) {
         self.hint_dynamic();
         self.link_or_cc_arg(path);
     }

compiler/rustc_error_codes/src/error_codes/E0455.md

@@ -1,7 +1,8 @@
 Some linking kinds are target-specific and not supported on all platforms.
 
-Linking with `kind=framework` is only supported when targeting macOS,
-as frameworks are specific to that operating system.
+Linking with `kind=framework`, `kind=weak_framework` or `kind=weak_dylib` is
+only supported when targeting macOS, as frameworks are specific to that
+operating system.
 
 Similarly, `kind=raw-dylib` is only supported when targeting Windows-like
 platforms.

compiler/rustc_error_codes/src/error_codes/E0458.md

@@ -11,5 +11,7 @@ Please specify a valid "kind" value, from one of the following:
 
 * static
 * dylib
+* weak_dylib
 * framework
+* weak_framework
 * raw-dylib

compiler/rustc_feature/src/unstable.rs

@@ -560,6 +560,8 @@ declare_features! (
     (unstable, naked_functions, "1.9.0", Some(90957)),
     /// Allows specifying the as-needed link modifier
     (unstable, native_link_modifiers_as_needed, "1.53.0", Some(81490)),
+    /// Allows specifying the weak link modifier.
+    (unstable, native_link_modifiers_weak, "CURRENT_RUSTC_VERSION", None),
     /// Allow negative trait implementations.
     (unstable, negative_impls, "1.44.0", Some(68318)),
     /// Allows the `!` pattern.

compiler/rustc_interface/src/tests.rs

@@ -382,7 +382,7 @@ fn test_native_libs_tracking_hash_different_values() {
         NativeLib {
             name: String::from("b"),
             new_name: None,
-            kind: NativeLibKind::Framework { as_needed: None },
+            kind: NativeLibKind::Framework { as_needed: None, weak: None },
             verbatim: None,
         },
         NativeLib {
@@ -404,7 +404,7 @@ fn test_native_libs_tracking_hash_different_values() {
         NativeLib {
             name: String::from("X"),
             new_name: None,
-            kind: NativeLibKind::Framework { as_needed: None },
+            kind: NativeLibKind::Framework { as_needed: None, weak: None },
             verbatim: None,
         },
         NativeLib {
@@ -448,7 +448,7 @@ fn test_native_libs_tracking_hash_different_values() {
         NativeLib {
             name: String::from("b"),
             new_name: Some(String::from("X")),
-            kind: NativeLibKind::Framework { as_needed: None },
+            kind: NativeLibKind::Framework { as_needed: None, weak: None },
             verbatim: None,
         },
         NativeLib {
@@ -470,7 +470,7 @@ fn test_native_libs_tracking_hash_different_values() {
         NativeLib {
             name: String::from("b"),
             new_name: None,
-            kind: NativeLibKind::Framework { as_needed: None },
+            kind: NativeLibKind::Framework { as_needed: None, weak: None },
             verbatim: Some(true),
         },
         NativeLib {
@@ -504,7 +504,7 @@ fn test_native_libs_tracking_hash_different_order() {
         NativeLib {
             name: String::from("b"),
             new_name: None,
-            kind: NativeLibKind::Framework { as_needed: None },
+            kind: NativeLibKind::Framework { as_needed: None, weak: None },
             verbatim: None,
         },
         NativeLib {
@@ -519,7 +519,7 @@ fn test_native_libs_tracking_hash_different_order() {
         NativeLib {
             name: String::from("b"),
             new_name: None,
-            kind: NativeLibKind::Framework { as_needed: None },
+            kind: NativeLibKind::Framework { as_needed: None, weak: None },
             verbatim: None,
         },
         NativeLib {
@@ -552,7 +552,7 @@ fn test_native_libs_tracking_hash_different_order() {
         NativeLib {
             name: String::from("b"),
             new_name: None,
-            kind: NativeLibKind::Framework { as_needed: None },
+            kind: NativeLibKind::Framework { as_needed: None, weak: None },
             verbatim: None,
         },
     ];

compiler/rustc_metadata/messages.ftl

@@ -120,7 +120,7 @@ metadata_install_missing_components =
     maybe you need to install the missing components with: `rustup component add rust-src rustc-dev llvm-tools-preview`
 
 metadata_invalid_link_modifier =
-    invalid linking modifier syntax, expected '+' or '-' prefix before one of: bundle, verbatim, whole-archive, as-needed
+    invalid linking modifier syntax, expected '+' or '-' prefix before one of: bundle, verbatim, whole-archive, as-needed, weak
 
 metadata_invalid_meta_files =
     found invalid metadata files for crate `{$crate_name}`{$add_info}
@@ -134,6 +134,9 @@ metadata_lib_framework_apple =
 metadata_lib_required =
     crate `{$crate_name}` required to be available in {$kind} format, but was not found in this form
 
+metadata_lib_weak_apple =
+    library modifier `weak` is only supported on Apple targets
+
 metadata_link_arg_unstable =
     link kind `link-arg` is unstable
 
@@ -165,6 +168,9 @@ metadata_link_requires_name =
     `#[link]` attribute requires a `name = "string"` argument
     .label = missing `name` argument
 
+metadata_link_weak_apple =
+    link modifier `weak` is only supported on Apple targets
+
 metadata_missing_native_library =
     could not find native static library `{$libname}`, perhaps an -L flag is missing?
 
@@ -282,7 +288,7 @@ metadata_unknown_link_kind =
     .label = unknown link kind
 
 metadata_unknown_link_modifier =
-    unknown linking modifier `{$modifier}`, expected one of: bundle, verbatim, whole-archive, as-needed
+    unknown linking modifier `{$modifier}`, expected one of: bundle, verbatim, whole-archive, as-needed, weak
 
 metadata_unsupported_abi =
     ABI not supported by `#[link(kind = "raw-dylib")]` on this architecture
@@ -293,5 +299,8 @@ metadata_unsupported_abi_i686 =
 metadata_wasm_import_form =
     wasm import module must be of the form `wasm_import_module = "string"`
 
+metadata_weak_compatibility =
+    linking modifier `weak` is only compatible with `dylib` and `framework` linking kinds
+
 metadata_whole_archive_needs_static =
     linking modifier `whole-archive` is only compatible with `static` linking kind

compiler/rustc_metadata/src/errors.rs

@@ -146,6 +146,13 @@ pub struct LinkFrameworkApple {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(metadata_link_weak_apple, code = E0455)]
+pub struct LinkWeakApple {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(metadata_raw_dylib_only_windows, code = E0455)]
 pub struct RawDylibOnlyWindows {
@@ -233,6 +240,13 @@ pub struct AsNeededCompatibility {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(metadata_weak_compatibility)]
+pub struct WeakCompatibility {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(metadata_unknown_link_modifier)]
 pub struct UnknownLinkModifier<'a> {
@@ -274,6 +288,10 @@ pub struct LinkOrdinalRawDylib {
 #[diag(metadata_lib_framework_apple)]
 pub struct LibFrameworkApple;
 
+#[derive(Diagnostic)]
+#[diag(metadata_lib_weak_apple)]
+pub struct LibWeakApple;
+
 #[derive(Diagnostic)]
 #[diag(metadata_empty_renaming_target)]
 pub struct EmptyRenamingTarget<'a> {