linker: Link dylib crates by path

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -2816,6 +2816,15 @@ fn rehome_sysroot_lib_dir(sess: &Session, lib_dir: &Path) -> PathBuf {
     }
 }
 
+fn rehome_lib_path(sess: &Session, path: &Path) -> PathBuf {
+    if let Some(dir) = path.parent() {
+        let file_name = path.file_name().expect("library path has no file name component");
+        rehome_sysroot_lib_dir(sess, dir).join(file_name)
+    } else {
+        fix_windows_verbatim_for_gcc(path)
+    }
+}
+
 // Adds the static "rlib" versions of all crates to the command line.
 // There's a bit of magic which happens here specifically related to LTO,
 // namely that we remove upstream object files.
@@ -2847,12 +2856,7 @@ fn add_static_crate(
     let cratepath = &src.rlib.as_ref().unwrap().0;
 
     let mut link_upstream = |path: &Path| {
-        let rlib_path = if let Some(dir) = path.parent() {
-            let file_name = path.file_name().expect("rlib path has no file name path component");
-            rehome_sysroot_lib_dir(sess, dir).join(file_name)
-        } else {
-            fix_windows_verbatim_for_gcc(path)
-        };
+        let rlib_path = rehome_lib_path(sess, path);
         cmd.link_staticlib_by_path(&rlib_path, false);
     };
 
@@ -2918,27 +2922,16 @@ 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) {
-    // Just need to tell the linker about where the library lives and
-    // what its name is
-    let parent = cratepath.parent();
     // When producing a dll, the MSVC linker may not actually emit a
     // `foo.lib` file if the dll doesn't actually export any symbols, so we
     // check to see if the file is there and just omit linking to it if it's
     // not present.
     if sess.target.is_like_msvc && !cratepath.with_extension("dll.lib").exists() {
         return;
     }
-    if let Some(dir) = parent {
-        cmd.include_path(&rehome_sysroot_lib_dir(sess, dir));
-    }
-    // "<dir>/name.dll -> name.dll" on windows-msvc
-    // "<dir>/name.dll -> name" on windows-gnu
-    // "<dir>/libname.<ext> -> name" elsewhere
-    let stem = if sess.target.is_like_msvc { cratepath.file_name() } else { cratepath.file_stem() };
-    let stem = stem.unwrap().to_str().unwrap();
-    // Convert library file-stem into a cc -l argument.
-    let prefix = if stem.starts_with("lib") && !sess.target.is_like_windows { 3 } else { 0 };
-    cmd.link_dylib_by_name(&stem[prefix..], false, true);
+
+    let dylib_path = rehome_lib_path(sess, cratepath);
+    cmd.link_dylib_by_path(&dylib_path, true);
 }
 
 fn relevant_lib(sess: &Session, lib: &NativeLib) -> bool {

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -170,6 +170,9 @@ pub trait Linker {
     fn cmd(&mut self) -> &mut Command;
     fn set_output_kind(&mut self, output_kind: LinkOutputKind, out_filename: &Path);
     fn link_dylib_by_name(&mut self, name: &str, verbatim: bool, as_needed: bool);
+    fn link_dylib_by_path(&mut self, path: &Path, _as_needed: bool) {
+        self.cmd().arg(path);
+    }
     fn link_framework_by_name(&mut self, _name: &str, _verbatim: bool, _as_needed: bool) {
         bug!("framework linked with unsupported linker")
     }
@@ -466,6 +469,29 @@ impl<'a> Linker for GccLinker<'a> {
         }
     }
 
+    fn link_dylib_by_path(&mut self, path: &Path, as_needed: bool) {
+        if !as_needed {
+            if self.sess.target.is_like_osx {
+                // FIXME(81490): ld64 doesn't support these flags but macOS 11
+                // has -needed-l{} / -needed_library {}
+                // but we have no way to detect that here.
+                self.sess.dcx().emit_warn(errors::Ld64UnimplementedModifier);
+            } else if self.is_gnu && !self.sess.target.is_like_windows {
+                self.linker_arg("--no-as-needed");
+            } else {
+                self.sess.dcx().emit_warn(errors::LinkerUnsupportedModifier);
+            }
+        }
+        self.cmd.arg(path);
+        if !as_needed {
+            if self.sess.target.is_like_osx {
+                // See above FIXME comment
+            } else if self.is_gnu && !self.sess.target.is_like_windows {
+                self.linker_arg("--as-needed");
+            }
+        }
+    }
+
     fn link_framework_by_name(&mut self, name: &str, _verbatim: bool, as_needed: bool) {
         self.hint_dynamic();
         if !as_needed {