Add regression test for shared-generics x dylibs (#67276).

src/test/run-make-fulldeps/share-generics-dylib/Makefile

@@ -0,0 +1,22 @@
+# This test makes sure all generic instances get re-exported from Rust dylibs for use by
+# `-Zshare-generics`. There are two rlibs (`instance_provider_a` and `instance_provider_b`)
+# which both provide an instance of `Cell<i32>::set`. There is `instance_user_dylib` which is
+# supposed to re-export both these instances, and then there are `instance_user_a_rlib` and
+# `instance_user_b_rlib` which each rely on a specific instance to be available.
+#
+# In the end everything is linked together into `linked_leaf`. If `instance_user_dylib` does
+# not export both then we'll get an `undefined reference` error for one of the instances.
+#
+# This is regression test for https://github.com/rust-lang/rust/issues/67276.
+
+-include ../../run-make-fulldeps/tools.mk
+
+COMMON_ARGS=-Cprefer-dynamic -Zshare-generics=yes -Ccodegen-units=1 -Zsymbol-mangling-version=v0
+
+all:
+	$(RUSTC) instance_provider_a.rs $(COMMON_ARGS) --crate-type=rlib
+	$(RUSTC) instance_provider_b.rs $(COMMON_ARGS) --crate-type=rlib
+	$(RUSTC) instance_user_dylib.rs $(COMMON_ARGS) --crate-type=dylib
+	$(RUSTC) instance_user_a_rlib.rs $(COMMON_ARGS) --crate-type=rlib
+	$(RUSTC) instance_user_b_rlib.rs $(COMMON_ARGS) --crate-type=rlib
+	$(RUSTC) linked_leaf.rs $(COMMON_ARGS) --crate-type=bin

src/test/run-make-fulldeps/share-generics-dylib/instance_provider_a.rs

@@ -0,0 +1,6 @@
+use std::cell::Cell;
+
+pub fn foo() {
+    let a: Cell<i32> = Cell::new(1);
+    a.set(123);
+}

src/test/run-make-fulldeps/share-generics-dylib/instance_provider_b.rs

@@ -0,0 +1,6 @@
+use std::cell::Cell;
+
+pub fn foo() {
+    let b: Cell<i32> = Cell::new(1);
+    b.set(123);
+}

src/test/run-make-fulldeps/share-generics-dylib/instance_user_a_rlib.rs

@@ -0,0 +1,9 @@
+extern crate instance_provider_a as upstream;
+use std::cell::Cell;
+
+pub fn foo() {
+    upstream::foo();
+
+    let b: Cell<i32> = Cell::new(1);
+    b.set(123);
+}

src/test/run-make-fulldeps/share-generics-dylib/instance_user_b_rlib.rs

@@ -0,0 +1,9 @@
+extern crate instance_provider_b as upstream;
+use std::cell::Cell;
+
+pub fn foo() {
+    upstream::foo();
+
+    let b: Cell<i32> = Cell::new(1);
+    b.set(123);
+}

src/test/run-make-fulldeps/share-generics-dylib/instance_user_dylib.rs

@@ -0,0 +1,7 @@
+extern crate instance_provider_a;
+extern crate instance_provider_b;
+
+pub fn foo() {
+    instance_provider_a::foo();
+    instance_provider_b::foo();
+}

src/test/run-make-fulldeps/share-generics-dylib/linked_leaf.rs

@@ -0,0 +1,15 @@
+extern crate instance_user_dylib;
+extern crate instance_user_a_rlib;
+extern crate instance_user_b_rlib;
+
+use std::cell::Cell;
+
+fn main() {
+
+    instance_user_a_rlib::foo();
+    instance_user_b_rlib::foo();
+    instance_user_dylib::foo();
+
+    let a: Cell<i32> = Cell::new(1);
+    a.set(123);
+}