Tests for external linkage symbol collision check.

Fix rust-lang#61232

Author: pnkfelix

src/test/ui/linkage-attr/auxiliary/def_colliding_external.rs

@@ -0,0 +1,7 @@
+#![feature(linkage)]
+#![crate_type = "lib"]
+
+extern {
+    #[linkage="external"]
+    pub static collision: *const i32;
+}

src/test/ui/linkage-attr/linkage-detect-extern-generated-name-collision.rs

@@ -0,0 +1,21 @@
+// rust-lang/rust#61232: We used to ICE when trying to detect a
+// collision on the symbol generated for the external linkage item in
+// an extern crate.
+
+// aux-build:def_colliding_external.rs
+
+extern crate def_colliding_external as dep1;
+
+#[no_mangle]
+pub static _rust_extern_with_linkage_collision: i32 = 0;
+
+mod dep2 {
+    #[no_mangle]
+    pub static collision: usize = 0;
+}
+
+fn main() {
+    unsafe {
+       println!("{:p}", &dep1::collision);
+    }
+}

src/test/ui/linkage-attr/linkage-detect-extern-generated-name-collision.stderr

@@ -0,0 +1,8 @@
+error: symbol `collision` is already defined
+  --> $DIR/auxiliary/def_colliding_external.rs:6:5
+   |
+LL |     pub static collision: *const i32;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/linkage-attr/linkage-detect-local-generated-name-collision.rs

@@ -0,0 +1,23 @@
+#![feature(linkage)]
+
+mod dep1 {
+    extern {
+        #[linkage="external"]
+        #[no_mangle]
+        pub static collision: *const i32; //~ ERROR symbol `collision` is already defined
+    }
+}
+
+#[no_mangle]
+pub static _rust_extern_with_linkage_collision: i32 = 0;
+
+mod dep2 {
+    #[no_mangle]
+    pub static collision: usize = 0;
+}
+
+fn main() {
+    unsafe {
+       println!("{:p}", &dep1::collision);
+    }
+}

src/test/ui/linkage-attr/linkage-detect-local-generated-name-collision.stderr

@@ -0,0 +1,8 @@
+error: symbol `collision` is already defined
+  --> $DIR/linkage-detect-local-generated-name-collision.rs:7:9
+   |
+LL |         pub static collision: *const i32;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+