Update existing test cases.

- Allow ClashingExternDecl for lint-dead-code-3
- Update test case for #5791
- Update test case for #1866
- Update extern-abi-from-macro test case

src/test/ui/issues/issue-1866.rs

@@ -1,6 +1,7 @@
 // build-pass
 #![allow(dead_code)]
 #![allow(non_camel_case_types)]
+#![warn(clashing_extern_decl)]
 
 // pretty-expanded FIXME #23616
 
@@ -20,6 +21,7 @@ mod b {
         use super::rust_task;
         extern {
             pub fn rust_task_is_unwinding(rt: *const rust_task) -> bool;
+            //~^ WARN `rust_task_is_unwinding` redeclared with a different signature
         }
     }
 }

src/test/ui/issues/issue-1866.stderr

@@ -0,0 +1,19 @@
+warning: `rust_task_is_unwinding` redeclared with a different signature
+  --> $DIR/issue-1866.rs:23:13
+   |
+LL |             pub fn rust_task_is_unwinding(rt: *const rust_task) -> bool;
+   |             ------------------------------------------------------------ `rust_task_is_unwinding` previously declared here
+...
+LL |             pub fn rust_task_is_unwinding(rt: *const rust_task) -> bool;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
+   |
+note: the lint level is defined here
+  --> $DIR/issue-1866.rs:4:9
+   |
+LL | #![warn(clashing_extern_decl)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+   = note: expected  `unsafe extern "C" fn(*const usize) -> bool`
+              found  `unsafe extern "C" fn(*const bool) -> bool`
+
+warning: 1 warning emitted
+

src/test/ui/issues/issue-5791.rs

@@ -1,11 +1,13 @@
 // run-pass
 #![allow(dead_code)]
+#![warn(clashing_extern_decl)]
 // pretty-expanded FIXME #23616
 
 extern {
     #[link_name = "malloc"]
     fn malloc1(len: i32) -> *const u8;
     #[link_name = "malloc"]
+    //~^ WARN `malloc2` redeclares `malloc` with a different signature
     fn malloc2(len: i32, foo: i32) -> *const u8;
 }
 

src/test/ui/issues/issue-5791.stderr

@@ -0,0 +1,21 @@
+warning: `malloc2` redeclares `malloc` with a different signature
+  --> $DIR/issue-5791.rs:9:5
+   |
+LL | /     #[link_name = "malloc"]
+LL | |     fn malloc1(len: i32) -> *const u8;
+   | |______________________________________- `malloc` previously declared here
+LL | /     #[link_name = "malloc"]
+LL | |
+LL | |     fn malloc2(len: i32, foo: i32) -> *const u8;
+   | |________________________________________________^ this signature doesn't match the previous declaration
+   |
+note: the lint level is defined here
+  --> $DIR/issue-5791.rs:3:9
+   |
+LL | #![warn(clashing_extern_decl)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+   = note: expected  `unsafe extern "C" fn(i32) -> *const u8`
+              found  `unsafe extern "C" fn(i32, i32) -> *const u8`
+
+warning: 1 warning emitted
+

src/test/ui/lint/dead-code/lint-dead-code-3.rs

@@ -1,5 +1,6 @@
 #![allow(unused_variables)]
 #![allow(non_camel_case_types)]
+#![allow(clashing_extern_decl)]
 #![deny(dead_code)]
 
 #![crate_type="lib"]

src/test/ui/lint/dead-code/lint-dead-code-3.stderr

@@ -1,35 +1,35 @@
 error: struct is never constructed: `Foo`
-  --> $DIR/lint-dead-code-3.rs:13:8
+  --> $DIR/lint-dead-code-3.rs:14:8
    |
 LL | struct Foo;
    |        ^^^
    |
 note: the lint level is defined here
-  --> $DIR/lint-dead-code-3.rs:3:9
+  --> $DIR/lint-dead-code-3.rs:4:9
    |
 LL | #![deny(dead_code)]
    |         ^^^^^^^^^
 
 error: associated function is never used: `foo`
-  --> $DIR/lint-dead-code-3.rs:15:8
+  --> $DIR/lint-dead-code-3.rs:16:8
    |
 LL |     fn foo(&self) {
    |        ^^^
 
 error: function is never used: `bar`
-  --> $DIR/lint-dead-code-3.rs:20:4
+  --> $DIR/lint-dead-code-3.rs:21:4
    |
 LL | fn bar() {
    |    ^^^
 
 error: enum is never used: `c_void`
-  --> $DIR/lint-dead-code-3.rs:59:6
+  --> $DIR/lint-dead-code-3.rs:60:6
    |
 LL | enum c_void {}
    |      ^^^^^^
 
 error: function is never used: `free`
-  --> $DIR/lint-dead-code-3.rs:61:5
+  --> $DIR/lint-dead-code-3.rs:62:5
    |
 LL |     fn free(p: *const c_void);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^

src/test/ui/parser/extern-abi-from-mac-literal-frag.rs

@@ -1,3 +1,4 @@
+#![allow(clashing_extern_decl)]
 // check-pass
 
 // In this test we check that the parser accepts an ABI string when it