Regression test.

src/test/ui/lint/issue-70819-dont-override-forbid-in-same-scope.rs

@@ -0,0 +1,49 @@
+// This test is checking that you cannot override a `forbid` by adding in other
+// attributes later in the same scope. (We already ensure that you cannot
+// override it in nested scopes).
+
+// If you turn off deduplicate diagnostics (which rustc turns on by default but
+// compiletest turns off when it runs ui tests), then the errors are
+// (unfortunately) repeated here because the checking is done as we read in the
+// errors, and curretly that happens two or three different times, depending on
+// compiler flags.
+//
+// I decided avoiding the redundant output was not worth the time in engineering
+// effort for bug like this, which 1. end users are unlikely to run into in the
+// first place, and 2. they won't see the redundant output anyway.
+
+// compile-flags: -Z deduplicate-diagnostics=yes
+
+fn forbid_first(num: i32) -> i32 {
+    #![forbid(unused)]
+    #![deny(unused)]
+    //~^ ERROR: deny(unused) incompatible with previous forbid in same scope [E0453]
+    #![warn(unused)]
+    //~^ ERROR: warn(unused) incompatible with previous forbid in same scope [E0453]
+    #![allow(unused)]
+    //~^ ERROR: allow(unused) incompatible with previous forbid in same scope [E0453]
+
+    num * num
+}
+
+fn forbid_last(num: i32) -> i32 {
+    #![deny(unused)]
+    #![warn(unused)]
+    #![allow(unused)]
+    #![forbid(unused)]
+
+    num * num
+}
+
+fn forbid_multiple(num: i32) -> i32 {
+    #![forbid(unused)]
+    #![forbid(unused)]
+
+    num * num
+}
+
+fn main() {
+    forbid_first(10);
+    forbid_last(10);
+    forbid_multiple(10);
+}

src/test/ui/lint/issue-70819-dont-override-forbid-in-same-scope.stderr

@@ -0,0 +1,29 @@
+error[E0453]: deny(unused) incompatible with previous forbid in same scope
+  --> $DIR/issue-70819-dont-override-forbid-in-same-scope.rs:19:13
+   |
+LL |     #![forbid(unused)]
+   |               ------ `forbid` level set here
+LL |     #![deny(unused)]
+   |             ^^^^^^
+
+error[E0453]: warn(unused) incompatible with previous forbid in same scope
+  --> $DIR/issue-70819-dont-override-forbid-in-same-scope.rs:21:13
+   |
+LL |     #![forbid(unused)]
+   |               ------ `forbid` level set here
+...
+LL |     #![warn(unused)]
+   |             ^^^^^^
+
+error[E0453]: allow(unused) incompatible with previous forbid in same scope
+  --> $DIR/issue-70819-dont-override-forbid-in-same-scope.rs:23:14
+   |
+LL |     #![forbid(unused)]
+   |               ------ `forbid` level set here
+...
+LL |     #![allow(unused)]
+   |              ^^^^^^
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0453`.