Add tests for command line lint control

Author: inquisitivecrystal

Diff for: src/test/ui/lint/cli-lint-override.forbid_warn.stderr

@@ -0,0 +1,11 @@
+error: extern declarations without an explicit ABI are deprecated
+  --> $DIR/cli-lint-override.rs:12:1
+   |
+LL | extern fn foo() {}
+   | ^^^^^^^^^^^^^^^ ABI should be specified here
+   |
+   = note: requested on the command line with `-F missing-abi`
+   = help: the default ABI is C
+
+error: aborting due to previous error
+

Diff for: src/test/ui/lint/cli-lint-override.force_warn_deny.stderr

@@ -0,0 +1,11 @@
+warning: extern declarations without an explicit ABI are deprecated
+  --> $DIR/cli-lint-override.rs:12:1
+   |
+LL | extern fn foo() {}
+   | ^^^^^^^^^^^^^^^ ABI should be specified here
+   |
+   = note: requested on the command line with `--force-warns missing-abi`
+   = help: the default ABI is C
+
+warning: 1 warning emitted
+

Diff for: src/test/ui/lint/cli-lint-override.rs

@@ -0,0 +1,17 @@
+// Tests that subsequent lints specified via the command line override
+// each other, except for ForceWarn and Forbid, which cannot be overriden.
+//
+// revisions: warn_deny forbid_warn force_warn_deny
+//
+//[warn_deny] compile-flags: --warn missing_abi --deny missing_abi
+//[forbid_warn] compile-flags: --warn missing_abi --forbid missing_abi
+//[force_warn_deny] compile-flags: -Z unstable-options --force-warns missing_abi --allow missing_abi
+//[force_warn_deny] check-pass
+
+
+extern fn foo() {}
+//[warn_deny]~^ ERROR extern declarations without an explicit ABI are deprecated
+//[forbid_warn]~^^ ERROR extern declarations without an explicit ABI are deprecated
+//[force_warn_deny]~^^^ WARN extern declarations without an explicit ABI are deprecated
+
+fn main() {}

Diff for: src/test/ui/lint/cli-lint-override.warn_deny.stderr

@@ -0,0 +1,11 @@
+error: extern declarations without an explicit ABI are deprecated
+  --> $DIR/cli-lint-override.rs:12:1
+   |
+LL | extern fn foo() {}
+   | ^^^^^^^^^^^^^^^ ABI should be specified here
+   |
+   = note: requested on the command line with `-D missing-abi`
+   = help: the default ABI is C
+
+error: aborting due to previous error
+

Diff for: src/test/ui/lint/cli-unknown-force-warn.rs

@@ -0,0 +1,7 @@
+// Checks that rustc correctly errors when passed an invalid lint with
+// `--force-warns`. This is a regression test for issue #86958.
+//
+// compile-flags: -Z unstable-options --force-warns foo-qux
+// error-pattern: unknown lint: `foo_qux`
+
+fn main() {}

Diff for: src/test/ui/lint/cli-unknown-force-warn.stderr

@@ -0,0 +1,15 @@
+error[E0602]: unknown lint: `foo_qux`
+   |
+   = note: requested on the command line with `--force-warns foo_qux`
+
+error[E0602]: unknown lint: `foo_qux`
+   |
+   = note: requested on the command line with `--force-warns foo_qux`
+
+error[E0602]: unknown lint: `foo_qux`
+   |
+   = note: requested on the command line with `--force-warns foo_qux`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0602`.