Fix issues and add test

compiler/rustc_lint/src/levels.rs

@@ -147,7 +147,7 @@ impl<'s> LintLevelsBuilder<'s> {
                     LintLevelSource::Default => false,
                     LintLevelSource::Node(symbol, _, _) => self.store.is_lint_group(symbol),
                     LintLevelSource::CommandLine(symbol, _) => self.store.is_lint_group(symbol),
-                    LintLevelSource::ForceWarn(symbol) => {
+                    LintLevelSource::ForceWarn(_symbol) => {
                         bug!("forced warn lint returned a forbid lint level")
                     }
                 };

compiler/rustc_middle/src/lint.rs

@@ -1,7 +1,6 @@
 use std::cmp;
 
 use crate::ich::StableHashingContext;
-use chalk_ir::Substitution;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_errors::{DiagnosticBuilder, DiagnosticId};

src/test/ui/lint/force-warn/force-allowed-group.rs

@@ -1,7 +1,7 @@
 // compile-flags: --force-warns bare_trait_objects
 // check-pass
 
-#![allow(rust_2018_compatibility)]
+#![allow(rust_2018_idioms)]
 
 pub trait SomeTrait {}
 

src/test/ui/lint/force-warn/force-warn-group.rs

@@ -0,0 +1,13 @@
+// ignore-test
+// compile-flags: --force-warns rust_2018_idioms
+// check-pass
+
+#![allow(rust_2018_idioms)]
+
+pub trait SomeTrait {}
+
+pub fn function(_x: Box<SomeTrait>) {}
+//~^ WARN trait objects without an explicit `dyn` are deprecated
+//~| WARN this was previously accepted by the compiler
+
+fn main() {}