Force warnings even when can_emit_warnings == false

compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs

@@ -145,8 +145,9 @@ impl AnnotateSnippetEmitterWriter {
                 title: Some(Annotation {
                     label: Some(&message),
                     id: code.as_ref().map(|c| match c {
-                        DiagnosticId::Error(val)
-                        | DiagnosticId::Lint { name: val, has_future_breakage: _ } => val.as_str(),
+                        DiagnosticId::Error(val) | DiagnosticId::Lint { name: val, .. } => {
+                            val.as_str()
+                        }
                     }),
                     annotation_type: annotation_type_for_level(*level),
                 }),

compiler/rustc_errors/src/diagnostic.rs

@@ -29,7 +29,7 @@ pub struct Diagnostic {
 #[derive(Clone, Debug, PartialEq, Eq, Hash, Encodable, Decodable)]
 pub enum DiagnosticId {
     Error(String),
-    Lint { name: String, has_future_breakage: bool },
+    Lint { name: String, has_future_breakage: bool, is_force_warn: bool },
 }
 
 /// A "sub"-diagnostic attached to a parent diagnostic.
@@ -109,6 +109,13 @@ impl Diagnostic {
         }
     }
 
+    pub fn is_force_warn(&self) -> bool {
+        match self.code {
+            Some(DiagnosticId::Lint { is_force_warn, .. }) => is_force_warn,
+            _ => false,
+        }
+    }
+
     /// Cancel the diagnostic (a structured diagnostic must either be emitted or
     /// canceled or it will panic when dropped).
     pub fn cancel(&mut self) {

compiler/rustc_errors/src/json.rs

@@ -559,7 +559,7 @@ impl DiagnosticCode {
         s.map(|s| {
             let s = match s {
                 DiagnosticId::Error(s) => s,
-                DiagnosticId::Lint { name, has_future_breakage: _ } => name,
+                DiagnosticId::Lint { name, .. } => name,
             };
             let je_result =
                 je.registry.as_ref().map(|registry| registry.try_find_description(&s)).unwrap();

compiler/rustc_errors/src/lib.rs

@@ -802,7 +802,10 @@ impl HandlerInner {
             self.future_breakage_diagnostics.push(diagnostic.clone());
         }
 
-        if diagnostic.level == Warning && !self.flags.can_emit_warnings {
+        if diagnostic.level == Warning
+            && !self.flags.can_emit_warnings
+            && !diagnostic.is_force_warn()
+        {
             if diagnostic.has_future_breakage() {
                 (*TRACK_DIAGNOSTICS)(diagnostic);
             }
@@ -874,7 +877,7 @@ impl HandlerInner {
 
         match (errors.len(), warnings.len()) {
             (0, 0) => return,
-            (0, _) => self.emit_diagnostic(&Diagnostic::new(Level::Warning, &warnings)),
+            (0, _) => self.emitter.emit_diagnostic(&Diagnostic::new(Level::Warning, &warnings)),
             (_, 0) => {
                 let _ = self.fatal(&errors);
             }

compiler/rustc_middle/src/lint.rs

@@ -246,6 +246,9 @@ pub fn struct_lint_level<'s, 'd>(
         let has_future_breakage =
             future_incompatible.map_or(false, |incompat| incompat.future_breakage.is_some());
 
+        let is_force_warn = matches!(level, Level::ForceWarn)
+            || matches!(src, LintLevelSource::CommandLine(_, Level::ForceWarn));
+
         let mut err = match (level, span) {
             (Level::Allow, span) => {
                 if has_future_breakage {
@@ -254,6 +257,16 @@ pub fn struct_lint_level<'s, 'd>(
                     } else {
                         sess.struct_allow("")
                     }
+                } else if is_force_warn {
+                    let mut err = if let Some(span) = span {
+                        sess.struct_span_warn(span, "")
+                    } else {
+                        sess.struct_warn("")
+                    };
+                    // Ensure force-warn warns even if the diagnostic has
+                    // been canceled for reasons like `--cap-lints`
+                    err.level = rustc_errors::Level::Warning;
+                    err
                 } else {
                     return;
                 }
@@ -349,7 +362,7 @@ pub fn struct_lint_level<'s, 'd>(
             }
         }
 
-        err.code(DiagnosticId::Lint { name, has_future_breakage });
+        err.code(DiagnosticId::Lint { name, has_future_breakage, is_force_warn });
 
         if let Some(future_incompatible) = future_incompatible {
             let explanation = if lint_id == LintId::of(builtin::UNSTABLE_NAME_COLLISIONS) {

compiler/rustc_session/src/session.rs

@@ -323,7 +323,7 @@ impl Session {
             .into_iter()
             .map(|diag| {
                 let lint_name = match &diag.code {
-                    Some(DiagnosticId::Lint { name, has_future_breakage: true }) => name,
+                    Some(DiagnosticId::Lint { name, has_future_breakage: true, .. }) => name,
                     _ => panic!("Unexpected code in diagnostic {:?}", diag),
                 };
                 let lint = lint_store.name_to_lint(&lint_name);

src/test/ui/lint/force-warn/force-warns-cap-lints.rs

@@ -0,0 +1,10 @@
+// compile-flags: --cap-lints allow  --force-warns bare_trait_objects -Zunstable-options
+// check-pass
+
+pub trait SomeTrait {}
+
+pub fn function(_x: Box<SomeTrait>) {}
+//~^ WARN trait objects without an explicit `dyn` are deprecated
+//~| WARN this is accepted in the current edition
+
+fn main() {}

src/test/ui/lint/force-warn/force-warns-cap-lints.stderr

@@ -0,0 +1,12 @@
+warning: trait objects without an explicit `dyn` are deprecated
+  --> $DIR/force-warns-cap-lints.rs:6:25
+   |
+LL | pub fn function(_x: Box<SomeTrait>) {}
+   |                         ^^^^^^^^^ help: use `dyn`: `dyn SomeTrait`
+   |
+   = note: requested on the command line with `--force-warns bare-trait-objects`
+   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2021!
+   = note: for more information, see issue #80165 <https://github.com/rust-lang/rust/issues/80165>
+
+warning: 1 warning emitted
+