New force_warn diagnostic builder and ensure cap-lints doesn't reduce force_warn level

compiler/rustc_errors/src/lib.rs

@@ -521,12 +521,24 @@ impl Handler {
     }
 
     /// Construct a builder at the `Warning` level at the given `span` and with the `msg`.
+    /// Cancel the builder if warnings cannot be emitted
     pub fn struct_span_warn(&self, span: impl Into<MultiSpan>, msg: &str) -> DiagnosticBuilder<'_> {
         let mut result = self.struct_warn(msg);
         result.set_span(span);
         result
     }
 
+    /// Construct a builder at the `Warning` level at the given `span` and with the `msg`.
+    pub fn struct_span_force_warn(
+        &self,
+        span: impl Into<MultiSpan>,
+        msg: &str,
+    ) -> DiagnosticBuilder<'_> {
+        let mut result = self.struct_force_warn(msg);
+        result.set_span(span);
+        result
+    }
+
     /// Construct a builder at the `Allow` level at the given `span` and with the `msg`.
     pub fn struct_span_allow(
         &self,
@@ -552,6 +564,7 @@ impl Handler {
     }
 
     /// Construct a builder at the `Warning` level with the `msg`.
+    /// Cancel the builder if warnings cannot be emitted
     pub fn struct_warn(&self, msg: &str) -> DiagnosticBuilder<'_> {
         let mut result = DiagnosticBuilder::new(self, Level::Warning, msg);
         if !self.flags.can_emit_warnings {
@@ -560,6 +573,11 @@ impl Handler {
         result
     }
 
+    /// Construct a builder at the `Warning` level with the `msg`.
+    pub fn struct_force_warn(&self, msg: &str) -> DiagnosticBuilder<'_> {
+        DiagnosticBuilder::new(self, Level::Warning, msg)
+    }
+
     /// Construct a builder at the `Allow` level with the `msg`.
     pub fn struct_allow(&self, msg: &str) -> DiagnosticBuilder<'_> {
         DiagnosticBuilder::new(self, Level::Allow, msg)