Add a (hint) diagnostic for unconfigured items

crates/hir/src/diagnostics.rs

@@ -1,5 +1,5 @@
 //! FIXME: write short doc here
-pub use hir_def::diagnostics::UnresolvedModule;
+pub use hir_def::diagnostics::{UnconfiguredCode, UnresolvedModule};
 pub use hir_expand::diagnostics::{Diagnostic, DiagnosticSink, DiagnosticSinkBuilder};
 pub use hir_ty::diagnostics::{
     IncorrectCase, MismatchedArgCount, MissingFields, MissingMatchArms, MissingOkInTailExpr,

crates/hir_def/src/diagnostics.rs

@@ -86,3 +86,28 @@ impl Diagnostic for UnresolvedImport {
         true
     }
 }
+
+// Diagnostic: unconfigured-code
+//
+// This diagnostic is shown for code with inactive `#[cfg]` attributes.
+#[derive(Debug)]
+pub struct UnconfiguredCode {
+    pub file: HirFileId,
+    pub node: SyntaxNodePtr,
+}
+
+impl Diagnostic for UnconfiguredCode {
+    fn code(&self) -> DiagnosticCode {
+        DiagnosticCode("unconfigured-code")
+    }
+    fn message(&self) -> String {
+        // FIXME: say *why* it is configured out
+        "configured out".to_string()
+    }
+    fn display_source(&self) -> InFile<SyntaxNodePtr> {
+        InFile::new(self.file, self.node.clone())
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static) {
+        self
+    }
+}

crates/hir_def/src/item_tree.rs

@@ -672,6 +672,24 @@ impl ModItem {
     pub fn downcast<N: ItemTreeNode>(self) -> Option<FileItemTreeId<N>> {
         N::id_from_mod_item(self)
     }
+
+    pub fn ast_id(&self, tree: &ItemTree) -> FileAstId<ast::Item> {
+        match self {
+            ModItem::Import(it) => tree[it.index].ast_id().upcast(),
+            ModItem::ExternCrate(it) => tree[it.index].ast_id().upcast(),
+            ModItem::Function(it) => tree[it.index].ast_id().upcast(),
+            ModItem::Struct(it) => tree[it.index].ast_id().upcast(),
+            ModItem::Union(it) => tree[it.index].ast_id().upcast(),
+            ModItem::Enum(it) => tree[it.index].ast_id().upcast(),
+            ModItem::Const(it) => tree[it.index].ast_id().upcast(),
+            ModItem::Static(it) => tree[it.index].ast_id().upcast(),
+            ModItem::Trait(it) => tree[it.index].ast_id().upcast(),
+            ModItem::Impl(it) => tree[it.index].ast_id().upcast(),
+            ModItem::TypeAlias(it) => tree[it.index].ast_id().upcast(),
+            ModItem::Mod(it) => tree[it.index].ast_id().upcast(),
+            ModItem::MacroCall(it) => tree[it.index].ast_id().upcast(),
+        }
+    }
 }
 
 #[derive(Debug, Copy, Clone, Eq, PartialEq)]

crates/hir_def/src/nameres.rs

@@ -286,7 +286,7 @@ mod diagnostics {
     use hir_expand::diagnostics::DiagnosticSink;
     use hir_expand::hygiene::Hygiene;
     use hir_expand::InFile;
-    use syntax::{ast, AstPtr};
+    use syntax::{ast, AstPtr, SyntaxNodePtr};
 
     use crate::path::ModPath;
     use crate::{db::DefDatabase, diagnostics::*, nameres::LocalModuleId, AstId};
@@ -298,6 +298,8 @@ mod diagnostics {
         UnresolvedExternCrate { ast: AstId<ast::ExternCrate> },
 
         UnresolvedImport { ast: AstId<ast::Use>, index: usize },
+
+        UnconfiguredCode { ast: InFile<SyntaxNodePtr> },
     }
 
     #[derive(Debug, PartialEq, Eq)]
@@ -336,6 +338,13 @@ mod diagnostics {
             Self { in_module: container, kind: DiagnosticKind::UnresolvedImport { ast, index } }
         }
 
+        pub(super) fn unconfigured_code(
+            container: LocalModuleId,
+            ast: InFile<SyntaxNodePtr>,
+        ) -> Self {
+            Self { in_module: container, kind: DiagnosticKind::UnconfiguredCode { ast } }
+        }
+
         pub(super) fn add_to(
             &self,
             db: &dyn DefDatabase,
@@ -385,6 +394,10 @@ mod diagnostics {
                         sink.push(UnresolvedImport { file: ast.file_id, node: AstPtr::new(&tree) });
                     }
                 }
+
+                DiagnosticKind::UnconfiguredCode { ast } => {
+                    sink.push(UnconfiguredCode { file: ast.file_id, node: ast.value.clone() });
+                }
             }
         }
     }

crates/hir_def/src/nameres/collector.rs

@@ -913,6 +913,7 @@ impl ModCollector<'_, '_> {
         for &item in items {
             let attrs = self.item_tree.attrs(item.into());
             if !self.is_cfg_enabled(attrs) {
+                self.emit_unconfigured_diagnostic(item);
                 continue;
             }
             let module =
@@ -1323,6 +1324,18 @@ impl ModCollector<'_, '_> {
     fn is_cfg_enabled(&self, attrs: &Attrs) -> bool {
         attrs.is_cfg_enabled(self.def_collector.cfg_options)
     }
+
+    fn emit_unconfigured_diagnostic(&mut self, item: ModItem) {
+        let ast_id = item.ast_id(self.item_tree);
+        let id_map = self.def_collector.db.ast_id_map(self.file_id);
+        let syntax_ptr = id_map.get(ast_id).syntax_node_ptr();
+
+        let ast_node = InFile::new(self.file_id, syntax_ptr);
+        self.def_collector
+            .def_map
+            .diagnostics
+            .push(DefDiagnostic::unconfigured_code(self.module_id, ast_node));
+    }
 }
 
 fn is_macro_rules(path: &ModPath) -> bool {

crates/ide/src/diagnostics.rs

@@ -10,7 +10,10 @@ mod field_shorthand;
 use std::cell::RefCell;
 
 use base_db::SourceDatabase;
-use hir::{diagnostics::DiagnosticSinkBuilder, Semantics};
+use hir::{
+    diagnostics::{Diagnostic as _, DiagnosticSinkBuilder},
+    Semantics,
+};
 use ide_db::RootDatabase;
 use itertools::Itertools;
 use rustc_hash::FxHashSet;
@@ -46,6 +49,10 @@ impl Diagnostic {
     fn with_fix(self, fix: Option<Fix>) -> Self {
         Self { fix, ..self }
     }
+
+    fn with_unused(self, unused: bool) -> Self {
+        Self { unused, ..self }
+    }
 }
 
 #[derive(Debug)]
@@ -115,6 +122,13 @@ pub(crate) fn diagnostics(
         .on::<hir::diagnostics::IncorrectCase, _>(|d| {
             res.borrow_mut().push(warning_with_fix(d, &sema));
         })
+        .on::<hir::diagnostics::UnconfiguredCode, _>(|d| {
+            // Override severity and mark as unused.
+            res.borrow_mut().push(
+                Diagnostic::hint(sema.diagnostics_display_range(d).range, d.message())
+                    .with_unused(true),
+            );
+        })
         // Only collect experimental diagnostics when they're enabled.
         .filter(|diag| !(diag.is_experimental() && config.disable_experimental))
         .filter(|diag| !config.disabled.contains(diag.code().as_str()));