2 files changed, 86 insertions, 0 deletions

diff --git a/crates/ra_hir_expand/src/diagnostics.rs b/crates/ra_hir_expand/src/diagnostics.rs
new file mode 100644
index 000000000..201884b95
--- /dev/null
+++ b/crates/ra_hir_expand/src/diagnostics.rs
@@ -0,0 +1,85 @@
+//! Semantic errors and warnings.
+//!
+//! The `Diagnostic` trait defines a trait object which can represent any
+//! diagnostic.
+//!
+//! `DiagnosticSink` struct is used as an emitter for diagnostic. When creating
+//! a `DiagnosticSink`, you supply a callback which can react to a `dyn
+//! Diagnostic` or to any concrete diagnostic (downcasting is sued internally).
+//!
+//! Because diagnostics store file offsets, it's a bad idea to store them
+//! directly in salsa. For this reason, every hir subsytem defines it's own
+//! strongly-typed closed set of diagnostics which use hir ids internally, are
+//! stored in salsa and do *not* implement the `Diagnostic` trait. Instead, a
+//! subsystem provides a separate, non-query-based API which can walk all stored
+//! values and transform them into instances of `Diagnostic`.
+
+use std::{any::Any, fmt};
+
+use ra_syntax::{SyntaxNode, SyntaxNodePtr, TextRange};
+
+use crate::{db::AstDatabase, Source};
+
+pub trait Diagnostic: Any + Send + Sync + fmt::Debug + 'static {
+    fn message(&self) -> String;
+    fn source(&self) -> Source<SyntaxNodePtr>;
+    fn highlight_range(&self) -> TextRange {
+        self.source().ast.range()
+    }
+    fn as_any(&self) -> &(dyn Any + Send + 'static);
+}
+
+pub trait AstDiagnostic {
+    type AST;
+    fn ast(&self, db: &impl AstDatabase) -> Self::AST;
+}
+
+impl dyn Diagnostic {
+    pub fn syntax_node(&self, db: &impl AstDatabase) -> SyntaxNode {
+        let node = db.parse_or_expand(self.source().file_id).unwrap();
+        self.source().ast.to_node(&node)
+    }
+
+    pub fn downcast_ref<D: Diagnostic>(&self) -> Option<&D> {
+        self.as_any().downcast_ref()
+    }
+}
+
+pub struct DiagnosticSink<'a> {
+    callbacks: Vec<Box<dyn FnMut(&dyn Diagnostic) -> Result<(), ()> + 'a>>,
+    default_callback: Box<dyn FnMut(&dyn Diagnostic) + 'a>,
+}
+
+impl<'a> DiagnosticSink<'a> {
+    /// FIXME: split `new` and `on` into a separate builder type
+    pub fn new(cb: impl FnMut(&dyn Diagnostic) + 'a) -> DiagnosticSink<'a> {
+        DiagnosticSink { callbacks: Vec::new(), default_callback: Box::new(cb) }
+    }
+
+    pub fn on<D: Diagnostic, F: FnMut(&D) + 'a>(mut self, mut cb: F) -> DiagnosticSink<'a> {
+        let cb = move |diag: &dyn Diagnostic| match diag.downcast_ref::<D>() {
+            Some(d) => {
+                cb(d);
+                Ok(())
+            }
+            None => Err(()),
+        };
+        self.callbacks.push(Box::new(cb));
+        self
+    }
+
+    pub fn push(&mut self, d: impl Diagnostic) {
+        let d: &dyn Diagnostic = &d;
+        self._push(d);
+    }
+
+    fn _push(&mut self, d: &dyn Diagnostic) {
+        for cb in self.callbacks.iter_mut() {
+            match cb(d) {
+                Ok(()) => return,
+                Err(()) => (),
+            }
+        }
+        (self.default_callback)(d)
+    }
+}
diff --git a/crates/ra_hir_expand/src/lib.rs b/crates/ra_hir_expand/src/lib.rs
index 85c2b22ac..dd07a16b4 100644
--- a/crates/ra_hir_expand/src/lib.rs
+++ b/crates/ra_hir_expand/src/lib.rs
@@ -9,6 +9,7 @@ pub mod ast_id_map;
 pub mod either;
 pub mod name;
 pub mod hygiene;
+pub mod diagnostics;
 
 use std::hash::{Hash, Hasher};