Hover tooltips

src/ide/mod.rs

@@ -1,5 +1,7 @@
 //! Capabilities for IDE support.
 
 mod highlight;
+mod tooltip;
 
 pub use highlight::*;
+pub use tooltip::*;

src/ide/tooltip.rs

@@ -0,0 +1,19 @@
+use crate::model::Value;
+use crate::syntax::{LinkedNode, Source, SyntaxKind};
+use crate::World;
+
+/// Produce a tooltip which can be shown when a cursor position is hovered.
+pub fn tooltip(world: &dyn World, source: &Source, cursor: usize) -> Option<String> {
+    let leaf = LinkedNode::new(source.root()).leaf_at(cursor)?;
+
+    // If a known identifier is under the cursor, provide its documentation.
+    if let SyntaxKind::Ident(ident) = leaf.kind() {
+        if let Some(value) = world.library().scope.get(ident) {
+            if let Value::Func(func) = value {
+                return func.doc().map(Into::into);
+            }
+        }
+    }
+
+    None
+}