2 files changed, 187 insertions, 0 deletions

diff --git a/crates/ra_ide_api/src/inlay_hints.rs b/crates/ra_ide_api/src/inlay_hints.rs
new file mode 100644
index 000000000..174662beb
--- /dev/null
+++ b/crates/ra_ide_api/src/inlay_hints.rs
@@ -0,0 +1,180 @@
+use crate::{db::RootDatabase, FileId};
+use hir::{HirDisplay, Ty};
+use ra_syntax::ast::Pat;
+use ra_syntax::{
+    algo::visit::{visitor, Visitor},
+    ast::{self, PatKind, TypeAscriptionOwner},
+    AstNode, SmolStr, SourceFile, SyntaxNode, TextRange,
+};
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum InlayKind {
+    LetBindingType,
+    ClosureParameterType,
+}
+
+#[derive(Debug)]
+pub struct InlayHint {
+    pub range: TextRange,
+    pub kind: InlayKind,
+    pub label: SmolStr,
+}
+
+pub(crate) fn inlay_hints(db: &RootDatabase, file_id: FileId, file: &SourceFile) -> Vec<InlayHint> {
+    file.syntax()
+        .descendants()
+        .map(|node| get_inlay_hints(db, file_id, &node).unwrap_or_default())
+        .flatten()
+        .collect()
+}
+
+fn get_inlay_hints(
+    db: &RootDatabase,
+    file_id: FileId,
+    node: &SyntaxNode,
+) -> Option<Vec<InlayHint>> {
+    visitor()
+        .visit(|let_statement: ast::LetStmt| {
+            let let_syntax = let_statement.syntax();
+
+            if let_statement.ascribed_type().is_some() {
+                return None;
+            }
+
+            let let_pat = let_statement.pat()?;
+            let inlay_type_string = get_node_displayable_type(db, file_id, let_syntax, &let_pat)?
+                .display(db)
+                .to_string()
+                .into();
+
+            let pat_range = match let_pat.kind() {
+                PatKind::BindPat(bind_pat) => bind_pat.syntax().text_range(),
+                PatKind::TuplePat(tuple_pat) => tuple_pat.syntax().text_range(),
+                _ => return None,
+            };
+
+            Some(vec![InlayHint {
+                range: pat_range,
+                kind: InlayKind::LetBindingType,
+                label: inlay_type_string,
+            }])
+        })
+        .visit(|closure_parameter: ast::LambdaExpr| match closure_parameter.param_list() {
+            Some(param_list) => Some(
+                param_list
+                    .params()
+                    .filter(|closure_param| closure_param.ascribed_type().is_none())
+                    .filter_map(|closure_param| {
+                        let closure_param_syntax = closure_param.syntax();
+                        let inlay_type_string = get_node_displayable_type(
+                            db,
+                            file_id,
+                            closure_param_syntax,
+                            &closure_param.pat()?,
+                        )?
+                        .display(db)
+                        .to_string()
+                        .into();
+
+                        Some(InlayHint {
+                            range: closure_param_syntax.text_range(),
+                            kind: InlayKind::ClosureParameterType,
+                            label: inlay_type_string,
+                        })
+                    })
+                    .collect(),
+            ),
+            None => None,
+        })
+        .accept(&node)?
+}
+
+fn get_node_displayable_type(
+    db: &RootDatabase,
+    file_id: FileId,
+    node_syntax: &SyntaxNode,
+    node_pat: &Pat,
+) -> Option<Ty> {
+    let analyzer = hir::SourceAnalyzer::new(db, file_id, node_syntax, None);
+    analyzer.type_of_pat(db, node_pat).and_then(|resolved_type| {
+        if let Ty::Apply(_) = resolved_type {
+            Some(resolved_type)
+        } else {
+            None
+        }
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::mock_analysis::single_file;
+    use insta::assert_debug_snapshot_matches;
+
+    #[test]
+    fn test_inlay_hints() {
+        let (analysis, file_id) = single_file(
+            r#"
+struct OuterStruct {}
+
+fn main() {
+    struct InnerStruct {}
+
+    let test = 54;
+    let test = InnerStruct {};
+    let test = OuterStruct {};
+    let test = vec![222];
+    let mut test = Vec::new();
+    test.push(333);
+    let test = test.into_iter().map(|i| i * i).collect::<Vec<_>>();
+    let mut test = 33;
+    let _ = 22;
+    let test: Vec<_> = (0..3).collect();
+
+    let _ = (0..23).map(|i: u32| {
+        let i_squared = i * i;
+        i_squared
+    });
+
+    let test: i32 = 33;
+
+    let (x, c) = (42, 'a');
+    let test = (42, 'a');
+}
+"#,
+        );
+
+        assert_debug_snapshot_matches!(analysis.inlay_hints(file_id).unwrap(), @r#"[
+    InlayHint {
+        range: [71; 75),
+        kind: LetBindingType,
+        label: "i32",
+    },
+    InlayHint {
+        range: [121; 125),
+        kind: LetBindingType,
+        label: "OuterStruct",
+    },
+    InlayHint {
+        range: [297; 305),
+        kind: LetBindingType,
+        label: "i32",
+    },
+    InlayHint {
+        range: [417; 426),
+        kind: LetBindingType,
+        label: "u32",
+    },
+    InlayHint {
+        range: [496; 502),
+        kind: LetBindingType,
+        label: "(i32, char)",
+    },
+    InlayHint {
+        range: [524; 528),
+        kind: LetBindingType,
+        label: "(i32, char)",
+    },
+]"#
+        );
+    }
+}
diff --git a/crates/ra_ide_api/src/lib.rs b/crates/ra_ide_api/src/lib.rs
index c54d574bc..16ffb03ce 100644
--- a/crates/ra_ide_api/src/lib.rs
+++ b/crates/ra_ide_api/src/lib.rs
@@ -38,6 +38,7 @@ mod join_lines;
 mod typing;
 mod matching_brace;
 mod display;
+mod inlay_hints;
 
 #[cfg(test)]
 mod marks;
@@ -64,6 +65,7 @@ pub use crate::{
     display::{file_structure, FunctionSignature, NavigationTarget, StructureNode},
     folding_ranges::{Fold, FoldKind},
     hover::HoverResult,
+    inlay_hints::{InlayHint, InlayKind},
     line_index::{LineCol, LineIndex},
     line_index_utils::translate_offset_with_edit,
     references::ReferenceSearchResult,
@@ -396,6 +398,11 @@ impl Analysis {
         file_structure(&parse.tree())
     }
 
+    /// Returns a list of the places in the file where type hints can be displayed.
+    pub fn inlay_hints(&self, file_id: FileId) -> Cancelable<Vec<InlayHint>> {
+        self.with_db(|db| inlay_hints::inlay_hints(db, file_id, &db.parse(file_id).tree()))
+    }
+
     /// Returns the set of folding ranges.
     pub fn folding_ranges(&self, file_id: FileId) -> Vec<Fold> {
         let parse = self.db.parse(file_id);