4 files changed, 220 insertions, 0 deletions

diff --git a/crates/ide_assists/src/handlers/add_type_ascription.rs b/crates/ide_assists/src/handlers/add_type_ascription.rs
new file mode 100644
index 000000000..e9dc37150
--- /dev/null
+++ b/crates/ide_assists/src/handlers/add_type_ascription.rs
@@ -0,0 +1,198 @@
+use ide_db::defs::{Definition, NameRefClass};
+use syntax::{ast, AstNode, SyntaxKind, T};
+use test_utils::mark;
+
+use crate::{
+    assist_context::{AssistContext, Assists},
+    AssistId, AssistKind,
+};
+
+// Assist: add_type_ascription
+//
+// Adds `: _` before the assignment operator to prompt the user for a type
+//
+// ```
+// fn make<T>() -> T { todo!() }
+// fn main() {
+//     let x = make$0();
+// }
+// ```
+// ->
+// ```
+// fn make<T>() -> T { todo!() }
+// fn main() {
+//     let x: ${0:_} = make();
+// }
+// ```
+pub(crate) fn add_type_ascription(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let let_stmt = ctx.find_node_at_offset::<ast::LetStmt>()?;
+    if let_stmt.colon_token().is_some() {
+        mark::hit!(add_type_ascription_already_typed);
+        return None
+    }
+
+    let ident = ctx.find_token_syntax_at_offset(SyntaxKind::IDENT).or_else(|| {
+        let arg_list = ctx.find_node_at_offset::<ast::ArgList>()?;
+        if arg_list.args().count() > 0 {
+            return None;
+        }
+        mark::hit!(add_type_ascription_after_call);
+        arg_list.l_paren_token()?.prev_token().filter(|it| it.kind() == SyntaxKind::IDENT)
+    })?;
+    let next_token = ident.next_token()?;
+    if next_token.kind() == T![::] {
+        mark::hit!(add_type_ascription_turbofished);
+        return None;
+    }
+    let name_ref = ast::NameRef::cast(ident.parent())?;
+    let def = match NameRefClass::classify(&ctx.sema, &name_ref)? {
+        NameRefClass::Definition(def) => def,
+        NameRefClass::ExternCrate(_) | NameRefClass::FieldShorthand { .. } => return None,
+    };
+    let fun = match def {
+        Definition::ModuleDef(hir::ModuleDef::Function(it)) => it,
+        _ => return None,
+    };
+    let generics = hir::GenericDef::Function(fun).params(ctx.sema.db);
+    if generics.is_empty() {
+        mark::hit!(add_type_ascription_non_generic);
+        return None;
+    }
+    let pat = let_stmt.pat()?.syntax().last_token()?.text_range().end();
+    acc.add(
+        AssistId("add_type_ascription", AssistKind::RefactorRewrite),
+        "Add `: _` before assignment operator",
+        ident.text_range(),
+        |builder| match ctx.config.snippet_cap {
+            Some(cap) => builder.insert_snippet(cap, pat, ": ${0:_}"),
+            None => builder.insert(pat, ": _"),
+        },
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+    use test_utils::mark;
+
+    #[test]
+    fn add_type_ascription_function() {
+        check_assist(
+            add_type_ascription,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x = make$0();
+}
+"#,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x: ${0:_} = make();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn add_type_ascription_after_call() {
+        mark::check!(add_type_ascription_after_call);
+        check_assist(
+            add_type_ascription,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x = make()$0;
+}
+"#,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x: ${0:_} = make();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn add_type_ascription_method() {
+        check_assist(
+            add_type_ascription,
+            r#"
+struct S;
+impl S {
+    fn make<T>(&self) -> T {}
+}
+fn main() {
+    let x = S.make$0();
+}
+"#,
+            r#"
+struct S;
+impl S {
+    fn make<T>(&self) -> T {}
+}
+fn main() {
+    let x: ${0:_} = S.make();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn add_type_ascription_turbofished() {
+        mark::check!(add_type_ascription_turbofished);
+        check_assist_not_applicable(
+            add_type_ascription,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x = make$0::<()>();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn add_type_ascription_already_typed() {
+        mark::check!(add_type_ascription_already_typed);
+        check_assist_not_applicable(
+            add_type_ascription,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    let x: () = make$0();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn add_type_ascription_non_generic() {
+        mark::check!(add_type_ascription_non_generic);
+        check_assist_not_applicable(
+            add_type_ascription,
+            r#"
+fn make() -> () {}
+fn main() {
+    let x = make$0();
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn add_type_ascription_no_let() {
+        check_assist_not_applicable(
+            add_type_ascription,
+            r#"
+fn make<T>() -> T {}
+fn main() {
+    make$0();
+}
+"#,
+        );
+    }
+}
diff --git a/crates/ide_assists/src/lib.rs b/crates/ide_assists/src/lib.rs
index 9c8148462..0248cb9c0 100644
--- a/crates/ide_assists/src/lib.rs
+++ b/crates/ide_assists/src/lib.rs
@@ -111,6 +111,7 @@ mod handlers {
     mod add_lifetime_to_type;
     mod add_missing_impl_members;
     mod add_turbo_fish;
+    mod add_type_ascription;
     mod apply_demorgan;
     mod auto_import;
     mod change_visibility;
@@ -175,6 +176,7 @@ mod handlers {
             add_explicit_type::add_explicit_type,
             add_lifetime_to_type::add_lifetime_to_type,
             add_turbo_fish::add_turbo_fish,
+            add_type_ascription::add_type_ascription,
             apply_demorgan::apply_demorgan,
             auto_import::auto_import,
             change_visibility::change_visibility,
diff --git a/crates/ide_assists/src/tests/generated.rs b/crates/ide_assists/src/tests/generated.rs
index 4f007aa48..439ee8b22 100644
--- a/crates/ide_assists/src/tests/generated.rs
+++ b/crates/ide_assists/src/tests/generated.rs
@@ -142,6 +142,25 @@ fn main() {
 }
 
 #[test]
+fn doctest_add_type_ascription() {
+    check_doc_test(
+        "add_type_ascription",
+        r#####"
+fn make<T>() -> T { todo!() }
+fn main() {
+    let x = make$0();
+}
+"#####,
+        r#####"
+fn make<T>() -> T { todo!() }
+fn main() {
+    let x: ${0:_} = make();
+}
+"#####,
+    )
+}
+
+#[test]
 fn doctest_apply_demorgan() {
     check_doc_test(
         "apply_demorgan",
diff --git a/xtask/src/tidy.rs b/xtask/src/tidy.rs
index 349ca14d0..91f1ee217 100644
--- a/xtask/src/tidy.rs
+++ b/xtask/src/tidy.rs
@@ -277,6 +277,7 @@ fn check_todo(path: &Path, text: &str) {
         "tests/tidy.rs",
         // Some of our assists generate `todo!()`.
         "handlers/add_turbo_fish.rs",
+        "handlers/add_type_ascription.rs",
         "handlers/generate_function.rs",
         // To support generating `todo!()` in assists, we have `expr_todo()` in
         // `ast::make`.