1 files changed, 100 insertions, 0 deletions

diff --git a/crates/assists/src/handlers/replace_let_with_if_let.rs b/crates/assists/src/handlers/replace_let_with_if_let.rs
new file mode 100644
index 000000000..ed6d0c29b
--- /dev/null
+++ b/crates/assists/src/handlers/replace_let_with_if_let.rs
@@ -0,0 +1,100 @@
+use std::iter::once;
+
+use syntax::{
+    ast::{
+        self,
+        edit::{AstNodeEdit, IndentLevel},
+        make,
+    },
+    AstNode, T,
+};
+
+use crate::{utils::TryEnum, AssistContext, AssistId, AssistKind, Assists};
+
+// Assist: replace_let_with_if_let
+//
+// Replaces `let` with an `if-let`.
+//
+// ```
+// # enum Option<T> { Some(T), None }
+//
+// fn main(action: Action) {
+//     <|>let x = compute();
+// }
+//
+// fn compute() -> Option<i32> { None }
+// ```
+// ->
+// ```
+// # enum Option<T> { Some(T), None }
+//
+// fn main(action: Action) {
+//     if let Some(x) = compute() {
+//     }
+// }
+//
+// fn compute() -> Option<i32> { None }
+// ```
+pub(crate) fn replace_let_with_if_let(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let let_kw = ctx.find_token_at_offset(T![let])?;
+    let let_stmt = let_kw.ancestors().find_map(ast::LetStmt::cast)?;
+    let init = let_stmt.initializer()?;
+    let original_pat = let_stmt.pat()?;
+    let ty = ctx.sema.type_of_expr(&init)?;
+    let happy_variant = TryEnum::from_ty(&ctx.sema, &ty).map(|it| it.happy_case());
+
+    let target = let_kw.text_range();
+    acc.add(
+        AssistId("replace_let_with_if_let", AssistKind::RefactorRewrite),
+        "Replace with if-let",
+        target,
+        |edit| {
+            let with_placeholder: ast::Pat = match happy_variant {
+                None => make::wildcard_pat().into(),
+                Some(var_name) => make::tuple_struct_pat(
+                    make::path_unqualified(make::path_segment(make::name_ref(var_name))),
+                    once(make::wildcard_pat().into()),
+                )
+                .into(),
+            };
+            let block =
+                make::block_expr(None, None).indent(IndentLevel::from_node(let_stmt.syntax()));
+            let if_ = make::expr_if(make::condition(init, Some(with_placeholder)), block);
+            let stmt = make::expr_stmt(if_);
+
+            let placeholder = stmt.syntax().descendants().find_map(ast::WildcardPat::cast).unwrap();
+            let stmt = stmt.replace_descendant(placeholder.into(), original_pat);
+
+            edit.replace_ast(ast::Stmt::from(let_stmt), ast::Stmt::from(stmt));
+        },
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::check_assist;
+
+    use super::*;
+
+    #[test]
+    fn replace_let_unknown_enum() {
+        check_assist(
+            replace_let_with_if_let,
+            r"
+enum E<T> { X(T), Y(T) }
+
+fn main() {
+    <|>let x = E::X(92);
+}
+            ",
+            r"
+enum E<T> { X(T), Y(T) }
+
+fn main() {
+    if let x = E::X(92) {
+    }
+}
+            ",
+        )
+    }
+}