6 files changed, 170 insertions, 5 deletions

diff --git a/crates/ra_assists/src/doc_tests/generated.rs b/crates/ra_assists/src/doc_tests/generated.rs
index 62dcb3808..c0486a8a9 100644
--- a/crates/ra_assists/src/doc_tests/generated.rs
+++ b/crates/ra_assists/src/doc_tests/generated.rs
@@ -608,6 +608,32 @@ fn handle(action: Action) {
 }
 
 #[test]
+fn doctest_replace_let_with_if_let() {
+    check(
+        "replace_let_with_if_let",
+        r#####"
+enum Option<T> { Some(T), None }
+
+fn main(action: Action) {
+    <|>let x = compute();
+}
+
+fn compute() -> Option<i32> { None }
+"#####,
+        r#####"
+enum Option<T> { Some(T), None }
+
+fn main(action: Action) {
+    if let Some(x) = compute() {
+    }
+}
+
+fn compute() -> Option<i32> { None }
+"#####,
+    )
+}
+
+#[test]
 fn doctest_replace_qualified_name_with_use() {
     check(
         "replace_qualified_name_with_use",
diff --git a/crates/ra_assists/src/handlers/replace_let_with_if_let.rs b/crates/ra_assists/src/handlers/replace_let_with_if_let.rs
new file mode 100644
index 000000000..10e41f97e
--- /dev/null
+++ b/crates/ra_assists/src/handlers/replace_let_with_if_let.rs
@@ -0,0 +1,108 @@
+use hir::Adt;
+use ra_syntax::{
+    ast::{self, make},
+    AstNode, T,
+};
+
+use crate::{
+    assist_ctx::{Assist, AssistCtx},
+    AssistId,
+};
+use ast::edit::{AstNodeEdit, IndentLevel};
+use std::iter::once;
+
+// Assist: replace_let_with_if_let
+//
+// Replaces `if let` with an else branch with a `match` expression.
+//
+// ```
+// # 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(ctx: AssistCtx) -> Option<Assist> {
+    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 enum_ = match ty.as_adt() {
+        Some(Adt::Enum(it)) => it,
+        _ => return None,
+    };
+    let happy_case =
+        [("Result", "Ok"), ("Option", "Some")].iter().find_map(|(known_type, happy_case)| {
+            if &enum_.name(ctx.db).to_string() == known_type {
+                return Some(happy_case);
+            }
+            None
+        });
+
+    ctx.add_assist(AssistId("replace_let_with_if_let"), "Replace with if-let", |edit| {
+        let with_placeholder: ast::Pat = match happy_case {
+            None => make::placeholder_pat().into(),
+            Some(var_name) => make::tuple_struct_pat(
+                make::path_unqualified(make::path_segment(make::name_ref(var_name))),
+                once(make::placeholder_pat().into()),
+            )
+            .into(),
+        };
+        let block =
+            IndentLevel::from_node(let_stmt.syntax()).increase_indent(make::block_expr(None, None));
+        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::PlaceholderPat::cast).unwrap();
+        let target_offset =
+            let_stmt.syntax().text_range().start() + placeholder.syntax().text_range().start();
+        let stmt = stmt.replace_descendant(placeholder.into(), original_pat);
+
+        edit.replace_ast(ast::Stmt::from(let_stmt), ast::Stmt::from(stmt));
+        edit.target(let_kw.text_range());
+        edit.set_cursor(target_offset);
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::helpers::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) {
+    }
+}
+            ",
+        )
+    }
+}
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index bcc9b3f10..67a58fc1f 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -118,6 +118,7 @@ mod handlers {
     mod remove_dbg;
     mod remove_mut;
     mod replace_if_let_with_match;
+    mod replace_let_with_if_let;
     mod replace_qualified_name_with_use;
     mod split_import;
 
@@ -153,6 +154,7 @@ mod handlers {
             remove_dbg::remove_dbg,
             remove_mut::remove_mut,
             replace_if_let_with_match::replace_if_let_with_match,
+            replace_let_with_if_let::replace_let_with_if_let,
             replace_qualified_name_with_use::replace_qualified_name_with_use,
             split_import::split_import,
         ]
diff --git a/crates/ra_syntax/src/ast/edit.rs b/crates/ra_syntax/src/ast/edit.rs
index f74c9f9c6..bdaecdc43 100644
--- a/crates/ra_syntax/src/ast/edit.rs
+++ b/crates/ra_syntax/src/ast/edit.rs
@@ -251,7 +251,7 @@ impl ast::UseItem {
     #[must_use]
     pub fn with_use_tree(&self, use_tree: ast::UseTree) -> ast::UseItem {
         if let Some(old) = self.use_tree() {
-            return self.replace_descendants(iter::once((old, use_tree)));
+            return self.replace_descendant(old, use_tree);
         }
         self.clone()
     }
@@ -283,7 +283,7 @@ impl ast::UseTree {
     #[must_use]
     pub fn with_path(&self, path: ast::Path) -> ast::UseTree {
         if let Some(old) = self.path() {
-            return self.replace_descendants(iter::once((old, path)));
+            return self.replace_descendant(old, path);
         }
         self.clone()
     }
@@ -291,7 +291,7 @@ impl ast::UseTree {
     #[must_use]
     pub fn with_use_tree_list(&self, use_tree_list: ast::UseTreeList) -> ast::UseTree {
         if let Some(old) = self.use_tree_list() {
-            return self.replace_descendants(iter::once((old, use_tree_list)));
+            return self.replace_descendant(old, use_tree_list);
         }
         self.clone()
     }
@@ -466,6 +466,11 @@ pub trait AstNodeEdit: AstNode + Sized {
     }
 
     #[must_use]
+    fn replace_descendant<D: AstNode>(&self, old: D, new: D) -> Self {
+        self.replace_descendants(iter::once((old, new)))
+    }
+
+    #[must_use]
     fn replace_descendants<D: AstNode>(
         &self,
         replacement_map: impl IntoIterator<Item = (D, D)>,
diff --git a/crates/ra_syntax/src/ast/make.rs b/crates/ra_syntax/src/ast/make.rs
index 6aee39203..c818bba55 100644
--- a/crates/ra_syntax/src/ast/make.rs
+++ b/crates/ra_syntax/src/ast/make.rs
@@ -127,7 +127,7 @@ pub fn condition(expr: ast::Expr, pattern: Option<ast::Pat>) -> ast::Condition {
     match pattern {
         None => ast_from_text(&format!("const _: () = while {} {{}};", expr)),
         Some(pattern) => {
-            ast_from_text(&format!("const _: () = while {} = {} {{}};", pattern, expr))
+            ast_from_text(&format!("const _: () = while let {} = {} {{}};", pattern, expr))
         }
     }
 }
@@ -245,7 +245,8 @@ pub fn let_stmt(pattern: ast::Pat, initializer: Option<ast::Expr>) -> ast::LetSt
     ast_from_text(&format!("fn f() {{ {} }}", text))
 }
 pub fn expr_stmt(expr: ast::Expr) -> ast::ExprStmt {
-    ast_from_text(&format!("fn f() {{ {}; }}", expr))
+    let semi = if expr.is_block_like() { "" } else { ";" };
+    ast_from_text(&format!("fn f() {{ {}{} (); }}", expr, semi))
 }
 
 pub fn token(kind: SyntaxKind) -> SyntaxToken {
diff --git a/docs/user/assists.md b/docs/user/assists.md
index f3ce6b0e0..3b7467a26 100644
--- a/docs/user/assists.md
+++ b/docs/user/assists.md
@@ -583,6 +583,29 @@ fn handle(action: Action) {
 }
 ```
 
+## `replace_let_with_if_let`
+
+Replaces `if let` with an else branch with a `match` expression.
+
+```rust
+// BEFORE
+
+fn main(action: Action) {
+    ┃let x = compute();
+}
+
+fn compute() -> Option<i32> { None }
+
+// AFTER
+
+fn main(action: Action) {
+    if let Some(x) = compute() {
+    }
+}
+
+fn compute() -> Option<i32> { None }
+```
+
 ## `replace_qualified_name_with_use`
 
 Adds a use statement for a given fully-qualified name.