1 files changed, 109 insertions, 0 deletions

diff --git a/crates/assists/src/handlers/invert_if.rs b/crates/assists/src/handlers/invert_if.rs
new file mode 100644
index 000000000..f0e047538
--- /dev/null
+++ b/crates/assists/src/handlers/invert_if.rs
@@ -0,0 +1,109 @@
+use syntax::{
+    ast::{self, AstNode},
+    T,
+};
+
+use crate::{
+    assist_context::{AssistContext, Assists},
+    utils::invert_boolean_expression,
+    AssistId, AssistKind,
+};
+
+// Assist: invert_if
+//
+// Apply invert_if
+// This transforms if expressions of the form `if !x {A} else {B}` into `if x {B} else {A}`
+// This also works with `!=`. This assist can only be applied with the cursor
+// on `if`.
+//
+// ```
+// fn main() {
+//     if<|> !y { A } else { B }
+// }
+// ```
+// ->
+// ```
+// fn main() {
+//     if y { B } else { A }
+// }
+// ```
+
+pub(crate) fn invert_if(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
+    let if_keyword = ctx.find_token_at_offset(T![if])?;
+    let expr = ast::IfExpr::cast(if_keyword.parent())?;
+    let if_range = if_keyword.text_range();
+    let cursor_in_range = if_range.contains_range(ctx.frange.range);
+    if !cursor_in_range {
+        return None;
+    }
+
+    // This assist should not apply for if-let.
+    if expr.condition()?.pat().is_some() {
+        return None;
+    }
+
+    let cond = expr.condition()?.expr()?;
+    let then_node = expr.then_branch()?.syntax().clone();
+    let else_block = match expr.else_branch()? {
+        ast::ElseBranch::Block(it) => it,
+        ast::ElseBranch::IfExpr(_) => return None,
+    };
+
+    let cond_range = cond.syntax().text_range();
+    let flip_cond = invert_boolean_expression(cond);
+    let else_node = else_block.syntax();
+    let else_range = else_node.text_range();
+    let then_range = then_node.text_range();
+    acc.add(AssistId("invert_if", AssistKind::RefactorRewrite), "Invert if", if_range, |edit| {
+        edit.replace(cond_range, flip_cond.syntax().text());
+        edit.replace(else_range, then_node.text());
+        edit.replace(then_range, else_node.text());
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    #[test]
+    fn invert_if_remove_inequality() {
+        check_assist(
+            invert_if,
+            "fn f() { i<|>f x != 3 { 1 } else { 3 + 2 } }",
+            "fn f() { if x == 3 { 3 + 2 } else { 1 } }",
+        )
+    }
+
+    #[test]
+    fn invert_if_remove_not() {
+        check_assist(
+            invert_if,
+            "fn f() { <|>if !cond { 3 * 2 } else { 1 } }",
+            "fn f() { if cond { 1 } else { 3 * 2 } }",
+        )
+    }
+
+    #[test]
+    fn invert_if_general_case() {
+        check_assist(
+            invert_if,
+            "fn f() { i<|>f cond { 3 * 2 } else { 1 } }",
+            "fn f() { if !cond { 1 } else { 3 * 2 } }",
+        )
+    }
+
+    #[test]
+    fn invert_if_doesnt_apply_with_cursor_not_on_if() {
+        check_assist_not_applicable(invert_if, "fn f() { if !<|>cond { 3 * 2 } else { 1 } }")
+    }
+
+    #[test]
+    fn invert_if_doesnt_apply_with_if_let() {
+        check_assist_not_applicable(
+            invert_if,
+            "fn f() { i<|>f let Some(_) = Some(1) { 1 } else { 0 } }",
+        )
+    }
+}