1 files changed, 149 insertions, 0 deletions

diff --git a/crates/ra_assists/src/flip_binexpr.rs b/crates/ra_assists/src/flip_binexpr.rs
new file mode 100644
index 000000000..8a0737b55
--- /dev/null
+++ b/crates/ra_assists/src/flip_binexpr.rs
@@ -0,0 +1,149 @@
+use hir::db::HirDatabase;
+use ra_syntax::ast::{AstNode, BinExpr, BinOp};
+
+use crate::{AssistCtx, Assist, AssistId};
+
+/// Flip binary comparison expressions (==, !=, >, >=, <, <=).
+pub(crate) fn flip_binexpr(mut ctx: AssistCtx<impl HirDatabase>) -> Option<Assist> {
+    let expr = ctx.node_at_offset::<BinExpr>()?;
+    let lhs = expr.lhs()?.syntax();
+    let rhs = expr.rhs()?.syntax();
+    let op_range = expr.op()?.range();
+    // The assist should be available only if the cursor is on the operator
+    let cursor_in_range = ctx.frange.range.is_subrange(&op_range);
+    // The assist should be available only for these binary operators
+    // (it should not change the meaning of the expression)
+    let allowed_ops = [
+        BinOp::EqualityTest,
+        BinOp::NegatedEqualityTest,
+        BinOp::GreaterTest,
+        BinOp::GreaterEqualTest,
+        BinOp::LesserTest,
+        BinOp::LesserEqualTest,
+    ];
+    let op_kind = expr.op_kind()?;
+    if !cursor_in_range || !allowed_ops.iter().any(|o| *o == op_kind) {
+        return None;
+    }
+    let new_op = match op_kind {
+        BinOp::GreaterTest => Some("<"),
+        BinOp::GreaterEqualTest => Some("<="),
+        BinOp::LesserTest => Some(">"),
+        BinOp::LesserEqualTest => Some(">="),
+        _ => None,
+    };
+    ctx.add_action(AssistId("flip_binexpr"), "flip binary expression", |edit| {
+        edit.target(op_range);
+        if let Some(new_op) = new_op {
+            edit.replace(op_range, new_op);
+        }
+        edit.replace(lhs.range(), rhs.text());
+        edit.replace(rhs.range(), lhs.text());
+    });
+
+    ctx.build()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::helpers::{check_assist, check_assist_target};
+
+    #[test]
+    fn flip_eq_operands_for_simple_stmt() {
+        check_assist(
+            flip_binexpr,
+            "fn f() { let res = 1 ==<|> 2; }",
+            "fn f() { let res = 2 ==<|> 1; }",
+        )
+    }
+
+    #[test]
+    fn flip_neq_operands_for_simple_stmt() {
+        check_assist(
+            flip_binexpr,
+            "fn f() { let res = 1 !=<|> 2; }",
+            "fn f() { let res = 2 !=<|> 1; }",
+        )
+    }
+
+    #[test]
+    fn flip_gt_operands_for_simple_stmt() {
+        check_assist(
+            flip_binexpr,
+            "fn f() { let res = 1 ><|> 2; }",
+            "fn f() { let res = 2 <<|> 1; }",
+        )
+    }
+
+    #[test]
+    fn flip_gteq_operands_for_simple_stmt() {
+        check_assist(
+            flip_binexpr,
+            "fn f() { let res = 1 >=<|> 2; }",
+            "fn f() { let res = 2 <=<|> 1; }",
+        )
+    }
+
+    #[test]
+    fn flip_lt_operands_for_simple_stmt() {
+        check_assist(
+            flip_binexpr,
+            "fn f() { let res = 1 <<|> 2; }",
+            "fn f() { let res = 2 ><|> 1; }",
+        )
+    }
+
+    #[test]
+    fn flip_lteq_operands_for_simple_stmt() {
+        check_assist(
+            flip_binexpr,
+            "fn f() { let res = 1 <=<|> 2; }",
+            "fn f() { let res = 2 >=<|> 1; }",
+        )
+    }
+
+    #[test]
+    fn flip_eq_operands_for_complex_stmt() {
+        check_assist(
+            flip_binexpr,
+            "fn f() { let res = (1 + 1) ==<|> (2 + 2); }",
+            "fn f() { let res = (2 + 2) ==<|> (1 + 1); }",
+        )
+    }
+
+    #[test]
+    fn flip_eq_operands_in_match_expr() {
+        check_assist(
+            flip_binexpr,
+            r#"
+            fn dyn_eq(&self, other: &dyn Diagnostic) -> bool {
+                match other.downcast_ref::<Self>() {
+                    None => false,
+                    Some(it) => it ==<|> self,
+                }
+            }
+            "#,
+            r#"
+            fn dyn_eq(&self, other: &dyn Diagnostic) -> bool {
+                match other.downcast_ref::<Self>() {
+                    None => false,
+                    Some(it) => self ==<|> it,
+                }
+            }
+            "#,
+        )
+    }
+
+    #[test]
+    fn flip_eq_operands_target() {
+        check_assist_target(flip_binexpr, "fn f() { let res = 1 ==<|> 2; }", "==")
+    }
+
+    #[test]
+    fn flip_gt_operands_target() {
+        check_assist_target(flip_binexpr, "fn f() { let res = 1 ><|> 2; }", ">")
+    }
+
+}