4 files changed, 135 insertions, 35 deletions

diff --git a/crates/ra_assists/src/flip_eq_operands.rs b/crates/ra_assists/src/flip_eq_operands.rs
new file mode 100644
index 000000000..df0bb689d
--- /dev/null
+++ b/crates/ra_assists/src/flip_eq_operands.rs
@@ -0,0 +1,86 @@
+use hir::db::HirDatabase;
+use ra_syntax::ast::{AstNode, BinExpr, BinOp};
+
+use crate::{AssistCtx, Assist, AssistId};
+
+pub(crate) fn flip_eq_operands(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();
+    let cursor_in_range = ctx.frange.range.is_subrange(&op_range);
+    let allowed_ops = [BinOp::EqualityTest, BinOp::NegatedEqualityTest];
+    let expr_op = expr.op_kind()?;
+    if !cursor_in_range || !allowed_ops.iter().any(|o| *o == expr_op) {
+        return None;
+    }
+    ctx.add_action(AssistId("flip_eq_operands"), "flip equality operands", |edit| {
+        edit.target(op_range);
+        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_eq_operands,
+            "fn f() { let res = 1 ==<|> 2; }",
+            "fn f() { let res = 2 ==<|> 1; }",
+        )
+    }
+
+    #[test]
+    fn flip_neq_operands_for_simple_stmt() {
+        check_assist(
+            flip_eq_operands,
+            "fn f() { let res = 1 !=<|> 2; }",
+            "fn f() { let res = 2 !=<|> 1; }",
+        )
+    }
+
+    #[test]
+    fn flip_eq_operands_for_complex_stmt() {
+        check_assist(
+            flip_eq_operands,
+            "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_eq_operands,
+            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_eq_operands, "fn f() { let res = 1 ==<|> 2; }", "==")
+    }
+}
diff --git a/crates/ra_assists/src/lib.rs b/crates/ra_assists/src/lib.rs
index fc36e8cc9..8900fbc4b 100644
--- a/crates/ra_assists/src/lib.rs
+++ b/crates/ra_assists/src/lib.rs
@@ -88,6 +88,7 @@ where
 mod add_derive;
 mod add_impl;
 mod flip_comma;
+mod flip_eq_operands;
 mod change_visibility;
 mod fill_match_arms;
 mod fill_struct_fields;
@@ -106,6 +107,7 @@ fn all_assists<DB: HirDatabase>() -> &'static [fn(AssistCtx<DB>) -> Option<Assis
         fill_match_arms::fill_match_arms,
         fill_struct_fields::fill_struct_fields,
         flip_comma::flip_comma,
+        flip_eq_operands::flip_eq_operands,
         introduce_variable::introduce_variable,
         replace_if_let_with_match::replace_if_let_with_match,
         split_import::split_import,
diff --git a/crates/ra_hir/src/expr.rs b/crates/ra_hir/src/expr.rs
index 703d99d9b..c37fd0454 100644
--- a/crates/ra_hir/src/expr.rs
+++ b/crates/ra_hir/src/expr.rs
@@ -680,7 +680,7 @@ impl ExprCollector {
             }
             ast::ExprKind::PrefixExpr(e) => {
                 let expr = self.collect_expr_opt(e.expr());
-                if let Some(op) = e.op() {
+                if let Some(op) = e.op_kind() {
                     self.alloc_expr(Expr::UnaryOp { expr, op }, syntax_ptr)
                 } else {
                     self.alloc_expr(Expr::Missing, syntax_ptr)
@@ -703,7 +703,7 @@ impl ExprCollector {
             ast::ExprKind::BinExpr(e) => {
                 let lhs = self.collect_expr_opt(e.lhs());
                 let rhs = self.collect_expr_opt(e.rhs());
-                let op = e.op();
+                let op = e.op_kind();
                 self.alloc_expr(Expr::BinaryOp { lhs, rhs, op }, syntax_ptr)
             }
             ast::ExprKind::TupleExpr(e) => {
diff --git a/crates/ra_syntax/src/ast.rs b/crates/ra_syntax/src/ast.rs
index d8c2cb063..226208700 100644
--- a/crates/ra_syntax/src/ast.rs
+++ b/crates/ra_syntax/src/ast.rs
@@ -521,7 +521,7 @@ pub enum PrefixOp {
 }
 
 impl PrefixExpr {
-    pub fn op(&self) -> Option<PrefixOp> {
+    pub fn op_kind(&self) -> Option<PrefixOp> {
         match self.syntax().first_child()?.kind() {
             STAR => Some(PrefixOp::Deref),
             EXCL => Some(PrefixOp::Not),
@@ -529,6 +529,10 @@ impl PrefixExpr {
             _ => None,
         }
     }
+
+    pub fn op(&self) -> Option<&SyntaxNode> {
+        self.syntax().first_child()
+    }
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
@@ -598,46 +602,54 @@ pub enum BinOp {
 }
 
 impl BinExpr {
-    pub fn op(&self) -> Option<BinOp> {
+    fn op_details(&self) -> Option<(&SyntaxNode, BinOp)> {
         self.syntax()
             .children()
             .filter_map(|c| match c.kind() {
-                PIPEPIPE => Some(BinOp::BooleanOr),
-                AMPAMP => Some(BinOp::BooleanAnd),
-                EQEQ => Some(BinOp::EqualityTest),
-                NEQ => Some(BinOp::NegatedEqualityTest),
-                LTEQ => Some(BinOp::LesserEqualTest),
-                GTEQ => Some(BinOp::GreaterEqualTest),
-                L_ANGLE => Some(BinOp::LesserTest),
-                R_ANGLE => Some(BinOp::GreaterTest),
-                PLUS => Some(BinOp::Addition),
-                STAR => Some(BinOp::Multiplication),
-                MINUS => Some(BinOp::Subtraction),
-                SLASH => Some(BinOp::Division),
-                PERCENT => Some(BinOp::Remainder),
-                SHL => Some(BinOp::LeftShift),
-                SHR => Some(BinOp::RightShift),
-                CARET => Some(BinOp::BitwiseXor),
-                PIPE => Some(BinOp::BitwiseOr),
-                AMP => Some(BinOp::BitwiseAnd),
-                DOTDOT => Some(BinOp::RangeRightOpen),
-                DOTDOTEQ => Some(BinOp::RangeRightClosed),
-                EQ => Some(BinOp::Assignment),
-                PLUSEQ => Some(BinOp::AddAssign),
-                SLASHEQ => Some(BinOp::DivAssign),
-                STAREQ => Some(BinOp::MulAssign),
-                PERCENTEQ => Some(BinOp::RemAssign),
-                SHREQ => Some(BinOp::ShrAssign),
-                SHLEQ => Some(BinOp::ShlAssign),
-                MINUSEQ => Some(BinOp::SubAssign),
-                PIPEEQ => Some(BinOp::BitOrAssign),
-                AMPEQ => Some(BinOp::BitAndAssign),
-                CARETEQ => Some(BinOp::BitXorAssign),
+                PIPEPIPE => Some((c, BinOp::BooleanOr)),
+                AMPAMP => Some((c, BinOp::BooleanAnd)),
+                EQEQ => Some((c, BinOp::EqualityTest)),
+                NEQ => Some((c, BinOp::NegatedEqualityTest)),
+                LTEQ => Some((c, BinOp::LesserEqualTest)),
+                GTEQ => Some((c, BinOp::GreaterEqualTest)),
+                L_ANGLE => Some((c, BinOp::LesserTest)),
+                R_ANGLE => Some((c, BinOp::GreaterTest)),
+                PLUS => Some((c, BinOp::Addition)),
+                STAR => Some((c, BinOp::Multiplication)),
+                MINUS => Some((c, BinOp::Subtraction)),
+                SLASH => Some((c, BinOp::Division)),
+                PERCENT => Some((c, BinOp::Remainder)),
+                SHL => Some((c, BinOp::LeftShift)),
+                SHR => Some((c, BinOp::RightShift)),
+                CARET => Some((c, BinOp::BitwiseXor)),
+                PIPE => Some((c, BinOp::BitwiseOr)),
+                AMP => Some((c, BinOp::BitwiseAnd)),
+                DOTDOT => Some((c, BinOp::RangeRightOpen)),
+                DOTDOTEQ => Some((c, BinOp::RangeRightClosed)),
+                EQ => Some((c, BinOp::Assignment)),
+                PLUSEQ => Some((c, BinOp::AddAssign)),
+                SLASHEQ => Some((c, BinOp::DivAssign)),
+                STAREQ => Some((c, BinOp::MulAssign)),
+                PERCENTEQ => Some((c, BinOp::RemAssign)),
+                SHREQ => Some((c, BinOp::ShrAssign)),
+                SHLEQ => Some((c, BinOp::ShlAssign)),
+                MINUSEQ => Some((c, BinOp::SubAssign)),
+                PIPEEQ => Some((c, BinOp::BitOrAssign)),
+                AMPEQ => Some((c, BinOp::BitAndAssign)),
+                CARETEQ => Some((c, BinOp::BitXorAssign)),
                 _ => None,
             })
             .next()
     }
 
+    pub fn op_kind(&self) -> Option<BinOp> {
+        self.op_details().map(|t| t.1)
+    }
+
+    pub fn op(&self) -> Option<&SyntaxNode> {
+        self.op_details().map(|t| t.0)
+    }
+
     pub fn lhs(&self) -> Option<&Expr> {
         children(self).nth(0)
     }