Auto merge of #13764 - WaffleLapkin:badassexprs, r=Veykril

fix: Correctly check for parentheses redundancy in `remove_parentheses` assist

This is quite a bunch of code and some hacks, but I _think_ this time it's correct.

I've added a lot of tests, most of which fail with the assist impl from #13733 :')

2 files changed, 440 insertions, 97 deletions

diff --git a/crates/ide-assists/src/handlers/remove_parentheses.rs b/crates/ide-assists/src/handlers/remove_parentheses.rs
index 185beda9d07..e9c7c6bae9c 100644
--- a/crates/ide-assists/src/handlers/remove_parentheses.rs
+++ b/crates/ide-assists/src/handlers/remove_parentheses.rs
@@ -29,9 +29,8 @@ pub(crate) fn remove_parentheses(acc: &mut Assists, ctx: &AssistContext<'_>) ->
 
     let expr = parens.expr()?;
 
-    let parent = ast::Expr::cast(parens.syntax().parent()?);
-    let is_ok_to_remove = expr.precedence() >= parent.as_ref().and_then(ast::Expr::precedence);
-    if !is_ok_to_remove {
+    let parent = parens.syntax().parent()?;
+    if expr.needs_parens_in(parent) {
         return None;
     }
 
@@ -59,6 +58,31 @@ mod tests {
     }
 
     #[test]
+    fn remove_parens_closure() {
+        check_assist(remove_parentheses, r#"fn f() { &$0(|| 42) }"#, r#"fn f() { &|| 42 }"#);
+
+        check_assist_not_applicable(remove_parentheses, r#"fn f() { $0(|| 42).f() }"#);
+    }
+
+    #[test]
+    fn remove_parens_if_let_chains() {
+        check_assist_not_applicable(
+            remove_parentheses,
+            r#"fn f() { if let true = $0(true && true) {} }"#,
+        );
+    }
+
+    #[test]
+    fn remove_parens_associativity() {
+        check_assist(
+            remove_parentheses,
+            r#"fn f() { $0(2 + 2) + 2; }"#,
+            r#"fn f() { 2 + 2 + 2; }"#,
+        );
+        check_assist_not_applicable(remove_parentheses, r#"fn f() { 2 + $0(2 + 2); }"#);
+    }
+
+    #[test]
     fn remove_parens_precedence() {
         check_assist(
             remove_parentheses,
@@ -88,4 +112,110 @@ mod tests {
         check_assist_not_applicable(remove_parentheses, r#"fn f() { (2 +$0 2) }"#);
         check_assist_not_applicable(remove_parentheses, r#"fn f() {$0 (2 + 2) }"#);
     }
+
+    #[test]
+    fn remove_parens_doesnt_apply_when_expr_would_be_turned_into_a_statement() {
+        check_assist_not_applicable(remove_parentheses, r#"fn x() -> u8 { $0({ 0 } + 1) }"#);
+        check_assist_not_applicable(
+            remove_parentheses,
+            r#"fn x() -> u8 { $0(if true { 0 } else { 1 } + 1) }"#,
+        );
+        check_assist_not_applicable(remove_parentheses, r#"fn x() -> u8 { $0(loop {} + 1) }"#);
+    }
+
+    #[test]
+    fn remove_parens_doesnt_apply_weird_syntax_and_adge_cases() {
+        // removing `()` would break code because {} would be counted as the loop/if body
+        check_assist_not_applicable(remove_parentheses, r#"fn f() { for _ in $0(0..{3}) {} }"#);
+        check_assist_not_applicable(remove_parentheses, r#"fn f() { for _ in $0(S {}) {} }"#);
+        check_assist_not_applicable(remove_parentheses, r#"fn f() { if $0(S {} == 2) {} }"#);
+        check_assist_not_applicable(remove_parentheses, r#"fn f() { if $0(return) {} }"#);
+    }
+
+    #[test]
+    fn remove_parens_return_with_value_followed_by_block() {
+        check_assist(
+            remove_parentheses,
+            r#"fn f() { if $0(return ()) {} }"#,
+            r#"fn f() { if return () {} }"#,
+        );
+    }
+
+    #[test]
+    fn remove_exprs_let_else_restrictions() {
+        // `}` is not allowed before `else` here
+        check_assist_not_applicable(
+            remove_parentheses,
+            r#"fn f() { let _ = $0(S{}) else { return }; }"#,
+        );
+
+        // logic operators can't directly appear in the let-else
+        check_assist_not_applicable(
+            remove_parentheses,
+            r#"fn f() { let _ = $0(false || false) else { return }; }"#,
+        );
+        check_assist_not_applicable(
+            remove_parentheses,
+            r#"fn f() { let _ = $0(true && true) else { return }; }"#,
+        );
+    }
+
+    #[test]
+    fn remove_parens_weird_places() {
+        check_assist(
+            remove_parentheses,
+            r#"fn f() { match () { _=>$0(()) } }"#,
+            r#"fn f() { match () { _=>() } }"#,
+        );
+
+        check_assist(
+            remove_parentheses,
+            r#"fn x() -> u8 { { [$0({ 0 } + 1)] } }"#,
+            r#"fn x() -> u8 { { [{ 0 } + 1] } }"#,
+        );
+    }
+
+    #[test]
+    fn remove_parens_return_dot_f() {
+        check_assist(
+            remove_parentheses,
+            r#"fn f() { $0(return).f() }"#,
+            r#"fn f() { return.f() }"#,
+        );
+    }
+
+    #[test]
+    fn remove_parens_prefix_then_return_something() {
+        check_assist(
+            remove_parentheses,
+            r#"fn f() { &$0(return ()) }"#,
+            r#"fn f() { &return () }"#,
+        );
+    }
+
+    #[test]
+    fn remove_parens_double_paren_stmt() {
+        check_assist(
+            remove_parentheses,
+            r#"fn x() -> u8 { $0(({ 0 } + 1)) }"#,
+            r#"fn x() -> u8 { ({ 0 } + 1) }"#,
+        );
+
+        check_assist(
+            remove_parentheses,
+            r#"fn x() -> u8 { (($0{ 0 } + 1)) }"#,
+            r#"fn x() -> u8 { ({ 0 } + 1) }"#,
+        );
+    }
+
+    #[test]
+    fn remove_parens_im_tired_of_naming_tests() {
+        check_assist(
+            remove_parentheses,
+            r#"fn f() { 2 + $0(return 2) }"#,
+            r#"fn f() { 2 + return 2 }"#,
+        );
+
+        check_assist_not_applicable(remove_parentheses, r#"fn f() { $0(return 2) + 2 }"#);
+    }
 }
diff --git a/crates/syntax/src/ast/prec.rs b/crates/syntax/src/ast/prec.rs
index 6253c4dc3e7..ac7ef45c1de 100644
--- a/crates/syntax/src/ast/prec.rs
+++ b/crates/syntax/src/ast/prec.rs
@@ -1,115 +1,328 @@
 //! Precedence representation.
 
-use crate::ast::{self, BinExpr, Expr};
-
-/// Precedence of an expression.
-#[derive(Debug, Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]
-pub enum ExprPrecedence {
-    // N.B.: Order is important
-    Closure,
-    Jump,
-    Range,
-    Bin(BinOpPresedence),
-    Prefix,
-    Postfix,
-    Paren,
-}
-
-/// Precedence of a binary operator.
-#[derive(Debug, Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]
-pub enum BinOpPresedence {
-    // N.B.: Order is important
-    /// `=`, `+=`, `-=`, `*=`, `/=`, `%=`, `|=`, `&=`
-    Assign,
-    /// `||`
-    LOr,
-    /// `&&`
-    LAnd,
-    /// `<`, `<=`, `>`, `>=`, `==` and `!=`
-    Cmp,
-    /// `|`
-    BitOr,
-    /// `^`
-    BitXor,
-    /// `&`
-    BitAnd,
-    /// `<<` and `>>`
-    Shift,
-    /// `+` and `-`
-    Add,
-    /// `*`, `/` and `%`
-    Mul,
-    /// `as`
-    As,
-}
+use crate::{
+    ast::{self, BinaryOp, Expr, HasArgList},
+    match_ast, AstNode, SyntaxNode,
+};
 
 impl Expr {
-    /// Returns precedence of this expression.
-    /// Usefull to preserve semantics in assists.
-    ///
-    /// Returns `None` if this is a [`BinExpr`] and its [`op_kind`] returns `None`.
-    ///
-    /// [`op_kind`]: BinExpr::op_kind
-    /// [`BinExpr`]: Expr::BinExpr
-    pub fn precedence(&self) -> Option<ExprPrecedence> {
-        // Copied from <https://github.com/rust-lang/rust/blob/b6852428a8ea9728369b64b9964cad8e258403d3/compiler/rustc_ast/src/util/parser.rs#L296>
+    // Implementation is based on
+    // - https://doc.rust-lang.org/reference/expressions.html#expression-precedence
+    // - https://matklad.github.io/2020/04/13/simple-but-powerful-pratt-parsing.html
+    // - rustc source, including, but not limited to
+    //   - https://github.com/rust-lang/rust/blob/b6852428a8ea9728369b64b9964cad8e258403d3/compiler/rustc_ast/src/util/parser.rs#L296
+
+    /// Returns `true` if `self` would need to be wrapped in parentheses given that its parent is `parent`.
+    pub fn needs_parens_in(&self, parent: SyntaxNode) -> bool {
+        match_ast! {
+            match parent {
+                ast::Expr(e) => self.needs_parens_in_expr(&e),
+                ast::Stmt(e) => self.needs_parens_in_stmt(Some(&e)),
+                ast::StmtList(_) => self.needs_parens_in_stmt(None),
+                ast::ArgList(_) => false,
+                ast::MatchArm(_) => false,
+                _ => false,
+            }
+        }
+    }
+
+    fn needs_parens_in_expr(&self, parent: &Expr) -> bool {
+        // Special-case block weirdness
+        if parent.child_is_followed_by_a_block() {
+            use Expr::*;
+            match self {
+                // Cases like `if return {}` (need parens or else `{}` is returned, instead of being `if`'s body)
+                ReturnExpr(e) if e.expr().is_none() => return true,
+                BreakExpr(e) if e.expr().is_none() => return true,
+                YieldExpr(e) if e.expr().is_none() => return true,
+
+                // Same but with `..{}`
+                RangeExpr(e) if matches!(e.end(), Some(BlockExpr(..))) => return true,
+
+                // Similarly with struct literals, e.g. `if S{} == 1 {}`
+                _ if self.contains_exterior_struct_lit() => return true,
+                _ => {}
+            }
+        }
+
+        // Special-case `return.f()`
+        if self.is_ret_like_with_no_value() && parent.is_postfix() {
+            return false;
+        }
+
+        if self.is_paren_like()
+            || parent.is_paren_like()
+            || self.is_prefix() && (parent.is_prefix() || !self.is_ordered_before(parent))
+            || self.is_postfix() && (parent.is_postfix() || self.is_ordered_before(parent))
+        {
+            return false;
+        }
+
+        let (left, right, inv) = match self.is_ordered_before(parent) {
+            true => (self, parent, false),
+            false => (parent, self, true),
+        };
+
+        let (_, left_right_bp) = left.binding_power();
+        let (right_left_bp, _) = right.binding_power();
+
+        (left_right_bp < right_left_bp) ^ inv
+    }
+
+    fn needs_parens_in_stmt(&self, stmt: Option<&ast::Stmt>) -> bool {
         use Expr::*;
 
-        let prec = match self {
-            ClosureExpr(_) => ExprPrecedence::Closure,
+        // Prevent false-positives in cases like `fn x() -> u8 { ({ 0 } + 1) }`,
+        // `{ { 0 } + 1 }` won't parse -- `{ 0 }` would be parsed as a self-contained stmt,
+        // leaving `+ 1` as a parse error.
+        let mut innermost = self.clone();
+        loop {
+            let next = match &innermost {
+                BinExpr(e) => e.lhs(),
+                CallExpr(e) => e.expr(),
+                CastExpr(e) => e.expr(),
+                IndexExpr(e) => e.base(),
+                _ => break,
+            };
 
-            ContinueExpr(_) | ReturnExpr(_) | YieldExpr(_) | BreakExpr(_) => ExprPrecedence::Jump,
+            if let Some(next) = next {
+                innermost = next;
+                if !innermost.requires_semi_to_be_stmt() {
+                    return true;
+                }
+            } else {
+                break;
+            }
+        }
 
-            RangeExpr(_) => ExprPrecedence::Range,
+        // Not every expression can be followed by `else` in the `let-else`
+        if let Some(ast::Stmt::LetStmt(e)) = stmt {
+            if e.let_else().is_some() {
+                match self {
+                    BinExpr(e)
+                        if e.op_kind()
+                            .map(|op| matches!(op, BinaryOp::LogicOp(_)))
+                            .unwrap_or(false) =>
+                    {
+                        return true
+                    }
+                    _ if self.clone().trailing_brace().is_some() => return true,
+                    _ => {}
+                }
+            }
+        }
 
-            BinExpr(bin_expr) => return bin_expr.precedence().map(ExprPrecedence::Bin),
-            CastExpr(_) => ExprPrecedence::Bin(BinOpPresedence::As),
+        false
+    }
+
+    /// Returns left and right so-called "binding powers" of this expression.
+    fn binding_power(&self) -> (u8, u8) {
+        use ast::{ArithOp::*, BinaryOp::*, Expr::*, LogicOp::*};
+
+        let dps = match self {
+            // (0, 0)   -- paren-like/nullary
+            // (0, N)   -- prefix
+            // (N, 0)   -- postfix
+            // (N, N)   -- infix, requires parens
+            // (N, N+1) -- infix, left to right associative
+            // (N+1, N) -- infix, right to left associative
+            // N is odd
+            //
+            ContinueExpr(_) => (0, 0),
+
+            ClosureExpr(_) | ReturnExpr(_) | YieldExpr(_) | BreakExpr(_) => (0, 1),
+
+            RangeExpr(_) => (5, 5),
+
+            BinExpr(e) => {
+                // Return a dummy value if we don't know the op
+                let Some(op) = e.op_kind() else { return (0, 0) };
+                match op {
+                    Assignment { .. } => (4, 3),
+                    //
+                    // Ranges are here in order :)
+                    //
+                    LogicOp(op) => match op {
+                        Or => (7, 8),
+                        And => (9, 10),
+                    },
+                    CmpOp(_) => (11, 11),
+                    ArithOp(op) => match op {
+                        BitOr => (13, 14),
+                        BitXor => (15, 16),
+                        BitAnd => (17, 18),
+                        Shl | Shr => (19, 20),
+                        Add | Sub => (21, 22),
+                        Mul | Div | Rem => (23, 24),
+                    },
+                }
+            }
+
+            CastExpr(_) => (25, 26),
+
+            BoxExpr(_) | RefExpr(_) | LetExpr(_) | PrefixExpr(_) => (0, 27),
 
-            BoxExpr(_) | RefExpr(_) | LetExpr(_) | PrefixExpr(_) => ExprPrecedence::Prefix,
+            AwaitExpr(_) | CallExpr(_) | MethodCallExpr(_) | IndexExpr(_) | TryExpr(_)
+            | MacroExpr(_) => (29, 0),
 
-            AwaitExpr(_) | CallExpr(_) | MethodCallExpr(_) | FieldExpr(_) | IndexExpr(_)
-            | TryExpr(_) | MacroExpr(_) => ExprPrecedence::Postfix,
+            FieldExpr(_) => (31, 32),
 
             ArrayExpr(_) | TupleExpr(_) | Literal(_) | PathExpr(_) | ParenExpr(_) | IfExpr(_)
             | WhileExpr(_) | ForExpr(_) | LoopExpr(_) | MatchExpr(_) | BlockExpr(_)
-            | RecordExpr(_) | UnderscoreExpr(_) => ExprPrecedence::Paren,
+            | RecordExpr(_) | UnderscoreExpr(_) => (0, 0),
         };
 
-        Some(prec)
+        dps
     }
-}
 
-impl BinExpr {
-    /// Returns precedence of this binary expression.
-    /// Usefull to preserve semantics in assists.
-    ///
-    /// Returns `None` if [`op_kind`] returns `None`.
-    ///
-    /// [`op_kind`]: BinExpr::op_kind
-    pub fn precedence(&self) -> Option<BinOpPresedence> {
-        use ast::{ArithOp::*, BinaryOp::*, LogicOp::*};
-
-        let prec = match self.op_kind()? {
-            LogicOp(op) => match op {
-                And => BinOpPresedence::LAnd,
-                Or => BinOpPresedence::LOr,
-            },
-            ArithOp(op) => match op {
-                Add => BinOpPresedence::Add,
-                Mul => BinOpPresedence::Mul,
-                Sub => BinOpPresedence::Add,
-                Div => BinOpPresedence::Mul,
-                Rem => BinOpPresedence::Mul,
-                Shl => BinOpPresedence::Shift,
-                Shr => BinOpPresedence::Shift,
-                BitXor => BinOpPresedence::BitXor,
-                BitOr => BinOpPresedence::BitOr,
-                BitAnd => BinOpPresedence::BitAnd,
-            },
-            CmpOp(_) => BinOpPresedence::Cmp,
-            Assignment { .. } => BinOpPresedence::Assign,
-        };
+    fn is_paren_like(&self) -> bool {
+        matches!(self.binding_power(), (0, 0))
+    }
+
+    fn is_prefix(&self) -> bool {
+        matches!(self.binding_power(), (0, 1..))
+    }
+
+    fn is_postfix(&self) -> bool {
+        matches!(self.binding_power(), (1.., 0))
+    }
+
+    /// Returns `true` if this expression can't be a standalone statement.
+    fn requires_semi_to_be_stmt(&self) -> bool {
+        use Expr::*;
+        !matches!(
+            self,
+            IfExpr(..) | MatchExpr(..) | BlockExpr(..) | WhileExpr(..) | LoopExpr(..) | ForExpr(..)
+        )
+    }
+
+    /// If an expression ends with `}`, returns the innermost expression ending in this `}`.
+    fn trailing_brace(mut self) -> Option<Expr> {
+        use Expr::*;
+
+        loop {
+            let rhs = match self {
+                RefExpr(e) => e.expr(),
+                BinExpr(e) => e.rhs(),
+                BoxExpr(e) => e.expr(),
+                BreakExpr(e) => e.expr(),
+                LetExpr(e) => e.expr(),
+                RangeExpr(e) => e.end(),
+                ReturnExpr(e) => e.expr(),
+                PrefixExpr(e) => e.expr(),
+                YieldExpr(e) => e.expr(),
+                ClosureExpr(e) => e.body(),
+
+                BlockExpr(..) | ForExpr(..) | IfExpr(..) | LoopExpr(..) | MatchExpr(..)
+                | RecordExpr(..) | WhileExpr(..) => break Some(self),
+                _ => break None,
+            };
+
+            self = rhs?;
+        }
+    }
+
+    /// Expressions that syntactically contain an "exterior" struct literal i.e., not surrounded by any
+    /// parens or other delimiters, e.g., `X { y: 1 }`, `X { y: 1 }.method()`, `foo == X { y: 1 }` and
+    /// `X { y: 1 } == foo` all do, but `(X { y: 1 }) == foo` does not.
+    fn contains_exterior_struct_lit(&self) -> bool {
+        return contains_exterior_struct_lit_inner(self).is_some();
+
+        fn contains_exterior_struct_lit_inner(expr: &Expr) -> Option<()> {
+            use Expr::*;
+
+            match expr {
+                RecordExpr(..) => Some(()),
+
+                // X { y: 1 } + X { y: 2 }
+                BinExpr(e) => e
+                    .lhs()
+                    .as_ref()
+                    .and_then(contains_exterior_struct_lit_inner)
+                    .or_else(|| e.rhs().as_ref().and_then(contains_exterior_struct_lit_inner)),
+
+                // `&X { y: 1 }`, `X { y: 1 }.y`, `X { y: 1 }.bar(...)`, etc
+                IndexExpr(e) => contains_exterior_struct_lit_inner(&e.base()?),
+                AwaitExpr(e) => contains_exterior_struct_lit_inner(&e.expr()?),
+                PrefixExpr(e) => contains_exterior_struct_lit_inner(&e.expr()?),
+                CastExpr(e) => contains_exterior_struct_lit_inner(&e.expr()?),
+                FieldExpr(e) => contains_exterior_struct_lit_inner(&e.expr()?),
+                MethodCallExpr(e) => contains_exterior_struct_lit_inner(&e.receiver()?),
+
+                _ => None,
+            }
+        }
+    }
+
+    /// Returns true if self is one of `return`, `break`, `continue` or `yield` with **no associated value**.
+    fn is_ret_like_with_no_value(&self) -> bool {
+        use Expr::*;
+
+        match self {
+            ReturnExpr(e) => e.expr().is_none(),
+            BreakExpr(e) => e.expr().is_none(),
+            ContinueExpr(_) => true,
+            YieldExpr(e) => e.expr().is_none(),
+            _ => false,
+        }
+    }
+
+    fn is_ordered_before(&self, other: &Expr) -> bool {
+        use Expr::*;
+
+        return order(self) < order(other);
+
+        /// Returns text range that can be used to compare two expression for order (which goes first).
+        fn order(this: &Expr) -> rowan::TextSize {
+            // For non-paren-like operators: get the operator itself
+            let token = match this {
+                RangeExpr(e) => e.op_token(),
+                BinExpr(e) => e.op_token(),
+                CastExpr(e) => e.as_token(),
+                FieldExpr(e) => e.dot_token(),
+                AwaitExpr(e) => e.dot_token(),
+                BoxExpr(e) => e.box_token(),
+                BreakExpr(e) => e.break_token(),
+                CallExpr(e) => e.arg_list().and_then(|args| args.l_paren_token()),
+                ClosureExpr(e) => e.param_list().and_then(|params| params.l_paren_token()),
+                ContinueExpr(e) => e.continue_token(),
+                IndexExpr(e) => e.l_brack_token(),
+                MethodCallExpr(e) => e.dot_token(),
+                PrefixExpr(e) => e.op_token(),
+                RefExpr(e) => e.amp_token(),
+                ReturnExpr(e) => e.return_token(),
+                TryExpr(e) => e.question_mark_token(),
+                YieldExpr(e) => e.yield_token(),
+                LetExpr(e) => e.let_token(),
+
+                ArrayExpr(_) | TupleExpr(_) | Literal(_) | PathExpr(_) | ParenExpr(_)
+                | IfExpr(_) | WhileExpr(_) | ForExpr(_) | LoopExpr(_) | MatchExpr(_)
+                | BlockExpr(_) | RecordExpr(_) | UnderscoreExpr(_) | MacroExpr(_) => None,
+            };
+
+            token.map(|t| t.text_range()).unwrap_or_else(|| this.syntax().text_range()).start()
+        }
+    }
+
+    fn child_is_followed_by_a_block(&self) -> bool {
+        use Expr::*;
+
+        match self {
+            ArrayExpr(_) | AwaitExpr(_) | BlockExpr(_) | CallExpr(_) | CastExpr(_)
+            | ClosureExpr(_) | FieldExpr(_) | IndexExpr(_) | Literal(_) | LoopExpr(_)
+            | MacroExpr(_) | MethodCallExpr(_) | ParenExpr(_) | PathExpr(_) | RecordExpr(_)
+            | TryExpr(_) | TupleExpr(_) | UnderscoreExpr(_) => false,
+
+            // For BinExpr and RangeExpr this is technically wrong -- the child can be on the left...
+            BinExpr(_) | RangeExpr(_) | BoxExpr(_) | BreakExpr(_) | ContinueExpr(_)
+            | PrefixExpr(_) | RefExpr(_) | ReturnExpr(_) | YieldExpr(_) | LetExpr(_) => self
+                .syntax()
+                .parent()
+                .and_then(Expr::cast)
+                .map(|e| e.child_is_followed_by_a_block())
+                .unwrap_or(false),
 
-        Some(prec)
+            ForExpr(_) | IfExpr(_) | MatchExpr(_) | WhileExpr(_) => true,
+        }
     }
 }