swc_ecma_minifier/compress/optimize/
dead_code.rs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
use swc_common::util::take::Take;
use swc_ecma_ast::*;

use super::Optimizer;
use crate::mode::Mode;

/// Methods related to option `dead_code`.
impl<M> Optimizer<'_, M>
where
    M: Mode,
{
    /// Optimize return value or argument of throw.
    ///
    /// This methods removes some useless assignments.
    ///
    /// # Example
    ///
    /// Note: `a` being declared in the function is important in the example
    /// below.
    ///
    /// ```ts
    /// function foo(){
    ///     var a;
    ///     throw a = x();
    /// }
    /// ```
    ///
    /// can be optimized as
    ///
    /// ```ts
    /// function foo(){
    ///     var a; // Will be dropped in next pass.
    ///     throw x();
    /// }
    /// ```
    pub(super) fn optimize_in_fn_termination(&mut self, e: &mut Expr) {
        if !self.options.dead_code {
            return;
        }

        // A return statement in a try block may not terminate function.
        if self.ctx.in_try_block {
            return;
        }

        if let Expr::Assign(assign @ AssignExpr { op: op!("="), .. }) = e {
            self.optimize_in_fn_termination(&mut assign.right);

            // We only handle identifiers on lhs for now.
            if let Some(lhs) = assign.left.as_ident() {
                //
                if self
                    .data
                    .vars
                    .get(&lhs.to_id())
                    .map(|var| var.is_fn_local && !var.declared_as_fn_param)
                    .unwrap_or(false)
                {
                    report_change!(
                        "dead_code: Dropping an assignment to a variable declared in function \
                         because function is being terminated"
                    );
                    self.changed = true;
                    *e = *assign.right.take();
                    return;
                }
            }
        }

        if let Expr::Assign(assign) = e {
            // x += 1 => x + 1
            if let Some(op) = assign.op.to_update() {
                if let Some(lhs) = assign.left.as_ident() {
                    //
                    if self
                        .data
                        .vars
                        .get(&lhs.to_id())
                        .map(|var| var.is_fn_local)
                        .unwrap_or(false)
                    {
                        report_change!(
                            "dead_code: Converting an assignment into a binary expression in \
                             function termination"
                        );

                        self.changed = true;
                        *e = Expr::Bin(BinExpr {
                            span: assign.span,
                            op,
                            left: Box::new(Expr::Ident(lhs.clone())),
                            right: assign.right.take(),
                        });
                    }
                }
            }
        }
    }
}