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use swc_common::util::take::Take;
use swc_ecma_ast::*;
use super::{BitCtx, Optimizer};
use crate::program_data::VarUsageInfoFlags;
/// Methods related to option `dead_code`.
impl Optimizer<'_> {
/// 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();
/// }
/// ```
/// # Returns
///
/// returns true if `e` is changed.
pub(super) fn optimize_last_expr_before_termination(&mut self, e: &mut Expr) -> bool {
if !self.options.dead_code {
return false;
}
// A return statement in a try block may not terminate function.
if self.ctx.bit_ctx.contains(BitCtx::InTryBlock) {
return false;
}
if let Expr::Assign(assign @ AssignExpr { op: op!("="), .. }) = e {
self.optimize_last_expr_before_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.flags.contains(
VarUsageInfoFlags::DECLARED.union(VarUsageInfoFlags::IS_FN_LOCAL),
) && !(self.data.used_arguments(self.ctx.scope)
&& var.flags.contains(VarUsageInfoFlags::DECLARED_AS_FN_PARAM))
&& !var.flags.intersects(VarUsageInfoFlags::EXPORTED)
})
.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 true;
}
}
}
if let Expr::Assign(assign) = e {
// x += 1 => x + 1
if let Some(op) = assign.op.to_update() {
if op == op!("**") {
return false;
}
if let Some(lhs) = assign.left.as_ident() {
if self
.data
.vars
.get(&lhs.to_id())
.map(|var| {
var.flags.contains(
VarUsageInfoFlags::DECLARED.union(VarUsageInfoFlags::IS_FN_LOCAL),
) && !(self.data.used_arguments(self.ctx.scope)
&& var.flags.contains(VarUsageInfoFlags::DECLARED_AS_FN_PARAM))
&& !var.flags.contains(VarUsageInfoFlags::EXPORTED)
})
.unwrap_or(false)
{
report_change!(
"dead_code: Converting an assignment into a binary expression in \
function termination"
);
self.changed = true;
*e = BinExpr {
span: assign.span,
op,
left: lhs.clone().into(),
right: assign.right.take(),
}
.into();
return true;
}
}
}
}
false
}
}