let_chains: Adjust lowering logic in lieu of ::Let.
This commit is contained in:
Mazdak Farrokhzad
parent
a7b00f5807
commit
c9fb639085
@ -4344,53 +4344,120 @@ impl<'a> LoweringContext<'a> {
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let ohs = P(self.lower_expr(ohs));
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hir::ExprKind::AddrOf(m, ohs)
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}
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// More complicated than you might expect because the else branch
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// might be `if let`.
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ExprKind::Let(..) => {
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// This should have been caught `ast_validation`!
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self.sess.span_err(e.span, "`let` expressions only supported in `if`");
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// ^-- FIXME(53667): Change to `delay_span_bug` when let_chains handled in lowering.
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self.sess.abort_if_errors();
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hir::ExprKind::Err
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}
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// FIXME(#53667): handle lowering of && and parens.
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ExprKind::If(ref cond, ref then, ref else_opt) => {
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// `true => then`:
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let then_pat = self.pat_bool(e.span, true);
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let then_blk = self.lower_block(then, false);
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let then_expr = self.expr_block(then_blk, ThinVec::new());
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let then_arm = self.arm(hir_vec![then_pat], P(then_expr));
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// `_ => else_block` where `else_block` is `{}` if there's `None`:
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let else_pat = self.pat_wild(e.span);
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let else_expr = match else_opt {
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None => self.expr_block_empty(e.span),
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Some(els) => match els.node {
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ExprKind::IfLet(..) => {
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// Wrap the `if let` expr in a block.
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let els = self.lower_expr(els);
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let blk = self.block_all(els.span, hir_vec![], Some(P(els)));
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self.expr_block(P(blk), ThinVec::new())
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}
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_ => self.lower_expr(els),
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}
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let (else_expr, contains_else_clause) = match else_opt {
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None => (self.expr_block_empty(e.span), false),
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Some(els) => (self.lower_expr(els), true),
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};
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let else_arm = self.arm(hir_vec![else_pat], P(else_expr));
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// Lower condition:
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let span_block = self.mark_span_with_reason(IfTemporary, cond.span, None);
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let cond = self.lower_expr(cond);
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// Wrap in a construct equivalent to `{ let _t = $cond; _t }` to preserve drop
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// semantics since `if cond { ... }` don't let temporaries live outside of `cond`.
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let cond = self.expr_drop_temps(span_block, P(cond), ThinVec::new());
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// Handle then + scrutinee:
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let then_blk = self.lower_block(then, false);
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let then_expr = self.expr_block(then_blk, ThinVec::new());
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let (then_pats, scrutinee, desugar) = match cond.node {
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// `<pat> => <then>`
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ExprKind::Let(ref pats, ref scrutinee) => {
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let scrutinee = self.lower_expr(scrutinee);
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let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
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let desugar = hir::MatchSource::IfLetDesugar { contains_else_clause };
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(pats, scrutinee, desugar)
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}
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// `true => then`:
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_ => {
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// Lower condition:
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let cond = self.lower_expr(cond);
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// Wrap in a construct equivalent to `{ let _t = $cond; _t }`
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// to preserve drop semantics since `if cond { ... }`
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// don't let temporaries live outside of `cond`.
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let span_block = self.mark_span_with_reason(IfTemporary, cond.span, None);
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// Wrap in a construct equivalent to `{ let _t = $cond; _t }`
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// to preserve drop semantics since `if cond { ... }` does not
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// let temporaries live outside of `cond`.
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let cond = self.expr_drop_temps(span_block, P(cond), ThinVec::new());
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hir::ExprKind::Match(
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P(cond),
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vec![then_arm, else_arm].into(),
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hir::MatchSource::IfDesugar {
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contains_else_clause: else_opt.is_some()
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},
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)
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let desugar = hir::MatchSource::IfDesugar { contains_else_clause };
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let pats = hir_vec![self.pat_bool(e.span, true)];
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(pats, cond, desugar)
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}
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};
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let then_arm = self.arm(then_pats, P(then_expr));
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hir::ExprKind::Match(P(scrutinee), vec![then_arm, else_arm].into(), desugar)
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}
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// FIXME(#53667): handle lowering of && and parens.
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ExprKind::While(ref cond, ref body, opt_label) => {
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// Desugar `ExprWhileLet`
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// from: `[opt_ident]: while let <pat> = <sub_expr> <body>`
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if let ExprKind::Let(ref pats, ref sub_expr) = cond.node {
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// to:
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//
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// [opt_ident]: loop {
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// match <sub_expr> {
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// <pat> => <body>,
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// _ => break
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// }
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// }
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// Note that the block AND the condition are evaluated in the loop scope.
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// This is done to allow `break` from inside the condition of the loop.
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let (body, break_expr, sub_expr) = self.with_loop_scope(e.id, |this| {
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(
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this.lower_block(body, false),
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this.expr_break(e.span, ThinVec::new()),
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this.with_loop_condition_scope(|this| P(this.lower_expr(sub_expr))),
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)
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});
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// `<pat> => <body>`
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let pat_arm = {
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let body_expr = P(self.expr_block(body, ThinVec::new()));
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let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
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self.arm(pats, body_expr)
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};
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// `_ => break`
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let break_arm = {
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let pat_under = self.pat_wild(e.span);
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self.arm(hir_vec![pat_under], break_expr)
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};
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// `match <sub_expr> { ... }`
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let arms = hir_vec![pat_arm, break_arm];
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let match_expr = self.expr(
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sub_expr.span,
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hir::ExprKind::Match(sub_expr, arms, hir::MatchSource::WhileLetDesugar),
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ThinVec::new(),
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);
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// `[opt_ident]: loop { ... }`
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let loop_block = P(self.block_expr(P(match_expr)));
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let loop_expr = hir::ExprKind::Loop(
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loop_block,
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self.lower_label(opt_label),
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hir::LoopSource::WhileLet,
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);
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// Add attributes to the outer returned expr node.
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loop_expr
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} else {
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self.with_loop_scope(e.id, |this| {
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hir::ExprKind::While(
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this.with_loop_condition_scope(|this| P(this.lower_expr(cond))),
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this.lower_block(body, false),
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this.lower_label(opt_label),
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)
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})
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}
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}
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ExprKind::While(ref cond, ref body, opt_label) => self.with_loop_scope(e.id, |this| {
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hir::ExprKind::While(
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this.with_loop_condition_scope(|this| P(this.lower_expr(cond))),
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this.lower_block(body, false),
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this.lower_label(opt_label),
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)
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}),
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ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
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hir::ExprKind::Loop(
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this.lower_block(body, false),
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@ -4703,105 +4770,6 @@ impl<'a> LoweringContext<'a> {
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ExprKind::Err => hir::ExprKind::Err,
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// Desugar `ExprIfLet`
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// from: `if let <pat> = <sub_expr> <body> [<else_opt>]`
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ExprKind::IfLet(ref pats, ref sub_expr, ref body, ref else_opt) => {
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// to:
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//
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// match <sub_expr> {
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// <pat> => <body>,
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// _ => [<else_opt> | ()]
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// }
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let mut arms = vec![];
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// `<pat> => <body>`
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{
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let body = self.lower_block(body, false);
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let body_expr = P(self.expr_block(body, ThinVec::new()));
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let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
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arms.push(self.arm(pats, body_expr));
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}
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// _ => [<else_opt>|{}]
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{
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let wildcard_arm: Option<&Expr> = else_opt.as_ref().map(|p| &**p);
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let wildcard_pattern = self.pat_wild(e.span);
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let body = if let Some(else_expr) = wildcard_arm {
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self.lower_expr(else_expr)
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} else {
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self.expr_block_empty(e.span)
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};
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arms.push(self.arm(hir_vec![wildcard_pattern], P(body)));
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}
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let contains_else_clause = else_opt.is_some();
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let sub_expr = P(self.lower_expr(sub_expr));
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hir::ExprKind::Match(
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sub_expr,
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arms.into(),
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hir::MatchSource::IfLetDesugar {
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contains_else_clause,
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},
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)
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}
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// Desugar `ExprWhileLet`
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// from: `[opt_ident]: while let <pat> = <sub_expr> <body>`
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ExprKind::WhileLet(ref pats, ref sub_expr, ref body, opt_label) => {
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// to:
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//
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// [opt_ident]: loop {
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// match <sub_expr> {
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// <pat> => <body>,
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// _ => break
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// }
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// }
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// Note that the block AND the condition are evaluated in the loop scope.
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// This is done to allow `break` from inside the condition of the loop.
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let (body, break_expr, sub_expr) = self.with_loop_scope(e.id, |this| {
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(
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this.lower_block(body, false),
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this.expr_break(e.span, ThinVec::new()),
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this.with_loop_condition_scope(|this| P(this.lower_expr(sub_expr))),
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)
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});
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// `<pat> => <body>`
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let pat_arm = {
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let body_expr = P(self.expr_block(body, ThinVec::new()));
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let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
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self.arm(pats, body_expr)
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};
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// `_ => break`
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let break_arm = {
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let pat_under = self.pat_wild(e.span);
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self.arm(hir_vec![pat_under], break_expr)
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};
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// `match <sub_expr> { ... }`
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let arms = hir_vec![pat_arm, break_arm];
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let match_expr = self.expr(
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sub_expr.span,
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hir::ExprKind::Match(sub_expr, arms, hir::MatchSource::WhileLetDesugar),
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ThinVec::new(),
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);
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// `[opt_ident]: loop { ... }`
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let loop_block = P(self.block_expr(P(match_expr)));
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let loop_expr = hir::ExprKind::Loop(
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loop_block,
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self.lower_label(opt_label),
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hir::LoopSource::WhileLet,
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);
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// Add attributes to the outer returned expr node.
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loop_expr
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}
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// Desugar `ExprForLoop`
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// from: `[opt_ident]: for <pat> in <head> <body>`
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ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
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