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Introduce hir::ExprKind::Let - Take 2 #80357

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Aug 16, 2021
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Introduce hir::ExprKind::Let - Take 2 #80357

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2d9f2ea
Use correct drop scopes for if expressions
matthewjasper Feb 25, 2021
6aa9937
Introduce hir::ExprKind::Let - Take 2
c410-f3r Aug 8, 2021
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4 changes: 3 additions & 1 deletion compiler/rustc_ast/src/ast.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1302,7 +1302,9 @@ pub enum ExprKind {
Type(P<Expr>, P<Ty>),
/// A `let pat = expr` expression that is only semantically allowed in the condition
/// of `if` / `while` expressions. (e.g., `if let 0 = x { .. }`).
Let(P<Pat>, P<Expr>),
///
/// `Span` represents the whole `let pat = expr` statement.
Let(P<Pat>, P<Expr>, Span),
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/// An `if` block, with an optional `else` block.
///
/// `if expr { block } else { expr }`
Expand Down
2 changes: 1 addition & 1 deletion compiler/rustc_ast/src/mut_visit.rs
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Expand Up @@ -1237,7 +1237,7 @@ pub fn noop_visit_expr<T: MutVisitor>(
vis.visit_ty(ty);
}
ExprKind::AddrOf(_, _, ohs) => vis.visit_expr(ohs),
ExprKind::Let(pat, scrutinee) => {
ExprKind::Let(pat, scrutinee, _) => {
vis.visit_pat(pat);
vis.visit_expr(scrutinee);
}
Expand Down
4 changes: 2 additions & 2 deletions compiler/rustc_ast/src/visit.rs
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Expand Up @@ -779,9 +779,9 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
visitor.visit_expr(subexpression);
visitor.visit_ty(typ)
}
ExprKind::Let(ref pat, ref scrutinee) => {
ExprKind::Let(ref pat, ref expr, _) => {
visitor.visit_pat(pat);
visitor.visit_expr(scrutinee);
visitor.visit_expr(expr);
}
ExprKind::If(ref head_expression, ref if_block, ref optional_else) => {
visitor.visit_expr(head_expression);
Expand Down
260 changes: 60 additions & 200 deletions compiler/rustc_ast_lowering/src/expr.rs
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Expand Up @@ -86,32 +86,12 @@ impl<'hir> LoweringContext<'_, 'hir> {
let ohs = self.lower_expr(ohs);
hir::ExprKind::AddrOf(k, m, ohs)
}
ExprKind::Let(ref pat, ref scrutinee) => {
self.lower_expr_let(e.span, pat, scrutinee)
ExprKind::Let(ref pat, ref scrutinee, span) => {
hir::ExprKind::Let(self.lower_pat(pat), self.lower_expr(scrutinee), span)
}
ExprKind::If(ref cond, ref then, ref else_opt) => {
self.lower_expr_if(cond, then, else_opt.as_deref())
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}
ExprKind::If(ref cond, ref then, ref else_opt) => match cond.kind {
ExprKind::Let(ref pat, ref scrutinee) => {
self.lower_expr_if_let(e.span, pat, scrutinee, then, else_opt.as_deref())
}
ExprKind::Paren(ref paren) => match paren.peel_parens().kind {
ExprKind::Let(ref pat, ref scrutinee) => {
// A user has written `if (let Some(x) = foo) {`, we want to avoid
// confusing them with mentions of nightly features.
// If this logic is changed, you will also likely need to touch
// `unused::UnusedParens::check_expr`.
self.if_let_expr_with_parens(cond, &paren.peel_parens());
self.lower_expr_if_let(
e.span,
pat,
scrutinee,
then,
else_opt.as_deref(),
)
}
_ => self.lower_expr_if(cond, then, else_opt.as_deref()),
},
_ => self.lower_expr_if(cond, then, else_opt.as_deref()),
},
ExprKind::While(ref cond, ref body, opt_label) => self
.with_loop_scope(e.id, |this| {
this.lower_expr_while_in_loop_scope(e.span, cond, body, opt_label)
Expand Down Expand Up @@ -368,188 +348,69 @@ impl<'hir> LoweringContext<'_, 'hir> {
hir::ExprKind::Call(f, self.lower_exprs(&real_args))
}

fn if_let_expr_with_parens(&mut self, cond: &Expr, paren: &Expr) {
let start = cond.span.until(paren.span);
let end = paren.span.shrink_to_hi().until(cond.span.shrink_to_hi());
self.sess
.struct_span_err(
vec![start, end],
"invalid parentheses around `let` expression in `if let`",
)
.multipart_suggestion(
"`if let` needs to be written without parentheses",
vec![(start, String::new()), (end, String::new())],
rustc_errors::Applicability::MachineApplicable,
)
.emit();
// Ideally, we'd remove the feature gating of a `let` expression since we are already
// complaining about it here, but `feature_gate::check_crate` has already run by now:
// self.sess.parse_sess.gated_spans.ungate_last(sym::let_chains, paren.span);
}

/// Emit an error and lower `ast::ExprKind::Let(pat, scrutinee)` into:
/// ```rust
/// match scrutinee { pats => true, _ => false }
/// ```
fn lower_expr_let(&mut self, span: Span, pat: &Pat, scrutinee: &Expr) -> hir::ExprKind<'hir> {
// If we got here, the `let` expression is not allowed.

if self.sess.opts.unstable_features.is_nightly_build() {
self.sess
.struct_span_err(span, "`let` expressions are not supported here")
.note(
"only supported directly without parentheses in conditions of `if`- and \
`while`-expressions, as well as in `let` chains within parentheses",
)
.emit();
} else {
self.sess
.struct_span_err(span, "expected expression, found statement (`let`)")
.note("variable declaration using `let` is a statement")
.emit();
}

// For better recovery, we emit:
// ```
// match scrutinee { pat => true, _ => false }
// ```
// While this doesn't fully match the user's intent, it has key advantages:
// 1. We can avoid using `abort_if_errors`.
// 2. We can typeck both `pat` and `scrutinee`.
// 3. `pat` is allowed to be refutable.
// 4. The return type of the block is `bool` which seems like what the user wanted.
let scrutinee = self.lower_expr(scrutinee);
let then_arm = {
let pat = self.lower_pat(pat);
let expr = self.expr_bool(span, true);
self.arm(pat, expr)
};
let else_arm = {
let pat = self.pat_wild(span);
let expr = self.expr_bool(span, false);
self.arm(pat, expr)
};
hir::ExprKind::Match(
scrutinee,
arena_vec![self; then_arm, else_arm],
hir::MatchSource::Normal,
)
}

fn lower_expr_if(
&mut self,
cond: &Expr,
then: &Block,
else_opt: Option<&Expr>,
) -> hir::ExprKind<'hir> {
let cond = self.lower_expr(cond);
let then = self.arena.alloc(self.lower_block_expr(then));
let els = else_opt.map(|els| self.lower_expr(els));
hir::ExprKind::If(cond, then, els)
}

fn lower_expr_if_let(
&mut self,
span: Span,
pat: &Pat,
scrutinee: &Expr,
then: &Block,
else_opt: Option<&Expr>,
) -> hir::ExprKind<'hir> {
// FIXME(#53667): handle lowering of && and parens.

// `_ => else_block` where `else_block` is `{}` if there's `None`:
let else_pat = self.pat_wild(span);
let (else_expr, contains_else_clause) = match else_opt {
None => (self.expr_block_empty(span.shrink_to_hi()), false),
Some(els) => (self.lower_expr(els), true),
};
let else_arm = self.arm(else_pat, else_expr);

// Handle then + scrutinee:
let scrutinee = self.lower_expr(scrutinee);
let then_pat = self.lower_pat(pat);

let lowered_cond = self.lower_expr(cond);
let new_cond = self.manage_let_cond(lowered_cond);
let then_expr = self.lower_block_expr(then);
let then_arm = self.arm(then_pat, self.arena.alloc(then_expr));
if let Some(rslt) = else_opt {
hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), Some(self.lower_expr(rslt)))
} else {
hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), None)
}
}

let desugar = hir::MatchSource::IfLetDesugar { contains_else_clause };
hir::ExprKind::Match(scrutinee, arena_vec![self; then_arm, else_arm], desugar)
// If `cond` kind is `let`, returns `let`. Otherwise, wraps and returns `cond`
// in a temporary block.
fn manage_let_cond(&mut self, cond: &'hir hir::Expr<'hir>) -> &'hir hir::Expr<'hir> {
match cond.kind {
hir::ExprKind::Let(..) => cond,
_ => {
let span_block =
self.mark_span_with_reason(DesugaringKind::CondTemporary, cond.span, None);
self.expr_drop_temps(span_block, cond, AttrVec::new())
}
}
}

// We desugar: `'label: while $cond $body` into:
//
// ```
// 'label: loop {
// if { let _t = $cond; _t } {
// $body
// }
// else {
// break;
// }
// }
// ```
//
// Wrap in a construct equivalent to `{ let _t = $cond; _t }`
// to preserve drop semantics since `while $cond { ... }` does not
// let temporaries live outside of `cond`.
fn lower_expr_while_in_loop_scope(
&mut self,
span: Span,
cond: &Expr,
body: &Block,
opt_label: Option<Label>,
) -> hir::ExprKind<'hir> {
// FIXME(#53667): handle lowering of && and parens.

// Note that the block AND the condition are evaluated in the loop scope.
// This is done to allow `break` from inside the condition of the loop.

// `_ => break`:
let else_arm = {
let else_pat = self.pat_wild(span);
let else_expr = self.expr_break(span, ThinVec::new());
self.arm(else_pat, else_expr)
};

// Handle then + scrutinee:
let (then_pat, scrutinee, desugar, source) = match cond.kind {
ExprKind::Let(ref pat, ref scrutinee) => {
// to:
//
// [opt_ident]: loop {
// match <sub_expr> {
// <pat> => <body>,
// _ => break
// }
// }
let scrutinee = self.with_loop_condition_scope(|t| t.lower_expr(scrutinee));
let pat = self.lower_pat(pat);
(pat, scrutinee, hir::MatchSource::WhileLetDesugar, hir::LoopSource::WhileLet)
}
_ => {
// We desugar: `'label: while $cond $body` into:
//
// ```
// 'label: loop {
// match drop-temps { $cond } {
// true => $body,
// _ => break,
// }
// }
// ```

// Lower condition:
let cond = self.with_loop_condition_scope(|this| this.lower_expr(cond));
let span_block =
self.mark_span_with_reason(DesugaringKind::CondTemporary, cond.span, None);
// Wrap in a construct equivalent to `{ let _t = $cond; _t }`
// to preserve drop semantics since `while cond { ... }` does not
// let temporaries live outside of `cond`.
let cond = self.expr_drop_temps(span_block, cond, ThinVec::new());
// `true => <then>`:
let pat = self.pat_bool(span, true);
(pat, cond, hir::MatchSource::WhileDesugar, hir::LoopSource::While)
}
};
let then_expr = self.lower_block_expr(body);
let then_arm = self.arm(then_pat, self.arena.alloc(then_expr));

// `match <scrutinee> { ... }`
let match_expr =
self.expr_match(span, scrutinee, arena_vec![self; then_arm, else_arm], desugar);

// `[opt_ident]: loop { ... }`
hir::ExprKind::Loop(
self.block_expr(self.arena.alloc(match_expr)),
opt_label,
source,
span.with_hi(cond.span.hi()),
)
let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
let new_cond = self.manage_let_cond(lowered_cond);
let then = self.lower_block_expr(body);
let expr_break = self.expr_break(span, ThinVec::new());
let stmt_break = self.stmt_expr(span, expr_break);
let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
let else_expr = self.arena.alloc(self.expr_block(else_blk, ThinVec::new()));
let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
let if_expr = self.expr(span, if_kind, ThinVec::new());
let block = self.block_expr(self.arena.alloc(if_expr));
hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span.with_hi(cond.span.hi()))
}

/// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
Expand Down Expand Up @@ -609,7 +470,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
let pat = self.lower_pat(&arm.pat);
let guard = arm.guard.as_ref().map(|cond| {
if let ExprKind::Let(ref pat, ref scrutinee) = cond.kind {
if let ExprKind::Let(ref pat, ref scrutinee, _) = cond.kind {
hir::Guard::IfLet(self.lower_pat(pat), self.lower_expr(scrutinee))
} else {
hir::Guard::If(self.lower_expr(cond))
Expand Down Expand Up @@ -1457,7 +1318,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
// `::std::option::Option::None => break`
let break_arm = {
let break_expr =
self.with_loop_scope(e.id, |this| this.expr_break(e.span, ThinVec::new()));
self.with_loop_scope(e.id, |this| this.expr_break_alloc(e.span, ThinVec::new()));
let pat = self.pat_none(e.span);
self.arm(pat, break_expr)
};
Expand Down Expand Up @@ -1670,12 +1531,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
// Helper methods for building HIR.
// =========================================================================

/// Constructs a `true` or `false` literal expression.
pub(super) fn expr_bool(&mut self, span: Span, val: bool) -> &'hir hir::Expr<'hir> {
let lit = Spanned { span, node: LitKind::Bool(val) };
self.arena.alloc(self.expr(span, hir::ExprKind::Lit(lit), ThinVec::new()))
}

/// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
///
/// In terms of drop order, it has the same effect as wrapping `expr` in
Expand Down Expand Up @@ -1710,9 +1565,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
}

fn expr_break(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
self.arena.alloc(self.expr(span, expr_break, attrs))
self.expr(span, expr_break, attrs)
}

fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
let expr_break = self.expr_break(span, attrs);
self.arena.alloc(expr_break)
}

fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
Expand Down
10 changes: 0 additions & 10 deletions compiler/rustc_ast_lowering/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2537,12 +2537,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
self.arena.alloc(blk)
}

/// Constructs a `true` or `false` literal pattern.
fn pat_bool(&mut self, span: Span, val: bool) -> &'hir hir::Pat<'hir> {
let expr = self.expr_bool(span, val);
self.pat(span, hir::PatKind::Lit(expr))
}

fn pat_cf_continue(&mut self, span: Span, pat: &'hir hir::Pat<'hir>) -> &'hir hir::Pat<'hir> {
let field = self.single_pat_field(span, pat);
self.pat_lang_item_variant(span, hir::LangItem::ControlFlowContinue, field)
Expand Down Expand Up @@ -2624,10 +2618,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
)
}

fn pat_wild(&mut self, span: Span) -> &'hir hir::Pat<'hir> {
self.pat(span, hir::PatKind::Wild)
}

fn pat(&mut self, span: Span, kind: hir::PatKind<'hir>) -> &'hir hir::Pat<'hir> {
self.arena.alloc(hir::Pat {
hir_id: self.next_id(),
Expand Down
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