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…chenkov

[let_chains, 2/6] Introduce `Let(..)` in AST, remove IfLet + WhileLet and parse let chains

Here we remove `ast::ExprKind::{IfLet, WhileLet}` and introduce `ast::ExprKind::Let`.
Moreover, we also:
+ connect the parsing logic for let chains
+ introduce the feature gate
+ rewire HIR lowering a bit.

However, this does not connect the new syntax to semantics in HIR.
That will be the subject of a subsequent PR.

Per rust-lang#53667 (comment).
Next step after rust-lang#59288.

cc @Manishearth re. Clippy.

r? @oli-obk
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bors committed Jun 23, 2019
2 parents a96ba96 + c75f7ec commit 5d677b2
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280 changes: 140 additions & 140 deletions src/librustc/hir/lowering.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -4344,53 +4344,147 @@ impl<'a> LoweringContext<'a> {
let ohs = P(self.lower_expr(ohs));
hir::ExprKind::AddrOf(m, ohs)
}
// More complicated than you might expect because the else branch
// might be `if let`.
ExprKind::Let(ref pats, ref scrutinee) => {
// If we got here, the `let` expression is not allowed.
self.sess
.struct_span_err(e.span, "`let` expressions are not supported here")
.note("only supported directly in conditions of `if`- and `while`-expressions")
.note("as well as when nested within `&&` and parenthesis in those conditions")
.emit();

// For better recovery, we emit:
// ```
// match scrutinee { pats => 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 `pats` and `scrutinee`.
// 3. `pats` 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 pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
let expr = self.expr_bool(e.span, true);
self.arm(pats, P(expr))
};
let else_arm = {
let pats = hir_vec![self.pat_wild(e.span)];
let expr = self.expr_bool(e.span, false);
self.arm(pats, P(expr))
};
hir::ExprKind::Match(
P(scrutinee),
vec![then_arm, else_arm].into(),
hir::MatchSource::Normal,
)
}
// FIXME(#53667): handle lowering of && and parens.
ExprKind::If(ref cond, ref then, ref else_opt) => {
// `true => then`:
let then_pat = self.pat_bool(e.span, true);
let then_blk = self.lower_block(then, false);
let then_expr = self.expr_block(then_blk, ThinVec::new());
let then_arm = self.arm(hir_vec![then_pat], P(then_expr));

// `_ => else_block` where `else_block` is `{}` if there's `None`:
let else_pat = self.pat_wild(e.span);
let else_expr = match else_opt {
None => self.expr_block_empty(e.span),
Some(els) => match els.node {
ExprKind::IfLet(..) => {
// Wrap the `if let` expr in a block.
let els = self.lower_expr(els);
let blk = self.block_all(els.span, hir_vec![], Some(P(els)));
self.expr_block(P(blk), ThinVec::new())
}
_ => self.lower_expr(els),
}
let (else_expr, contains_else_clause) = match else_opt {
None => (self.expr_block_empty(e.span), false),
Some(els) => (self.lower_expr(els), true),
};
let else_arm = self.arm(hir_vec![else_pat], P(else_expr));

// Lower condition:
let span_block = self.mark_span_with_reason(IfTemporary, cond.span, None);
let cond = self.lower_expr(cond);
// Wrap in a construct equivalent to `{ let _t = $cond; _t }` to preserve drop
// semantics since `if cond { ... }` don't let temporaries live outside of `cond`.
let cond = self.expr_drop_temps(span_block, P(cond), ThinVec::new());
// Handle then + scrutinee:
let then_blk = self.lower_block(then, false);
let then_expr = self.expr_block(then_blk, ThinVec::new());
let (then_pats, scrutinee, desugar) = match cond.node {
// `<pat> => <then>`
ExprKind::Let(ref pats, ref scrutinee) => {
let scrutinee = self.lower_expr(scrutinee);
let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
let desugar = hir::MatchSource::IfLetDesugar { contains_else_clause };
(pats, scrutinee, desugar)
}
// `true => then`:
_ => {
// Lower condition:
let cond = self.lower_expr(cond);
// Wrap in a construct equivalent to `{ let _t = $cond; _t }`
// to preserve drop semantics since `if cond { ... }`
// don't let temporaries live outside of `cond`.
let span_block = self.mark_span_with_reason(IfTemporary, cond.span, None);
// Wrap in a construct equivalent to `{ let _t = $cond; _t }`
// to preserve drop semantics since `if cond { ... }` does not
// let temporaries live outside of `cond`.
let cond = self.expr_drop_temps(span_block, P(cond), ThinVec::new());

let desugar = hir::MatchSource::IfDesugar { contains_else_clause };
let pats = hir_vec![self.pat_bool(e.span, true)];
(pats, cond, desugar)
}
};
let then_arm = self.arm(then_pats, P(then_expr));

hir::ExprKind::Match(
P(cond),
vec![then_arm, else_arm].into(),
hir::MatchSource::IfDesugar {
contains_else_clause: else_opt.is_some()
},
)
hir::ExprKind::Match(P(scrutinee), vec![then_arm, else_arm].into(), desugar)
}
// FIXME(#53667): handle lowering of && and parens.
ExprKind::While(ref cond, ref body, opt_label) => {
// Desugar `ExprWhileLet`
// from: `[opt_ident]: while let <pat> = <sub_expr> <body>`
if let ExprKind::Let(ref pats, ref sub_expr) = cond.node {
// to:
//
// [opt_ident]: loop {
// match <sub_expr> {
// <pat> => <body>,
// _ => break
// }
// }

// 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.
let (body, break_expr, sub_expr) = self.with_loop_scope(e.id, |this| {
(
this.lower_block(body, false),
this.expr_break(e.span, ThinVec::new()),
this.with_loop_condition_scope(|this| P(this.lower_expr(sub_expr))),
)
});

// `<pat> => <body>`
let pat_arm = {
let body_expr = P(self.expr_block(body, ThinVec::new()));
let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
self.arm(pats, body_expr)
};

// `_ => break`
let break_arm = {
let pat_under = self.pat_wild(e.span);
self.arm(hir_vec![pat_under], break_expr)
};

// `match <sub_expr> { ... }`
let arms = hir_vec![pat_arm, break_arm];
let match_expr = self.expr(
sub_expr.span,
hir::ExprKind::Match(sub_expr, arms, hir::MatchSource::WhileLetDesugar),
ThinVec::new(),
);

// `[opt_ident]: loop { ... }`
let loop_block = P(self.block_expr(P(match_expr)));
let loop_expr = hir::ExprKind::Loop(
loop_block,
self.lower_label(opt_label),
hir::LoopSource::WhileLet,
);
// Add attributes to the outer returned expr node.
loop_expr
} else {
self.with_loop_scope(e.id, |this| {
hir::ExprKind::While(
this.with_loop_condition_scope(|this| P(this.lower_expr(cond))),
this.lower_block(body, false),
this.lower_label(opt_label),
)
})
}
}
ExprKind::While(ref cond, ref body, opt_label) => self.with_loop_scope(e.id, |this| {
hir::ExprKind::While(
this.with_loop_condition_scope(|this| P(this.lower_expr(cond))),
this.lower_block(body, false),
this.lower_label(opt_label),
)
}),
ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
hir::ExprKind::Loop(
this.lower_block(body, false),
Expand Down Expand Up @@ -4703,105 +4797,6 @@ impl<'a> LoweringContext<'a> {

ExprKind::Err => hir::ExprKind::Err,

// Desugar `ExprIfLet`
// from: `if let <pat> = <sub_expr> <body> [<else_opt>]`
ExprKind::IfLet(ref pats, ref sub_expr, ref body, ref else_opt) => {
// to:
//
// match <sub_expr> {
// <pat> => <body>,
// _ => [<else_opt> | ()]
// }

let mut arms = vec![];

// `<pat> => <body>`
{
let body = self.lower_block(body, false);
let body_expr = P(self.expr_block(body, ThinVec::new()));
let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
arms.push(self.arm(pats, body_expr));
}

// _ => [<else_opt>|{}]
{
let wildcard_arm: Option<&Expr> = else_opt.as_ref().map(|p| &**p);
let wildcard_pattern = self.pat_wild(e.span);
let body = if let Some(else_expr) = wildcard_arm {
self.lower_expr(else_expr)
} else {
self.expr_block_empty(e.span)
};
arms.push(self.arm(hir_vec![wildcard_pattern], P(body)));
}

let contains_else_clause = else_opt.is_some();

let sub_expr = P(self.lower_expr(sub_expr));

hir::ExprKind::Match(
sub_expr,
arms.into(),
hir::MatchSource::IfLetDesugar {
contains_else_clause,
},
)
}

// Desugar `ExprWhileLet`
// from: `[opt_ident]: while let <pat> = <sub_expr> <body>`
ExprKind::WhileLet(ref pats, ref sub_expr, ref body, opt_label) => {
// to:
//
// [opt_ident]: loop {
// match <sub_expr> {
// <pat> => <body>,
// _ => break
// }
// }

// 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.
let (body, break_expr, sub_expr) = self.with_loop_scope(e.id, |this| {
(
this.lower_block(body, false),
this.expr_break(e.span, ThinVec::new()),
this.with_loop_condition_scope(|this| P(this.lower_expr(sub_expr))),
)
});

// `<pat> => <body>`
let pat_arm = {
let body_expr = P(self.expr_block(body, ThinVec::new()));
let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
self.arm(pats, body_expr)
};

// `_ => break`
let break_arm = {
let pat_under = self.pat_wild(e.span);
self.arm(hir_vec![pat_under], break_expr)
};

// `match <sub_expr> { ... }`
let arms = hir_vec![pat_arm, break_arm];
let match_expr = self.expr(
sub_expr.span,
hir::ExprKind::Match(sub_expr, arms, hir::MatchSource::WhileLetDesugar),
ThinVec::new(),
);

// `[opt_ident]: loop { ... }`
let loop_block = P(self.block_expr(P(match_expr)));
let loop_expr = hir::ExprKind::Loop(
loop_block,
self.lower_label(opt_label),
hir::LoopSource::WhileLet,
);
// Add attributes to the outer returned expr node.
loop_expr
}

// Desugar `ExprForLoop`
// from: `[opt_ident]: for <pat> in <head> <body>`
ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
Expand Down Expand Up @@ -5463,10 +5458,15 @@ impl<'a> LoweringContext<'a> {
)
}

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

/// Constructs a `true` or `false` literal pattern.
fn pat_bool(&mut self, span: Span, val: bool) -> P<hir::Pat> {
let lit = Spanned { span, node: LitKind::Bool(val) };
let expr = self.expr(span, hir::ExprKind::Lit(lit), ThinVec::new());
let expr = self.expr_bool(span, val);
self.pat(span, hir::PatKind::Lit(P(expr)))
}

Expand Down
36 changes: 21 additions & 15 deletions src/librustc_lint/unused.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -324,20 +324,28 @@ impl UnusedParens {
value: &ast::Expr,
msg: &str,
followed_by_block: bool) {
if let ast::ExprKind::Paren(ref inner) = value.node {
let necessary = followed_by_block && match inner.node {
ast::ExprKind::Ret(_) | ast::ExprKind::Break(..) => true,
_ => parser::contains_exterior_struct_lit(&inner),
};
if !necessary {
let expr_text = if let Ok(snippet) = cx.sess().source_map()
.span_to_snippet(value.span) {
snippet
} else {
pprust::expr_to_string(value)
};
Self::remove_outer_parens(cx, value.span, &expr_text, msg);
match value.node {
ast::ExprKind::Paren(ref inner) => {
let necessary = followed_by_block && match inner.node {
ast::ExprKind::Ret(_) | ast::ExprKind::Break(..) => true,
_ => parser::contains_exterior_struct_lit(&inner),
};
if !necessary {
let expr_text = if let Ok(snippet) = cx.sess().source_map()
.span_to_snippet(value.span) {
snippet
} else {
pprust::expr_to_string(value)
};
Self::remove_outer_parens(cx, value.span, &expr_text, msg);
}
}
ast::ExprKind::Let(_, ref expr) => {
// FIXME(#60336): Properly handle `let true = (false && true)`
// actually needing the parenthesis.
self.check_unused_parens_expr(cx, expr, "`let` head expression", followed_by_block);
}
_ => {}
}
}

Expand Down Expand Up @@ -399,8 +407,6 @@ impl EarlyLintPass for UnusedParens {
let (value, msg, followed_by_block) = match e.node {
If(ref cond, ..) => (cond, "`if` condition", true),
While(ref cond, ..) => (cond, "`while` condition", true),
IfLet(_, ref cond, ..) => (cond, "`if let` head expression", true),
WhileLet(_, ref cond, ..) => (cond, "`while let` head expression", true),
ForLoop(_, ref cond, ..) => (cond, "`for` head expression", true),
Match(ref head, _) => (head, "`match` head expression", true),
Ret(Some(ref value)) => (value, "`return` value", false),
Expand Down
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