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Rollup merge of rust-lang#98574 - dingxiangfei2009:let-else-thir, r=o…
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…li-obk

Lower let-else in MIR

This MR will switch to lower let-else statements in MIR building instead.

To lower let-else in MIR, we build a mini-switch two branches. One branch leads to the matching case, and the other leads to the `else` block. This arrangement will allow temporary lifetime analysis running as-is so that the temporaries are properly extended according to the same rule applied to regular `let` statements.

cc rust-lang#87335

Fix rust-lang#98672
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Dylan-DPC authored Jul 13, 2022
2 parents 7b57152 + 947cbda commit 6fe82f2
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Showing 33 changed files with 576 additions and 265 deletions.
95 changes: 22 additions & 73 deletions compiler/rustc_ast_lowering/src/block.rs
Original file line number Diff line number Diff line change
@@ -1,8 +1,8 @@
use crate::{ImplTraitContext, ImplTraitPosition, LoweringContext};
use rustc_ast::{AttrVec, Block, BlockCheckMode, Expr, Local, LocalKind, Stmt, StmtKind};
use rustc_ast::{Block, BlockCheckMode, Local, LocalKind, Stmt, StmtKind};
use rustc_hir as hir;
use rustc_session::parse::feature_err;
use rustc_span::{sym, DesugaringKind};
use rustc_span::sym;

use smallvec::SmallVec;

Expand Down Expand Up @@ -36,21 +36,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
match s.kind {
StmtKind::Local(ref local) => {
let hir_id = self.lower_node_id(s.id);
match &local.kind {
LocalKind::InitElse(init, els) => {
let e = self.lower_let_else(hir_id, local, init, els, tail);
expr = Some(e);
// remaining statements are in let-else expression
break;
}
_ => {
let local = self.lower_local(local);
self.alias_attrs(hir_id, local.hir_id);
let kind = hir::StmtKind::Local(local);
let span = self.lower_span(s.span);
stmts.push(hir::Stmt { hir_id, kind, span });
}
}
let local = self.lower_local(local);
self.alias_attrs(hir_id, local.hir_id);
let kind = hir::StmtKind::Local(local);
let span = self.lower_span(s.span);
stmts.push(hir::Stmt { hir_id, kind, span });
}
StmtKind::Item(ref it) => {
stmts.extend(self.lower_item_ref(it).into_iter().enumerate().map(
Expand Down Expand Up @@ -101,10 +91,24 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
let init = l.kind.init().map(|init| self.lower_expr(init));
let hir_id = self.lower_node_id(l.id);
let pat = self.lower_pat(&l.pat);
let els = if let LocalKind::InitElse(_, els) = &l.kind {
if !self.tcx.features().let_else {
feature_err(
&self.tcx.sess.parse_sess,
sym::let_else,
l.span,
"`let...else` statements are unstable",
)
.emit();
}
Some(self.lower_block(els, false))
} else {
None
};
let span = self.lower_span(l.span);
let source = hir::LocalSource::Normal;
self.lower_attrs(hir_id, &l.attrs);
self.arena.alloc(hir::Local { hir_id, ty, pat, init, span, source })
self.arena.alloc(hir::Local { hir_id, ty, pat, init, els, span, source })
}

fn lower_block_check_mode(&mut self, b: &BlockCheckMode) -> hir::BlockCheckMode {
Expand All @@ -115,59 +119,4 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
}
}
}

fn lower_let_else(
&mut self,
stmt_hir_id: hir::HirId,
local: &Local,
init: &Expr,
els: &Block,
tail: &[Stmt],
) -> &'hir hir::Expr<'hir> {
let ty = local
.ty
.as_ref()
.map(|t| self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Variable)));
let span = self.lower_span(local.span);
let span = self.mark_span_with_reason(DesugaringKind::LetElse, span, None);
let init = self.lower_expr(init);
let local_hir_id = self.lower_node_id(local.id);
self.lower_attrs(local_hir_id, &local.attrs);
let let_expr = {
let lex = self.arena.alloc(hir::Let {
hir_id: local_hir_id,
pat: self.lower_pat(&local.pat),
ty,
init,
span,
});
self.arena.alloc(self.expr(span, hir::ExprKind::Let(lex), AttrVec::new()))
};
let then_expr = {
let (stmts, expr) = self.lower_stmts(tail);
let block = self.block_all(span, stmts, expr);
self.arena.alloc(self.expr_block(block, AttrVec::new()))
};
let else_expr = {
let block = self.lower_block(els, false);
self.arena.alloc(self.expr_block(block, AttrVec::new()))
};
self.alias_attrs(let_expr.hir_id, local_hir_id);
self.alias_attrs(else_expr.hir_id, local_hir_id);
let if_expr = self.arena.alloc(hir::Expr {
hir_id: stmt_hir_id,
span,
kind: hir::ExprKind::If(let_expr, then_expr, Some(else_expr)),
});
if !self.tcx.features().let_else {
feature_err(
&self.tcx.sess.parse_sess,
sym::let_else,
local.span,
"`let...else` statements are unstable",
)
.emit();
}
if_expr
}
}
10 changes: 9 additions & 1 deletion compiler/rustc_ast_lowering/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -2146,7 +2146,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
debug_assert!(!a.is_empty());
self.attrs.insert(hir_id.local_id, a);
}
let local = hir::Local { hir_id, init, pat, source, span: self.lower_span(span), ty: None };
let local = hir::Local {
hir_id,
init,
pat,
els: None,
source,
span: self.lower_span(span),
ty: None,
};
self.stmt(span, hir::StmtKind::Local(self.arena.alloc(local)))
}

Expand Down
2 changes: 2 additions & 0 deletions compiler/rustc_hir/src/hir.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1316,6 +1316,8 @@ pub struct Local<'hir> {
pub ty: Option<&'hir Ty<'hir>>,
/// Initializer expression to set the value, if any.
pub init: Option<&'hir Expr<'hir>>,
/// Else block for a `let...else` binding.
pub els: Option<&'hir Block<'hir>>,
pub hir_id: HirId,
pub span: Span,
/// Can be `ForLoopDesugar` if the `let` statement is part of a `for` loop
Expand Down
3 changes: 3 additions & 0 deletions compiler/rustc_hir/src/intravisit.rs
Original file line number Diff line number Diff line change
Expand Up @@ -472,6 +472,9 @@ pub fn walk_local<'v, V: Visitor<'v>>(visitor: &mut V, local: &'v Local<'v>) {
walk_list!(visitor, visit_expr, &local.init);
visitor.visit_id(local.hir_id);
visitor.visit_pat(&local.pat);
if let Some(els) = local.els {
visitor.visit_block(els);
}
walk_list!(visitor, visit_ty, &local.ty);
}

Expand Down
18 changes: 15 additions & 3 deletions compiler/rustc_hir_pretty/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -883,7 +883,12 @@ impl<'a> State<'a> {
self.ann.post(self, AnnNode::SubItem(ii.hir_id()))
}

pub fn print_local(&mut self, init: Option<&hir::Expr<'_>>, decl: impl Fn(&mut Self)) {
pub fn print_local(
&mut self,
init: Option<&hir::Expr<'_>>,
els: Option<&hir::Block<'_>>,
decl: impl Fn(&mut Self),
) {
self.space_if_not_bol();
self.ibox(INDENT_UNIT);
self.word_nbsp("let");
Expand All @@ -897,14 +902,21 @@ impl<'a> State<'a> {
self.word_space("=");
self.print_expr(init);
}

if let Some(els) = els {
self.nbsp();
self.word_space("else");
self.print_block(els);
}

self.end()
}

pub fn print_stmt(&mut self, st: &hir::Stmt<'_>) {
self.maybe_print_comment(st.span.lo());
match st.kind {
hir::StmtKind::Local(loc) => {
self.print_local(loc.init, |this| this.print_local_decl(loc));
self.print_local(loc.init, loc.els, |this| this.print_local_decl(loc));
}
hir::StmtKind::Item(item) => self.ann.nested(self, Nested::Item(item)),
hir::StmtKind::Expr(expr) => {
Expand Down Expand Up @@ -1404,7 +1416,7 @@ impl<'a> State<'a> {

// Print `let _t = $init;`:
let temp = Ident::from_str("_t");
self.print_local(Some(init), |this| this.print_ident(temp));
self.print_local(Some(init), None, |this| this.print_ident(temp));
self.word(";");

// Print `_t`:
Expand Down
3 changes: 3 additions & 0 deletions compiler/rustc_middle/src/thir.rs
Original file line number Diff line number Diff line change
Expand Up @@ -182,6 +182,9 @@ pub enum StmtKind<'tcx> {
/// `let pat: ty = <INIT>`
initializer: Option<ExprId>,

/// `let pat: ty = <INIT> else { <ELSE> }
else_block: Option<Block>,

/// The lint level for this `let` statement.
lint_level: LintLevel,
},
Expand Down
4 changes: 4 additions & 0 deletions compiler/rustc_middle/src/thir/visit.rs
Original file line number Diff line number Diff line change
Expand Up @@ -167,11 +167,15 @@ pub fn walk_stmt<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, stmt: &Stm
init_scope: _,
ref pattern,
lint_level: _,
else_block,
} => {
if let Some(init) = initializer {
visitor.visit_expr(&visitor.thir()[*init]);
}
visitor.visit_pat(pattern);
if let Some(block) = else_block {
visitor.visit_block(block)
}
}
}
}
Expand Down
33 changes: 23 additions & 10 deletions compiler/rustc_mir_build/src/build/block.rs
Original file line number Diff line number Diff line change
Expand Up @@ -99,6 +99,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
ref pattern,
initializer,
lint_level,
else_block,
} => {
let ignores_expr_result = matches!(*pattern.kind, PatKind::Wild);
this.block_context.push(BlockFrame::Statement { ignores_expr_result });
Expand All @@ -124,18 +125,30 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
|this| {
let scope = (*init_scope, source_info);
this.in_scope(scope, *lint_level, |this| {
this.declare_bindings(
visibility_scope,
remainder_span,
pattern,
ArmHasGuard(false),
Some((None, initializer_span)),
);
this.expr_into_pattern(block, pattern.clone(), init)
if let Some(else_block) = else_block {
this.ast_let_else(
block,
init,
initializer_span,
else_block,
visibility_scope,
remainder_span,
pattern,
)
} else {
this.declare_bindings(
visibility_scope,
remainder_span,
pattern,
ArmHasGuard(false),
Some((None, initializer_span)),
);
this.expr_into_pattern(block, pattern.clone(), init) // irrefutable pattern
}
})
}
},
)
);
)
} else {
let scope = (*init_scope, source_info);
unpack!(this.in_scope(scope, *lint_level, |this| {
Expand Down
77 changes: 74 additions & 3 deletions compiler/rustc_mir_build/src/build/matches/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1615,7 +1615,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
// those N possible outcomes, create a (initially empty)
// vector of candidates. Those are the candidates that still
// apply if the test has that particular outcome.
debug!("match_candidates: test={:?} match_pair={:?}", test, match_pair);
debug!("test_candidates: test={:?} match_pair={:?}", test, match_pair);
let mut target_candidates: Vec<Vec<&mut Candidate<'pat, 'tcx>>> = vec![];
target_candidates.resize_with(test.targets(), Default::default);

Expand All @@ -1635,8 +1635,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
}
// at least the first candidate ought to be tested
assert!(total_candidate_count > candidates.len());
debug!("tested_candidates: {}", total_candidate_count - candidates.len());
debug!("untested_candidates: {}", candidates.len());
debug!("test_candidates: tested_candidates: {}", total_candidate_count - candidates.len());
debug!("test_candidates: untested_candidates: {}", candidates.len());

// HACK(matthewjasper) This is a closure so that we can let the test
// create its blocks before the rest of the match. This currently
Expand Down Expand Up @@ -2274,4 +2274,75 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
debug!("declare_binding: vars={:?}", locals);
self.var_indices.insert(var_id, locals);
}

pub(crate) fn ast_let_else(
&mut self,
mut block: BasicBlock,
init: &Expr<'tcx>,
initializer_span: Span,
else_block: &Block,
visibility_scope: Option<SourceScope>,
remainder_span: Span,
pattern: &Pat<'tcx>,
) -> BlockAnd<()> {
let scrutinee = unpack!(block = self.lower_scrutinee(block, init, initializer_span));
let pat = Pat { ty: init.ty, span: else_block.span, kind: Box::new(PatKind::Wild) };
let mut wildcard = Candidate::new(scrutinee.clone(), &pat, false);
self.declare_bindings(
visibility_scope,
remainder_span,
pattern,
ArmHasGuard(false),
Some((None, initializer_span)),
);
let mut candidate = Candidate::new(scrutinee.clone(), pattern, false);
let fake_borrow_temps = self.lower_match_tree(
block,
initializer_span,
pattern.span,
false,
&mut [&mut candidate, &mut wildcard],
);
// This block is for the matching case
let matching = self.bind_pattern(
self.source_info(pattern.span),
candidate,
None,
&fake_borrow_temps,
initializer_span,
None,
None,
None,
);
// This block is for the failure case
let failure = self.bind_pattern(
self.source_info(else_block.span),
wildcard,
None,
&fake_borrow_temps,
initializer_span,
None,
None,
None,
);
// This place is not really used because this destination place
// should never be used to take values at the end of the failure
// block.
let dummy_place = Place { local: RETURN_PLACE, projection: ty::List::empty() };
let failure_block;
unpack!(
failure_block = self.ast_block(
dummy_place,
failure,
else_block,
self.source_info(else_block.span),
)
);
self.cfg.terminate(
failure_block,
self.source_info(else_block.span),
TerminatorKind::Unreachable,
);
matching.unit()
}
}
3 changes: 3 additions & 0 deletions compiler/rustc_mir_build/src/thir/cx/block.rs
Original file line number Diff line number Diff line change
Expand Up @@ -74,6 +74,8 @@ impl<'tcx> Cx<'tcx> {
)),
};

let else_block = local.els.map(|els| self.mirror_block(els));

let mut pattern = self.pattern_from_hir(local.pat);
debug!(?pattern);

Expand Down Expand Up @@ -110,6 +112,7 @@ impl<'tcx> Cx<'tcx> {
},
pattern,
initializer: local.init.map(|init| self.mirror_expr(init)),
else_block,
lint_level: LintLevel::Explicit(local.hir_id),
},
opt_destruction_scope: opt_dxn_ext,
Expand Down
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