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Auto merge of rust-lang#87688 - camsteffen:let-else, r=cjgillot
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Introduce `let...else`

Tracking issue: rust-lang#87335

The trickiest part for me was enforcing the diverging else block with clear diagnostics. Perhaps the obvious solution is to expand to `let _: ! = ..`, but I decided against this because, when a "mismatched type" error is found in typeck, there is no way to trace where in the HIR the expected type originated, AFAICT. In order to pass down this information, I believe we should introduce `Expectation::LetElseNever(HirId)` or maybe add `HirId` to `Expectation::HasType`, but I left that as a future enhancement. For now, I simply assert that the block is `!` with a custom `ObligationCauseCode`, and I think this is clear enough, at least to start. The downside here is that the error points at the entire block rather than the specific expression with the wrong type. I left a todo to this effect.

Overall, I believe this PR is feature-complete with regard to the RFC.
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bors committed Sep 1, 2021
2 parents a395610 + 3ff1d6b commit c2a4088
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Showing 59 changed files with 901 additions and 232 deletions.
33 changes: 31 additions & 2 deletions compiler/rustc_ast/src/ast.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1005,13 +1005,42 @@ pub struct Local {
pub id: NodeId,
pub pat: P<Pat>,
pub ty: Option<P<Ty>>,
/// Initializer expression to set the value, if any.
pub init: Option<P<Expr>>,
pub kind: LocalKind,
pub span: Span,
pub attrs: AttrVec,
pub tokens: Option<LazyTokenStream>,
}

#[derive(Clone, Encodable, Decodable, Debug)]
pub enum LocalKind {
/// Local declaration.
/// Example: `let x;`
Decl,
/// Local declaration with an initializer.
/// Example: `let x = y;`
Init(P<Expr>),
/// Local declaration with an initializer and an `else` clause.
/// Example: `let Some(x) = y else { return };`
InitElse(P<Expr>, P<Block>),
}

impl LocalKind {
pub fn init(&self) -> Option<&Expr> {
match self {
Self::Decl => None,
Self::Init(i) | Self::InitElse(i, _) => Some(i),
}
}

pub fn init_else_opt(&self) -> Option<(&Expr, Option<&Block>)> {
match self {
Self::Decl => None,
Self::Init(init) => Some((init, None)),
Self::InitElse(init, els) => Some((init, Some(els))),
}
}
}

/// An arm of a 'match'.
///
/// E.g., `0..=10 => { println!("match!") }` as in
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13 changes: 11 additions & 2 deletions compiler/rustc_ast/src/mut_visit.rs
Original file line number Diff line number Diff line change
Expand Up @@ -571,11 +571,20 @@ pub fn noop_visit_parenthesized_parameter_data<T: MutVisitor>(
}

pub fn noop_visit_local<T: MutVisitor>(local: &mut P<Local>, vis: &mut T) {
let Local { id, pat, ty, init, span, attrs, tokens } = local.deref_mut();
let Local { id, pat, ty, kind, span, attrs, tokens } = local.deref_mut();
vis.visit_id(id);
vis.visit_pat(pat);
visit_opt(ty, |ty| vis.visit_ty(ty));
visit_opt(init, |init| vis.visit_expr(init));
match kind {
LocalKind::Decl => {}
LocalKind::Init(init) => {
vis.visit_expr(init);
}
LocalKind::InitElse(init, els) => {
vis.visit_expr(init);
vis.visit_block(els);
}
}
vis.visit_span(span);
visit_thin_attrs(attrs, vis);
visit_lazy_tts(tokens, vis);
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27 changes: 27 additions & 0 deletions compiler/rustc_ast/src/util/classify.rs
Original file line number Diff line number Diff line change
Expand Up @@ -23,3 +23,30 @@ pub fn expr_requires_semi_to_be_stmt(e: &ast::Expr) -> bool {
| ast::ExprKind::TryBlock(..)
)
}

/// If an expression ends with `}`, returns the innermost expression ending in the `}`
pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<&ast::Expr> {
use ast::ExprKind::*;

loop {
match &expr.kind {
AddrOf(_, _, e)
| Assign(_, e, _)
| AssignOp(_, _, e)
| Binary(_, _, e)
| Box(e)
| Break(_, Some(e))
| Closure(.., e, _)
| Let(_, e, _)
| Range(_, Some(e), _)
| Ret(Some(e))
| Unary(_, e)
| Yield(Some(e)) => {
expr = e;
}
Async(..) | Block(..) | ForLoop(..) | If(..) | Loop(..) | Match(..) | Struct(..)
| TryBlock(..) | While(..) => break Some(expr),
_ => break None,
}
}
}
5 changes: 4 additions & 1 deletion compiler/rustc_ast/src/visit.rs
Original file line number Diff line number Diff line change
Expand Up @@ -242,7 +242,10 @@ pub fn walk_local<'a, V: Visitor<'a>>(visitor: &mut V, local: &'a Local) {
}
visitor.visit_pat(&local.pat);
walk_list!(visitor, visit_ty, &local.ty);
walk_list!(visitor, visit_expr, &local.init);
if let Some((init, els)) = local.kind.init_else_opt() {
visitor.visit_expr(init);
walk_list!(visitor, visit_block, els);
}
}

pub fn walk_label<'a, V: Visitor<'a>>(visitor: &mut V, label: &'a Label) {
Expand Down
185 changes: 185 additions & 0 deletions compiler/rustc_ast_lowering/src/block.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,185 @@
use crate::{ImplTraitContext, ImplTraitPosition, LoweringContext};
use rustc_ast::{AttrVec, Block, BlockCheckMode, Expr, Local, LocalKind, Stmt, StmtKind};
use rustc_hir as hir;
use rustc_session::parse::feature_err;
use rustc_span::symbol::Ident;
use rustc_span::{sym, DesugaringKind};

use smallvec::SmallVec;

impl<'a, 'hir> LoweringContext<'a, 'hir> {
pub(super) fn lower_block(
&mut self,
b: &Block,
targeted_by_break: bool,
) -> &'hir hir::Block<'hir> {
self.arena.alloc(self.lower_block_noalloc(b, targeted_by_break))
}

pub(super) fn lower_block_noalloc(
&mut self,
b: &Block,
targeted_by_break: bool,
) -> hir::Block<'hir> {
let (stmts, expr) = self.lower_stmts(&b.stmts);
let rules = self.lower_block_check_mode(&b.rules);
let hir_id = self.lower_node_id(b.id);
hir::Block { hir_id, stmts, expr, rules, span: self.lower_span(b.span), targeted_by_break }
}

fn lower_stmts(
&mut self,
mut ast_stmts: &[Stmt],
) -> (&'hir [hir::Stmt<'hir>], Option<&'hir hir::Expr<'hir>>) {
let mut stmts = SmallVec::<[hir::Stmt<'hir>; 8]>::new();
let mut expr = None;
while let [s, tail @ ..] = ast_stmts {
match s.kind {
StmtKind::Local(ref local) => {
let hir_id = self.lower_node_id(s.id);
match &local.kind {
LocalKind::InitElse(init, els) => {
let (s, e) = self.lower_let_else(hir_id, local, init, els, tail);
stmts.push(s);
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 });
}
}
}
StmtKind::Item(ref it) => {
stmts.extend(self.lower_item_id(it).into_iter().enumerate().map(
|(i, item_id)| {
let hir_id = match i {
0 => self.lower_node_id(s.id),
_ => self.next_id(),
};
let kind = hir::StmtKind::Item(item_id);
let span = self.lower_span(s.span);
hir::Stmt { hir_id, kind, span }
},
));
}
StmtKind::Expr(ref e) => {
let e = self.lower_expr(e);
if tail.is_empty() {
expr = Some(e);
} else {
let hir_id = self.lower_node_id(s.id);
self.alias_attrs(hir_id, e.hir_id);
let kind = hir::StmtKind::Expr(e);
let span = self.lower_span(s.span);
stmts.push(hir::Stmt { hir_id, kind, span });
}
}
StmtKind::Semi(ref e) => {
let e = self.lower_expr(e);
let hir_id = self.lower_node_id(s.id);
self.alias_attrs(hir_id, e.hir_id);
let kind = hir::StmtKind::Semi(e);
let span = self.lower_span(s.span);
stmts.push(hir::Stmt { hir_id, kind, span });
}
StmtKind::Empty => {}
StmtKind::MacCall(..) => panic!("shouldn't exist here"),
}
ast_stmts = &ast_stmts[1..];
}
(self.arena.alloc_from_iter(stmts), expr)
}

fn lower_local(&mut self, l: &Local) -> &'hir hir::Local<'hir> {
let ty = l
.ty
.as_ref()
.map(|t| self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Binding)));
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 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 })
}

fn lower_block_check_mode(&mut self, b: &BlockCheckMode) -> hir::BlockCheckMode {
match *b {
BlockCheckMode::Default => hir::BlockCheckMode::DefaultBlock,
BlockCheckMode::Unsafe(u) => {
hir::BlockCheckMode::UnsafeBlock(self.lower_unsafe_source(u))
}
}
}

fn lower_let_else(
&mut self,
stmt_hir_id: hir::HirId,
local: &Local,
init: &Expr,
els: &Block,
tail: &[Stmt],
) -> (hir::Stmt<'hir>, &'hir hir::Expr<'hir>) {
let ty = local
.ty
.as_ref()
.map(|t| self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Binding)));
let span = self.lower_span(local.span);
let span = self.mark_span_with_reason(DesugaringKind::LetElse, span, None);
let init = Some(self.lower_expr(init));
let val = Ident::with_dummy_span(sym::val);
let (pat, val_id) =
self.pat_ident_binding_mode(span, val, hir::BindingAnnotation::Unannotated);
let local_hir_id = self.lower_node_id(local.id);
self.lower_attrs(local_hir_id, &local.attrs);
// first statement which basically exists for the type annotation
let stmt = {
let local = self.arena.alloc(hir::Local {
hir_id: local_hir_id,
ty,
pat,
init,
span,
source: hir::LocalSource::Normal,
});
let kind = hir::StmtKind::Local(local);
hir::Stmt { hir_id: stmt_hir_id, kind, span }
};
let let_expr = {
let scrutinee = self.expr_ident(span, val, val_id);
let let_kind = hir::ExprKind::Let(self.lower_pat(&local.pat), scrutinee, span);
self.arena.alloc(self.expr(span, let_kind, 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(else_expr.hir_id, local_hir_id);
let if_expr = self.arena.alloc(hir::Expr {
hir_id: self.next_id(),
span,
kind: hir::ExprKind::If(let_expr, then_expr, Some(else_expr)),
});
if !self.sess.features_untracked().let_else {
feature_err(
&self.sess.parse_sess,
sym::let_else,
local.span,
"`let...else` statements are unstable",
)
.emit();
}
(stmt, if_expr)
}
}
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