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Auto merge of #87688 - camsteffen:let-else, r=cjgillot

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Introduce `let...else`

Tracking issue: #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.
This commit is contained in:
bors 2021-09-01 01:02:42 +00:00
commit c2a408840a
59 changed files with 901 additions and 232 deletions
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185
compiler/rustc_ast_lowering/src/block.rs Normal file
View file

@ -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)
}
}

97
compiler/rustc_ast_lowering/src/lib.rs
View file

@ -64,7 +64,7 @@ use rustc_span::symbol::{kw, sym, Ident, Symbol};
use rustc_span::{Span, DUMMY_SP};
use rustc_target::spec::abi::Abi;
use smallvec::{smallvec, SmallVec};
use smallvec::SmallVec;
use std::collections::BTreeMap;
use std::mem;
use tracing::{debug, trace};
@ -77,6 +77,7 @@ macro_rules! arena_vec {
}
mod asm;
mod block;
mod expr;
mod item;
mod pat;
@ -1793,24 +1794,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
)
}
fn lower_local(&mut self, l: &Local) -> hir::Local<'hir> {
let ty = l
.ty
.as_ref()
.map(|t| self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Binding)));
let init = l.init.as_ref().map(|e| self.lower_expr(e));
let hir_id = self.lower_node_id(l.id);
self.lower_attrs(hir_id, &l.attrs);
hir::Local {
hir_id,
ty,
pat: self.lower_pat(&l.pat),
init,
span: self.lower_span(l.span),
source: hir::LocalSource::Normal,
}
}
fn lower_fn_params_to_names(&mut self, decl: &FnDecl) -> &'hir [Ident] {
// Skip the `...` (`CVarArgs`) trailing arguments from the AST,
// as they are not explicit in HIR/Ty function signatures.
@ -2396,23 +2379,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
bounds.iter().map(move |bound| self.lower_param_bound(bound, itctx.reborrow()))
}
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))
}
fn lower_block_noalloc(&mut self, b: &Block, targeted_by_break: bool) -> hir::Block<'hir> {
let (stmts, expr) = match &*b.stmts {
[stmts @ .., Stmt { kind: StmtKind::Expr(e), .. }] => (stmts, Some(&*e)),
stmts => (stmts, None),
};
let stmts = self.arena.alloc_from_iter(stmts.iter().flat_map(|stmt| self.lower_stmt(stmt)));
let expr = expr.map(|e| self.lower_expr(e));
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 }
}
/// Lowers a block directly to an expression, presuming that it
/// has no attributes and is not targeted by a `break`.
fn lower_block_expr(&mut self, b: &Block) -> hir::Expr<'hir> {
@ -2427,65 +2393,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
})
}
fn lower_stmt(&mut self, s: &Stmt) -> SmallVec<[hir::Stmt<'hir>; 1]> {
let (hir_id, kind) = match s.kind {
StmtKind::Local(ref l) => {
let l = self.lower_local(l);
let hir_id = self.lower_node_id(s.id);
self.alias_attrs(hir_id, l.hir_id);
return smallvec![hir::Stmt {
hir_id,
kind: hir::StmtKind::Local(self.arena.alloc(l)),
span: self.lower_span(s.span),
}];
}
StmtKind::Item(ref it) => {
// Can only use the ID once.
let mut id = Some(s.id);
return self
.lower_item_id(it)
.into_iter()
.map(|item_id| {
let hir_id = id
.take()
.map(|id| self.lower_node_id(id))
.unwrap_or_else(|| self.next_id());
hir::Stmt {
hir_id,
kind: hir::StmtKind::Item(item_id),
span: self.lower_span(s.span),
}
})
.collect();
}
StmtKind::Expr(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);
(hir_id, hir::StmtKind::Expr(e))
}
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);
(hir_id, hir::StmtKind::Semi(e))
}
StmtKind::Empty => return smallvec![],
StmtKind::MacCall(..) => panic!("shouldn't exist here"),
};
smallvec![hir::Stmt { hir_id, kind, span: self.lower_span(s.span) }]
}
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_unsafe_source(&mut self, u: UnsafeSource) -> hir::UnsafeSource {
match u {
CompilerGenerated => hir::UnsafeSource::CompilerGenerated,