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…=oli-obk

Implement syntax for `impl Trait` to specify its captures explicitly (`feature(precise_capturing)`)

Implements `impl use<'a, 'b, T, U> Sized` syntax that allows users to explicitly list the captured parameters for an opaque, rather than inferring it from the opaque's bounds (or capturing *all* lifetimes under 2024-edition capture rules). This allows us to exclude some implicit captures, so this syntax may be used as a migration strategy for changes due to rust-lang#117587.

We represent this list of captured params as `PreciseCapturingArg` in AST and HIR, resolving them between `rustc_resolve` and `resolve_bound_vars`. Later on, we validate that the opaques only capture the parameters in this list.

We artificially limit the feature to *require* mentioning all type and const parameters, since we don't currently have support for non-lifetime bivariant generics. This can be relaxed in the future.

We also may need to limit this to require naming *all* lifetime parameters for RPITIT, since GATs have no variance. I have to investigate this. This can also be relaxed in the future.

r? `@oli-obk`

Tracking issue:

- rust-lang#123432
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bors committed Apr 16, 2024
2 parents 63f70b3 + 8ddd280 commit c002c0a
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Showing 55 changed files with 1,033 additions and 71 deletions.
12 changes: 10 additions & 2 deletions compiler/rustc_ast/src/ast.rs
Original file line number Diff line number Diff line change
Expand Up @@ -63,7 +63,7 @@ impl fmt::Debug for Label {

/// A "Lifetime" is an annotation of the scope in which variable
/// can be used, e.g. `'a` in `&'a i32`.
#[derive(Clone, Encodable, Decodable, Copy, PartialEq, Eq)]
#[derive(Clone, Encodable, Decodable, Copy, PartialEq, Eq, Hash)]
pub struct Lifetime {
pub id: NodeId,
pub ident: Ident,
Expand Down Expand Up @@ -2132,7 +2132,7 @@ pub enum TyKind {
/// The `NodeId` exists to prevent lowering from having to
/// generate `NodeId`s on the fly, which would complicate
/// the generation of opaque `type Foo = impl Trait` items significantly.
ImplTrait(NodeId, GenericBounds),
ImplTrait(NodeId, GenericBounds, Option<P<(ThinVec<PreciseCapturingArg>, Span)>>),
/// No-op; kept solely so that we can pretty-print faithfully.
Paren(P<Ty>),
/// Unused for now.
Expand Down Expand Up @@ -2188,6 +2188,14 @@ pub enum TraitObjectSyntax {
None,
}

#[derive(Clone, Encodable, Decodable, Debug)]
pub enum PreciseCapturingArg {
/// Lifetime parameter
Lifetime(Lifetime),
/// Type or const parameter
Arg(Path, NodeId),
}

/// Inline assembly operand explicit register or register class.
///
/// E.g., `"eax"` as in `asm!("mov eax, 2", out("eax") result)`.
Expand Down
23 changes: 22 additions & 1 deletion compiler/rustc_ast/src/mut_visit.rs
Original file line number Diff line number Diff line change
Expand Up @@ -259,6 +259,10 @@ pub trait MutVisitor: Sized {
noop_visit_param_bound(tpb, self);
}

fn visit_precise_capturing_arg(&mut self, arg: &mut PreciseCapturingArg) {
noop_visit_precise_capturing_arg(arg, self);
}

fn visit_mt(&mut self, mt: &mut MutTy) {
noop_visit_mt(mt, self);
}
Expand Down Expand Up @@ -518,9 +522,14 @@ pub fn noop_visit_ty<T: MutVisitor>(ty: &mut P<Ty>, vis: &mut T) {
TyKind::TraitObject(bounds, _syntax) => {
visit_vec(bounds, |bound| vis.visit_param_bound(bound))
}
TyKind::ImplTrait(id, bounds) => {
TyKind::ImplTrait(id, bounds, precise_capturing) => {
vis.visit_id(id);
visit_vec(bounds, |bound| vis.visit_param_bound(bound));
if let Some((precise_capturing, _span)) = precise_capturing.as_deref_mut() {
for arg in precise_capturing {
vis.visit_precise_capturing_arg(arg);
}
}
}
TyKind::MacCall(mac) => vis.visit_mac_call(mac),
TyKind::AnonStruct(id, fields) | TyKind::AnonUnion(id, fields) => {
Expand Down Expand Up @@ -914,6 +923,18 @@ pub fn noop_visit_param_bound<T: MutVisitor>(pb: &mut GenericBound, vis: &mut T)
}
}

pub fn noop_visit_precise_capturing_arg<T: MutVisitor>(arg: &mut PreciseCapturingArg, vis: &mut T) {
match arg {
PreciseCapturingArg::Lifetime(lt) => {
vis.visit_lifetime(lt);
}
PreciseCapturingArg::Arg(path, id) => {
vis.visit_path(path);
vis.visit_id(id);
}
}
}

pub fn noop_flat_map_generic_param<T: MutVisitor>(
mut param: GenericParam,
vis: &mut T,
Expand Down
24 changes: 23 additions & 1 deletion compiler/rustc_ast/src/visit.rs
Original file line number Diff line number Diff line change
Expand Up @@ -184,6 +184,9 @@ pub trait Visitor<'ast>: Sized {
fn visit_param_bound(&mut self, bounds: &'ast GenericBound, _ctxt: BoundKind) -> Self::Result {
walk_param_bound(self, bounds)
}
fn visit_precise_capturing_arg(&mut self, arg: &'ast PreciseCapturingArg) {
walk_precise_capturing_arg(self, arg);
}
fn visit_poly_trait_ref(&mut self, t: &'ast PolyTraitRef) -> Self::Result {
walk_poly_trait_ref(self, t)
}
Expand Down Expand Up @@ -457,8 +460,13 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) -> V::Result {
TyKind::TraitObject(bounds, ..) => {
walk_list!(visitor, visit_param_bound, bounds, BoundKind::TraitObject);
}
TyKind::ImplTrait(_, bounds) => {
TyKind::ImplTrait(_, bounds, precise_capturing) => {
walk_list!(visitor, visit_param_bound, bounds, BoundKind::Impl);
if let Some((precise_capturing, _span)) = precise_capturing.as_deref() {
for arg in precise_capturing {
try_visit!(visitor.visit_precise_capturing_arg(arg));
}
}
}
TyKind::Typeof(expression) => try_visit!(visitor.visit_anon_const(expression)),
TyKind::Infer | TyKind::ImplicitSelf | TyKind::Dummy | TyKind::Err(_) => {}
Expand Down Expand Up @@ -637,6 +645,20 @@ pub fn walk_param_bound<'a, V: Visitor<'a>>(visitor: &mut V, bound: &'a GenericB
}
}

pub fn walk_precise_capturing_arg<'a, V: Visitor<'a>>(
visitor: &mut V,
arg: &'a PreciseCapturingArg,
) {
match arg {
PreciseCapturingArg::Lifetime(lt) => {
visitor.visit_lifetime(lt, LifetimeCtxt::GenericArg);
}
PreciseCapturingArg::Arg(path, id) => {
visitor.visit_path(path, *id);
}
}
}

pub fn walk_generic_param<'a, V: Visitor<'a>>(
visitor: &mut V,
param: &'a GenericParam,
Expand Down
2 changes: 2 additions & 0 deletions compiler/rustc_ast_lowering/messages.ftl
Original file line number Diff line number Diff line change
Expand Up @@ -127,6 +127,8 @@ ast_lowering_never_pattern_with_guard =
a guard on a never pattern will never be run
.suggestion = remove this guard
ast_lowering_no_precise_captures_on_apit = `use<...>` precise capturing syntax not allowed on argument-position `impl Trait`
ast_lowering_previously_used_here = previously used here
ast_lowering_register1 = register `{$reg1_name}`
Expand Down
7 changes: 7 additions & 0 deletions compiler/rustc_ast_lowering/src/errors.rs
Original file line number Diff line number Diff line change
Expand Up @@ -414,3 +414,10 @@ pub(crate) struct AsyncBoundOnlyForFnTraits {
#[primary_span]
pub span: Span,
}

#[derive(Diagnostic)]
#[diag(ast_lowering_no_precise_captures_on_apit)]
pub(crate) struct NoPreciseCapturesOnApit {
#[primary_span]
pub span: Span,
}
17 changes: 17 additions & 0 deletions compiler/rustc_ast_lowering/src/index.rs
Original file line number Diff line number Diff line change
Expand Up @@ -385,4 +385,21 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
fn visit_pattern_type_pattern(&mut self, p: &'hir hir::Pat<'hir>) {
self.visit_pat(p)
}

fn visit_precise_capturing_arg(
&mut self,
arg: &'hir PreciseCapturingArg<'hir>,
) -> Self::Result {
match arg {
PreciseCapturingArg::Lifetime(_) => {
// This is represented as a `Node::Lifetime`, intravisit will get to it below.
}
PreciseCapturingArg::Param(param) => self.insert(
param.ident.span,
param.hir_id,
Node::PreciseCapturingNonLifetimeArg(param),
),
}
intravisit::walk_precise_capturing_arg(self, arg);
}
}
136 changes: 97 additions & 39 deletions compiler/rustc_ast_lowering/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -48,6 +48,7 @@ use rustc_ast::{self as ast, *};
use rustc_ast_pretty::pprust;
use rustc_data_structures::captures::Captures;
use rustc_data_structures::fingerprint::Fingerprint;
use rustc_data_structures::fx::FxIndexSet;
use rustc_data_structures::sorted_map::SortedMap;
use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
use rustc_data_structures::sync::Lrc;
Expand Down Expand Up @@ -1398,7 +1399,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
});
hir::TyKind::TraitObject(bounds, lifetime_bound, *kind)
}
TyKind::ImplTrait(def_node_id, bounds) => {
TyKind::ImplTrait(def_node_id, bounds, precise_capturing) => {
let span = t.span;
match itctx {
ImplTraitContext::OpaqueTy { origin, fn_kind } => self.lower_opaque_impl_trait(
Expand All @@ -1408,8 +1409,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
bounds,
fn_kind,
itctx,
precise_capturing.as_deref().map(|(args, _)| args.as_slice()),
),
ImplTraitContext::Universal => {
if let Some(&(_, span)) = precise_capturing.as_deref() {
self.tcx.dcx().emit_err(errors::NoPreciseCapturesOnApit { span });
};
let span = t.span;

// HACK: pprust breaks strings with newlines when the type
Expand Down Expand Up @@ -1520,6 +1525,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
bounds: &GenericBounds,
fn_kind: Option<FnDeclKind>,
itctx: ImplTraitContext,
precise_capturing_args: Option<&[PreciseCapturingArg]>,
) -> hir::TyKind<'hir> {
// Make sure we know that some funky desugaring has been going on here.
// This is a first: there is code in other places like for loop
Expand All @@ -1528,42 +1534,59 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
// frequently opened issues show.
let opaque_ty_span = self.mark_span_with_reason(DesugaringKind::OpaqueTy, span, None);

let captured_lifetimes_to_duplicate = match origin {
hir::OpaqueTyOrigin::TyAlias { .. } => {
// type alias impl trait and associated type position impl trait were
// decided to capture all in-scope lifetimes, which we collect for
// all opaques during resolution.
self.resolver
.take_extra_lifetime_params(opaque_ty_node_id)
.into_iter()
.map(|(ident, id, _)| Lifetime { id, ident })
let captured_lifetimes_to_duplicate =
if let Some(precise_capturing) = precise_capturing_args {
// We'll actually validate these later on; all we need is the list of
// lifetimes to duplicate during this portion of lowering.
precise_capturing
.iter()
.filter_map(|arg| match arg {
PreciseCapturingArg::Lifetime(lt) => Some(*lt),
PreciseCapturingArg::Arg(..) => None,
})
// Add in all the lifetimes mentioned in the bounds. We will error
// them out later, but capturing them here is important to make sure
// they actually get resolved in resolve_bound_vars.
.chain(lifetime_collector::lifetimes_in_bounds(self.resolver, bounds))
.collect()
}
hir::OpaqueTyOrigin::FnReturn(..) => {
if matches!(
fn_kind.expect("expected RPITs to be lowered with a FnKind"),
FnDeclKind::Impl | FnDeclKind::Trait
) || self.tcx.features().lifetime_capture_rules_2024
|| span.at_least_rust_2024()
{
// return-position impl trait in trait was decided to capture all
// in-scope lifetimes, which we collect for all opaques during resolution.
self.resolver
.take_extra_lifetime_params(opaque_ty_node_id)
.into_iter()
.map(|(ident, id, _)| Lifetime { id, ident })
.collect()
} else {
// in fn return position, like the `fn test<'a>() -> impl Debug + 'a`
// example, we only need to duplicate lifetimes that appear in the
// bounds, since those are the only ones that are captured by the opaque.
lifetime_collector::lifetimes_in_bounds(self.resolver, bounds)
} else {
match origin {
hir::OpaqueTyOrigin::TyAlias { .. } => {
// type alias impl trait and associated type position impl trait were
// decided to capture all in-scope lifetimes, which we collect for
// all opaques during resolution.
self.resolver
.take_extra_lifetime_params(opaque_ty_node_id)
.into_iter()
.map(|(ident, id, _)| Lifetime { id, ident })
.collect()
}
hir::OpaqueTyOrigin::FnReturn(..) => {
if matches!(
fn_kind.expect("expected RPITs to be lowered with a FnKind"),
FnDeclKind::Impl | FnDeclKind::Trait
) || self.tcx.features().lifetime_capture_rules_2024
|| span.at_least_rust_2024()
{
// return-position impl trait in trait was decided to capture all
// in-scope lifetimes, which we collect for all opaques during resolution.
self.resolver
.take_extra_lifetime_params(opaque_ty_node_id)
.into_iter()
.map(|(ident, id, _)| Lifetime { id, ident })
.collect()
} else {
// in fn return position, like the `fn test<'a>() -> impl Debug + 'a`
// example, we only need to duplicate lifetimes that appear in the
// bounds, since those are the only ones that are captured by the opaque.
lifetime_collector::lifetimes_in_bounds(self.resolver, bounds)
}
}
hir::OpaqueTyOrigin::AsyncFn(..) => {
unreachable!("should be using `lower_async_fn_ret_ty`")
}
}
}
hir::OpaqueTyOrigin::AsyncFn(..) => {
unreachable!("should be using `lower_async_fn_ret_ty`")
}
};
};
debug!(?captured_lifetimes_to_duplicate);

self.lower_opaque_inner(
Expand All @@ -1573,6 +1596,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
captured_lifetimes_to_duplicate,
span,
opaque_ty_span,
precise_capturing_args,
|this| this.lower_param_bounds(bounds, itctx),
)
}
Expand All @@ -1582,9 +1606,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
opaque_ty_node_id: NodeId,
origin: hir::OpaqueTyOrigin,
in_trait: bool,
captured_lifetimes_to_duplicate: Vec<Lifetime>,
captured_lifetimes_to_duplicate: FxIndexSet<Lifetime>,
span: Span,
opaque_ty_span: Span,
precise_capturing_args: Option<&[PreciseCapturingArg]>,
lower_item_bounds: impl FnOnce(&mut Self) -> &'hir [hir::GenericBound<'hir>],
) -> hir::TyKind<'hir> {
let opaque_ty_def_id = self.create_def(
Expand Down Expand Up @@ -1671,8 +1696,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
// Install the remapping from old to new (if any). This makes sure that
// any lifetimes that would have resolved to the def-id of captured
// lifetimes are remapped to the new *synthetic* lifetimes of the opaque.
let bounds = this
.with_remapping(captured_to_synthesized_mapping, |this| lower_item_bounds(this));
let (bounds, precise_capturing_args) =
this.with_remapping(captured_to_synthesized_mapping, |this| {
(
lower_item_bounds(this),
precise_capturing_args.map(|precise_capturing| {
this.lower_precise_capturing_args(precise_capturing)
}),
)
});

let generic_params =
this.arena.alloc_from_iter(synthesized_lifetime_definitions.iter().map(
Expand Down Expand Up @@ -1717,6 +1749,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
origin,
lifetime_mapping,
in_trait,
precise_capturing_args,
};

// Generate an `type Foo = impl Trait;` declaration.
Expand Down Expand Up @@ -1749,6 +1782,30 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
)
}

fn lower_precise_capturing_args(
&mut self,
precise_capturing_args: &[PreciseCapturingArg],
) -> &'hir [hir::PreciseCapturingArg<'hir>] {
self.arena.alloc_from_iter(precise_capturing_args.iter().map(|arg| match arg {
PreciseCapturingArg::Lifetime(lt) => {
hir::PreciseCapturingArg::Lifetime(self.lower_lifetime(lt))
}
PreciseCapturingArg::Arg(path, id) => {
let [segment] = path.segments.as_slice() else {
panic!();
};
let res = self.resolver.get_partial_res(*id).map_or(Res::Err, |partial_res| {
partial_res.full_res().expect("no partial res expected for precise capture arg")
});
hir::PreciseCapturingArg::Param(hir::PreciseCapturingNonLifetimeArg {
hir_id: self.lower_node_id(*id),
ident: self.lower_ident(segment.ident),
res: self.lower_res(res),
})
}
}))
}

fn lower_fn_params_to_names(&mut self, decl: &FnDecl) -> &'hir [Ident] {
self.arena.alloc_from_iter(decl.inputs.iter().map(|param| match param.pat.kind {
PatKind::Ident(_, ident, _) => self.lower_ident(ident),
Expand Down Expand Up @@ -1889,7 +1946,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
let opaque_ty_span =
self.mark_span_with_reason(DesugaringKind::Async, span, allowed_features);

let captured_lifetimes: Vec<_> = self
let captured_lifetimes = self
.resolver
.take_extra_lifetime_params(opaque_ty_node_id)
.into_iter()
Expand All @@ -1903,6 +1960,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
captured_lifetimes,
span,
opaque_ty_span,
None,
|this| {
let bound = this.lower_coroutine_fn_output_type_to_bound(
output,
Expand Down
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