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delegation: support mapping of all arguments with Self type #158690

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98973e4
Support mapping of all arguments with `Self` type
aerooneqq Jul 2, 2026
8f0fe76
Add #[cfg(debug_assertions)]
aerooneqq Jul 2, 2026
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123 changes: 90 additions & 33 deletions compiler/rustc_ast_lowering/src/delegation.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -64,6 +64,7 @@ use crate::diagnostics::{
};
use crate::{
AllowReturnTypeNotation, ImplTraitContext, ImplTraitPosition, LoweringContext, ParamMode,
re_lowering,
};

mod generics;
Expand Down Expand Up @@ -248,6 +249,17 @@ impl<'hir> LoweringContext<'_, 'hir> {
return false;
}

// If there are definitions inside and we can't delete target expression, so report an error.
// FIXME(fn_delegation): support deletion of target expression with defs inside.
if !should_generate_block && self.any_defs_in_block(block) {
self.dcx().emit_err(DelegationAttemptedBlockWithDefsDeletion { span: block.span });
return false;
}

true
}

fn any_defs_in_block(&self, block: &Block) -> bool {
struct DefinitionsFinder<'a> {
all_owners: &'a NodeMap<PerOwnerResolverData<'a>>,
// `self.owner.node_id_to_def_id`
Expand Down Expand Up @@ -284,16 +296,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
nested_def_ids: &self.owner.node_id_to_def_id,
};

let contains_defs = collector.visit_block(block).is_break();

// If there are definitions inside and we can't delete target expression, so report an error.
// FIXME(fn_delegation): support deletion of target expression with defs inside.
if !should_generate_block && contains_defs {
self.dcx().emit_err(DelegationAttemptedBlockWithDefsDeletion { span: block.span });
return false;
}

true
collector.visit_block(block).is_break()
}

fn should_generate_block(
Expand Down Expand Up @@ -529,7 +532,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
let block_id = self.lower_body(|this| {
let mut parameters: Vec<hir::Param<'_>> = Vec::with_capacity(param_count);
let mut args: Vec<hir::Expr<'_>> = Vec::with_capacity(param_count);
let mut stmts: &[hir::Stmt<'hir>] = &[];
let mut stmts = vec![];

let is_method = this.is_method(sig_id, span);
let should_generate_block = this.should_generate_block(delegation, sig_id, is_method);
Expand All @@ -546,25 +549,20 @@ impl<'hir> LoweringContext<'_, 'hir> {
let generate_arg =
|this: &mut Self| this.generate_arg(is_method, idx, param.pat.hir_id, span);

let param_local_id = param.pat.hir_id.local_id;
let arg = if let Some(block) = block
&& idx == 0
&& should_generate_block
&& (idx == 0 && should_generate_block
|| this.can_map_argument(delegation, sig_id, idx)
&& !this.any_defs_in_block(block))
{
let mut self_resolver = SelfResolver {
ctxt: this,
path_id: delegation.id,
self_param_id: pat_node_id,
};
self_resolver.visit_block(block);
// Target expr needs to lower `self` path.
this.ident_and_label_to_local_id.insert(pat_node_id, param.pat.hir_id.local_id);

// Lower with `HirId::INVALID` as we will use only expr and stmts.
// FIXME(fn_delegation): Alternatives for target expression lowering:
// https://github.com/rust-lang/rfcs/pull/3530#issuecomment-2197170600.
let block = this.lower_block_noalloc(HirId::INVALID, block, false);

stmts = block.stmts;
let block = this.lower_block_maybe_more_than_once(
block,
pat_node_id,
param_local_id,
delegation.id,
);

stmts.push(block.stmts);

// The behavior of the delegation's target expression differs from the
// behavior of the usual block, where if there is no final expression
Expand All @@ -579,8 +577,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
args.push(arg);
}

let (final_expr, hir_id) =
this.finalize_body_lowering(delegation, stmts, args, generics, span);
let (final_expr, hir_id) = this.finalize_body_lowering(
delegation,
sig_id,
this.arena.alloc_from_iter(stmts.into_iter().flatten().copied()),
args,
generics,
span,
);

call_expr_id = hir_id;

Expand All @@ -592,9 +596,50 @@ impl<'hir> LoweringContext<'_, 'hir> {
(block_id, call_expr_id, unused_target_expr)
}

fn can_map_argument(&self, delegation: &Delegation, sig_id: DefId, arg_index: usize) -> bool {
self.can_perform_mapping(delegation)
.filter(|_| {
self.tcx.fn_sig(sig_id).skip_binder().input(arg_index).skip_binder().is_param(0)
})
.is_some()
}

fn lower_block_maybe_more_than_once(
&mut self,
block: &Block,
pat_node_id: NodeId,
param_local_id: hir::ItemLocalId,
delegation_id: NodeId,
) -> hir::Block<'hir> {
let mut self_resolver = SelfResolver {
ctxt: self,
path_id: delegation_id,
self_param_id: pat_node_id,
overwrites: vec![],
};

self_resolver.visit_block(block);

let overwrites = self_resolver.overwrites;

// Target expr needs to lower `self` path.
self.ident_and_label_to_local_id.insert(pat_node_id, param_local_id);

let block = re_lowering::ReloweringChecker::allow_relowering(self, |this| {
this.lower_block_noalloc(HirId::INVALID, block, false)
});

for id in overwrites {
self.partial_res_overrides.remove(&id);
}

block
}

fn finalize_body_lowering(
&mut self,
delegation: &Delegation,
sig_id: DefId,
stmts: &'hir [hir::Stmt<'hir>],
args: Vec<hir::Expr<'hir>>,
generics: &mut GenericsGenerationResults<'hir>,
Expand Down Expand Up @@ -666,7 +711,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
let args = self.arena.alloc_from_iter(args);
let call = self.mk_expr(hir::ExprKind::Call(callee_path, args), span);

let expr = if let Some((parent, of_trait)) = self.should_wrap_return_value(delegation) {
let expr = if let Some((parent, of_trait)) =
self.should_wrap_return_value(delegation, sig_id)
{
let res = Res::SelfTyAlias { alias_to: parent.to_def_id(), is_trait_impl: of_trait };
let ident = Ident::new(kw::SelfUpper, span);
let path = self.create_resolved_path(res, ident, span);
Expand Down Expand Up @@ -701,7 +748,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
(self.mk_expr(hir::ExprKind::Block(block, None), span), call.hir_id)
}

fn should_wrap_return_value(&self, delegation: &Delegation) -> Option<(LocalDefId, bool)> {
fn can_perform_mapping(&self, delegation: &Delegation) -> Option<(LocalDefId, bool)> {
// Heuristic: don't do wrapping if there is no target expression.
if delegation.body.is_none() {
return None;
Expand Down Expand Up @@ -735,11 +782,19 @@ impl<'hir> LoweringContext<'_, 'hir> {
// After previous check we are sure that `sig_id` and `delegation.id`
// point to the same function.
&& tcx.def_kind(tcx.parent(id)) == DefKind::Trait
&& tcx.fn_sig(id).skip_binder().output().skip_binder().is_param(0)
})
})
}

fn should_wrap_return_value(
&self,
delegation: &Delegation,
sig_id: DefId,
) -> Option<(LocalDefId, bool)> {
self.can_perform_mapping(delegation)
.filter(|_| self.tcx.fn_sig(sig_id).skip_binder().output().skip_binder().is_param(0))
}

fn process_segment(
&mut self,
span: Span,
Expand Down Expand Up @@ -851,6 +906,7 @@ struct SelfResolver<'a, 'b, 'hir> {
ctxt: &'a mut LoweringContext<'b, 'hir>,
path_id: NodeId,
self_param_id: NodeId,
overwrites: Vec<NodeId>,
}

impl SelfResolver<'_, '_, '_> {
Expand All @@ -859,6 +915,7 @@ impl SelfResolver<'_, '_, '_> {
&& let Some(Res::Local(sig_id)) = res.full_res()
&& sig_id == self.path_id
{
self.overwrites.push(id);
self.ctxt.partial_res_overrides.insert(id, self.self_param_id);
}
}
Expand Down
62 changes: 50 additions & 12 deletions compiler/rustc_ast_lowering/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -100,6 +100,48 @@ pub fn provide(providers: &mut Providers) {
providers.lower_to_hir = lower_to_hir;
}

pub(crate) mod re_lowering {
use rustc_ast::NodeId;
use rustc_ast::node_id::NodeMap;
use rustc_hir::{self as hir};

use crate::LoweringContext;

#[derive(Debug, Default)]
pub(crate) struct ReloweringChecker {
node_id_to_local_id: NodeMap<hir::ItemLocalId>,
relowering_permissions_count: usize,
}

impl ReloweringChecker {
pub(crate) fn check(&mut self, ast_node_id: NodeId, local_id: hir::ItemLocalId) {
if self.relowering_permissions_count == 0 {
let old = self.node_id_to_local_id.insert(ast_node_id, local_id);
assert_eq!(old, None);
}
}

pub(crate) fn allow_relowering<'a, 'hir, TRes>(
ctx: &mut LoweringContext<'a, 'hir>,
op: impl FnOnce(&mut LoweringContext<'a, 'hir>) -> TRes,
) -> TRes {
#[cfg(debug_assertions)]
{
ctx.relowering_checker.relowering_permissions_count += 1;
}

let res = op(ctx);

#[cfg(debug_assertions)]
{
ctx.relowering_checker.relowering_permissions_count -= 1;
}

res
}
}
}

struct LoweringContext<'a, 'hir> {
tcx: TyCtxt<'hir>,
resolver: &'a ResolverAstLowering<'hir>,
Expand Down Expand Up @@ -146,7 +188,7 @@ struct LoweringContext<'a, 'hir> {
ident_and_label_to_local_id: NodeMap<hir::ItemLocalId>,
/// NodeIds that are lowered inside the current HIR owner. Only used for duplicate lowering check.
#[cfg(debug_assertions)]
node_id_to_local_id: NodeMap<hir::ItemLocalId>,
relowering_checker: re_lowering::ReloweringChecker,
/// The `NodeId` space is split in two.
/// `0..resolver.next_node_id` are created by the resolver on the AST.
/// The higher part `resolver.next_node_id..next_node_id` are created during lowering.
Expand Down Expand Up @@ -205,8 +247,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
// and we never call `lower_node_id(owner)`.
item_local_id_counter: hir::ItemLocalId::new(1),
ident_and_label_to_local_id: Default::default(),

#[cfg(debug_assertions)]
node_id_to_local_id: Default::default(),
relowering_checker: Default::default(),

trait_map: Default::default(),
next_node_id: resolver.next_node_id,
node_id_to_def_id: NodeMap::default(),
Expand Down Expand Up @@ -808,7 +852,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
let current_ident_and_label_to_local_id = mem::take(&mut self.ident_and_label_to_local_id);

#[cfg(debug_assertions)]
let current_node_id_to_local_id = mem::take(&mut self.node_id_to_local_id);
let current_relowering_checker = mem::take(&mut self.relowering_checker);
let current_trait_map = mem::take(&mut self.trait_map);
let current_owner = mem::replace(&mut self.current_hir_id_owner, owner_id);
let current_local_counter =
Expand All @@ -824,10 +868,7 @@ impl<'hir> LoweringContext<'_, 'hir> {

// Always allocate the first `HirId` for the owner itself.
#[cfg(debug_assertions)]
{
let _old = self.node_id_to_local_id.insert(owner, hir::ItemLocalId::ZERO);
debug_assert_eq!(_old, None);
}
self.relowering_checker.check(owner, hir::ItemLocalId::ZERO);

let item = f(self);
assert_eq!(owner_id, item.def_id());
Expand All @@ -845,7 +886,7 @@ impl<'hir> LoweringContext<'_, 'hir> {

#[cfg(debug_assertions)]
{
self.node_id_to_local_id = current_node_id_to_local_id;
self.relowering_checker = current_relowering_checker;
}
self.trait_map = current_trait_map;
self.current_hir_id_owner = current_owner;
Expand Down Expand Up @@ -936,10 +977,7 @@ impl<'hir> LoweringContext<'_, 'hir> {

// Check whether the same `NodeId` is lowered more than once.
#[cfg(debug_assertions)]
{
let old = self.node_id_to_local_id.insert(ast_node_id, local_id);
assert_eq!(old, None);
}
self.relowering_checker.check(ast_node_id, local_id);

hir_id
}
Expand Down
2 changes: 0 additions & 2 deletions tests/ui/delegation/self-coercion-static-free.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -22,7 +22,6 @@ impl Trait for S {
reuse <F as Trait>::{static_value, static_mut_ref, static_ref} {
//~^ ERROR: mismatched types
//~| ERROR: mismatched types
//~| ERROR: mismatched types
//~| ERROR: unused target expression is specified for glob or list delegation
let _ = self;
S::static_self()
Expand All @@ -35,7 +34,6 @@ impl Trait for S1 {
reuse <F as Trait>::{static_value, static_mut_ref, static_ref} {
//~^ ERROR: mismatched types
//~| ERROR: mismatched types
//~| ERROR: mismatched types
//~| ERROR: unused target expression is specified for glob or list delegation
let _ = self;
S1::static_self()
Expand Down
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