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Make suggest_deref_closure_return more idiomatic/easier to understand #123990

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Apr 16, 2024
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Make suggest_deref_closure_return more idiomatic/easier to understand #123990

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137 changes: 64 additions & 73 deletions compiler/rustc_borrowck/src/diagnostics/region_errors.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -26,7 +26,6 @@ use rustc_middle::ty::{self, RegionVid, Ty};
use rustc_middle::ty::{Region, TyCtxt};
use rustc_span::symbol::{kw, Ident};
use rustc_span::Span;
use rustc_trait_selection::infer::type_variable::TypeVariableOrigin;
use rustc_trait_selection::infer::InferCtxtExt;
use rustc_trait_selection::traits::{Obligation, ObligationCtxt};

Expand Down Expand Up @@ -813,7 +812,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
self.suggest_adding_lifetime_params(&mut diag, *fr, *outlived_fr);
self.suggest_move_on_borrowing_closure(&mut diag);
self.suggest_deref_closure_value(&mut diag);
self.suggest_deref_closure_return(&mut diag);

diag
}
Expand Down Expand Up @@ -1048,115 +1047,107 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
/// When encountering a lifetime error caused by the return type of a closure, check the
/// corresponding trait bound and see if dereferencing the closure return value would satisfy
/// them. If so, we produce a structured suggestion.
fn suggest_deref_closure_value(&self, diag: &mut Diag<'_>) {
fn suggest_deref_closure_return(&self, diag: &mut Diag<'_>) {
let tcx = self.infcx.tcx;
let map = tcx.hir();

// Get the closure return value and type.
let body_id = map.body_owned_by(self.mir_def_id());
let body = &map.body(body_id);
let value = &body.value.peel_blocks();
let hir::Node::Expr(closure_expr) = tcx.hir_node_by_def_id(self.mir_def_id()) else {
let closure_def_id = self.mir_def_id();
let hir::Node::Expr(
closure_expr @ hir::Expr {
kind: hir::ExprKind::Closure(hir::Closure { body, .. }), ..
},
) = tcx.hir_node_by_def_id(closure_def_id)
else {
return;
};
let fn_call_id = tcx.parent_hir_id(self.mir_hir_id());
let hir::Node::Expr(expr) = tcx.hir_node(fn_call_id) else { return };
let def_id = map.enclosing_body_owner(fn_call_id);
let tables = tcx.typeck(def_id);
let Some(return_value_ty) = tables.node_type_opt(value.hir_id) else { return };
let return_value_ty = self.infcx.resolve_vars_if_possible(return_value_ty);
let ty::Closure(_, args) = *tcx.type_of(closure_def_id).instantiate_identity().kind()
else {
return;
};
let args = args.as_closure();

// Make sure that the parent expression is a method call.
let parent_expr_id = tcx.parent_hir_id(self.mir_hir_id());
let hir::Node::Expr(
parent_expr @ hir::Expr {
kind: hir::ExprKind::MethodCall(_, rcvr, call_args, _), ..
},
) = tcx.hir_node(parent_expr_id)
else {
return;
};
let typeck_results = tcx.typeck(self.mir_def_id());

// We don't use `ty.peel_refs()` to get the number of `*`s needed to get the root type.
let mut ty = return_value_ty;
let liberated_sig = tcx.liberate_late_bound_regions(closure_def_id.to_def_id(), args.sig());
let mut peeled_ty = liberated_sig.output();
let mut count = 0;
while let ty::Ref(_, t, _) = ty.kind() {
ty = *t;
while let ty::Ref(_, ref_ty, _) = *peeled_ty.kind() {
peeled_ty = ref_ty;
count += 1;
}
if !self.infcx.type_is_copy_modulo_regions(self.param_env, ty) {
if !self.infcx.type_is_copy_modulo_regions(self.param_env, peeled_ty) {
return;
}

// Build a new closure where the return type is an owned value, instead of a ref.
let Some(ty::Closure(did, args)) =
tables.node_type_opt(closure_expr.hir_id).as_ref().map(|ty| ty.kind())
else {
return;
};
let sig = args.as_closure().sig();
let closure_sig_as_fn_ptr_ty = Ty::new_fn_ptr(
tcx,
sig.map_bound(|s| {
let unsafety = hir::Unsafety::Normal;
use rustc_target::spec::abi;
tcx.mk_fn_sig(
[s.inputs()[0]],
s.output().peel_refs(),
s.c_variadic,
unsafety,
abi::Abi::Rust,
)
}),
);
let parent_args = GenericArgs::identity_for_item(
tcx,
tcx.typeck_root_def_id(self.mir_def_id().to_def_id()),
ty::Binder::dummy(tcx.mk_fn_sig(
liberated_sig.inputs().iter().copied(),
peeled_ty,
liberated_sig.c_variadic,
hir::Unsafety::Normal,
rustc_target::spec::abi::Abi::Rust,
)),
);
let closure_kind = args.as_closure().kind();
let closure_kind_ty = Ty::from_closure_kind(tcx, closure_kind);
let tupled_upvars_ty = self
.infcx
.next_ty_var(TypeVariableOrigin { param_def_id: None, span: closure_expr.span });
let closure_args = ty::ClosureArgs::new(
let closure_ty = Ty::new_closure(
tcx,
ty::ClosureArgsParts {
parent_args,
closure_kind_ty,
closure_sig_as_fn_ptr_ty,
tupled_upvars_ty,
},
closure_def_id.to_def_id(),
ty::ClosureArgs::new(
tcx,
ty::ClosureArgsParts {
parent_args: args.parent_args(),
closure_kind_ty: args.kind_ty(),
tupled_upvars_ty: args.tupled_upvars_ty(),
closure_sig_as_fn_ptr_ty,
},
)
.args,
);
let closure_ty = Ty::new_closure(tcx, *did, closure_args.args);
let closure_ty = tcx.erase_regions(closure_ty);

let hir::ExprKind::MethodCall(_, rcvr, args, _) = expr.kind else { return };
let Some(pos) = args
.iter()
.enumerate()
.find(|(_, arg)| arg.hir_id == closure_expr.hir_id)
.map(|(i, _)| i)

let Some((closure_arg_pos, _)) =
call_args.iter().enumerate().find(|(_, arg)| arg.hir_id == closure_expr.hir_id)
else {
return;
};
// The found `Self` type of the method call.
let Some(possible_rcvr_ty) = tables.node_type_opt(rcvr.hir_id) else { return };
let Some(method_def_id) = tables.type_dependent_def_id(expr.hir_id) else { return };

// Get the type for the parameter corresponding to the argument the closure with the
// lifetime error we had.
let Some(input) = tcx
let Some(method_def_id) = typeck_results.type_dependent_def_id(parent_expr.hir_id) else {
return;
};
let Some(input_arg) = tcx
.fn_sig(method_def_id)
.instantiate_identity()
.skip_binder()
.inputs()
.skip_binder()
// Methods have a `self` arg, so `pos` is actually `+ 1` to match the method call arg.
.get(pos + 1)
.get(closure_arg_pos + 1)
else {
return;
};

trace!(?input);

let ty::Param(closure_param) = input.kind() else { return };
// If this isn't a param, then we can't substitute a new closure.
let ty::Param(closure_param) = input_arg.kind() else { return };

// Get the arguments for the found method, only specifying that `Self` is the receiver type.
let Some(possible_rcvr_ty) = typeck_results.node_type_opt(rcvr.hir_id) else { return };
let args = GenericArgs::for_item(tcx, method_def_id, |param, _| {
if param.index == 0 {
possible_rcvr_ty.into()
} else if param.index == closure_param.index {
closure_ty.into()
} else {
self.infcx.var_for_def(expr.span, param)
self.infcx.var_for_def(parent_expr.span, param)
}
});

Expand All @@ -1170,7 +1161,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {

if ocx.select_all_or_error().is_empty() {
diag.span_suggestion_verbose(
value.span.shrink_to_lo(),
tcx.hir().body(*body).value.peel_blocks().span.shrink_to_lo(),
"dereference the return value",
"*".repeat(count),
Applicability::MachineApplicable,
Expand Down
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