bjoernager/rust
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Note about object lifetime defaults in does not live long enough error

Browse source 
This is a aspect of Rust that frequently trips up people who are not
aware of it yet. This diagnostic attempts to explain what's happening
and why the lifetime constraint, that was never mentioned in the source,
arose.
This commit is contained in:
Nilstrieb 2023-11-12 13:48:47 +01:00
parent ed086d86b8
commit e5c330ac48
8 changed files with 132 additions and 11 deletions
Download patch file Download diff file Expand all files Collapse all files

60
compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs
View file

@ -5,12 +5,13 @@ use rustc_hir as hir;
use rustc_hir::intravisit::Visitor;
use rustc_index::IndexSlice;
use rustc_infer::infer::NllRegionVariableOrigin;
use rustc_middle::middle::resolve_bound_vars::ObjectLifetimeDefault;
use rustc_middle::mir::{
Body, CallSource, CastKind, ConstraintCategory, FakeReadCause, Local, LocalInfo, Location,
Operand, Place, Rvalue, Statement, StatementKind, TerminatorKind,
};
use rustc_middle::ty::adjustment::PointerCoercion;
use rustc_middle::ty::{self, RegionVid, TyCtxt};
use rustc_middle::ty::{self, RegionVid, Ty, TyCtxt};
use rustc_span::symbol::{kw, Symbol};
use rustc_span::{sym, DesugaringKind, Span};
use rustc_trait_selection::traits::error_reporting::FindExprBySpan;
@ -290,12 +291,69 @@ impl<'tcx> BorrowExplanation<'tcx> {
}
}
if let ConstraintCategory::Cast { unsize_to: Some(unsize_ty) } = category {
self.add_object_lifetime_default_note(tcx, err, unsize_ty);
}
self.add_lifetime_bound_suggestion_to_diagnostic(err, &category, span, region_name);
}
_ => {}
}
}
fn add_object_lifetime_default_note(
&self,
tcx: TyCtxt<'tcx>,
err: &mut Diagnostic,
unsize_ty: Ty<'tcx>,
) {
if let ty::Adt(def, args) = unsize_ty.kind() {
// We try to elaborate the object lifetime defaults and present those to the user. This should
// make it clear where the region constraint is coming from.
let generics = tcx.generics_of(def.did());
let mut has_dyn = false;
let mut failed = false;
let elaborated_args = std::iter::zip(*args, &generics.params).map(|(arg, param)| {
if let Some(ty::Dynamic(obj, _, ty::DynKind::Dyn)) = arg.as_type().map(Ty::kind) {
let default = tcx.object_lifetime_default(param.def_id);
let re_static = tcx.lifetimes.re_static;
let implied_region = match default {
// This is not entirely precise.
ObjectLifetimeDefault::Empty => re_static,
ObjectLifetimeDefault::Ambiguous => {
failed = true;
re_static
}
ObjectLifetimeDefault::Param(param_def_id) => {
let index = generics.param_def_id_to_index[&param_def_id] as usize;
args.get(index).and_then(|arg| arg.as_region()).unwrap_or_else(|| {
failed = true;
re_static
})
}
ObjectLifetimeDefault::Static => re_static,
};
has_dyn = true;
Ty::new_dynamic(tcx, obj, implied_region, ty::DynKind::Dyn).into()
} else {
arg
}
});
let elaborated_ty = Ty::new_adt(tcx, *def, tcx.mk_args_from_iter(elaborated_args));
if has_dyn && !failed {
err.note(format!(
"due to object lifetime defaults, `{unsize_ty}` actually means `{elaborated_ty}`"
));
}
}
}
fn add_lifetime_bound_suggestion_to_diagnostic(
&self,
err: &mut Diagnostic,

2
compiler/rustc_borrowck/src/diagnostics/region_errors.rs
View file

@ -53,7 +53,7 @@ impl<'tcx> ConstraintDescription for ConstraintCategory<'tcx> {
ConstraintCategory::Yield => "yielding this value ",
ConstraintCategory::UseAsConst => "using this value as a constant ",
ConstraintCategory::UseAsStatic => "using this value as a static ",
ConstraintCategory::Cast => "cast ",
ConstraintCategory::Cast { .. } => "cast ",
ConstraintCategory::CallArgument(_) => "argument ",
ConstraintCategory::TypeAnnotation => "type annotation ",
ConstraintCategory::ClosureBounds => "closure body ",

24
compiler/rustc_borrowck/src/type_check/mod.rs
View file

@ -1934,7 +1934,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
*ty,
ty_fn_ptr_from,
location.to_locations(),
ConstraintCategory::Cast,
ConstraintCategory::Cast { unsize_to: None },
) {
span_mirbug!(
self,
@ -1959,7 +1959,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
*ty,
ty_fn_ptr_from,
location.to_locations(),
ConstraintCategory::Cast,
ConstraintCategory::Cast { unsize_to: None },
) {
span_mirbug!(
self,
@ -1988,7 +1988,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
*ty,
ty_fn_ptr_from,
location.to_locations(),
ConstraintCategory::Cast,
ConstraintCategory::Cast { unsize_to: None },
) {
span_mirbug!(
self,
@ -2013,7 +2013,15 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
self.prove_trait_ref(
trait_ref,
location.to_locations(),
ConstraintCategory::Cast,
ConstraintCategory::Cast {
unsize_to: Some(tcx.fold_regions(ty, |r, _| {
if let ty::ReVar(_) = r.kind() {
tcx.lifetimes.re_erased
} else {
r
}
})),
},
);
}
@ -2033,7 +2041,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
.iter()
.map(|predicate| predicate.with_self_ty(tcx, self_ty)),
location.to_locations(),
ConstraintCategory::Cast,
ConstraintCategory::Cast { unsize_to: None },
);
let outlives_predicate = tcx.mk_predicate(Binder::dummy(
@ -2044,7 +2052,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
self.prove_predicate(
outlives_predicate,
location.to_locations(),
ConstraintCategory::Cast,
ConstraintCategory::Cast { unsize_to: None },
);
}
@ -2065,7 +2073,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
*ty_from,
*ty_to,
location.to_locations(),
ConstraintCategory::Cast,
ConstraintCategory::Cast { unsize_to: None },
) {
span_mirbug!(
self,
@ -2131,7 +2139,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
*ty_elem,
*ty_to,
location.to_locations(),
ConstraintCategory::Cast,
ConstraintCategory::Cast { unsize_to: None },
) {
span_mirbug!(
self,