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Unrolled build for rust-lang#125407
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Rollup merge of rust-lang#125407 - pacak:no-lending-iterators, r=pnkfelix

Detect when user is trying to create a lending `Iterator` and give a custom explanation

The scope for this diagnostic is to detect lending iterators specifically and it's main goal is to help beginners to understand that what they are trying to implement might not be possible for `Iterator` trait specifically.

I ended up to changing the wording from originally proposed in the ticket because it might be misleading otherwise: `Data` might have a lifetime parameter but it can be unrelated to items user is planning to return.

Fixes rust-lang#125337
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rust-timer authored Jun 5, 2024
2 parents c1dba09 + b70fb41 commit 752dc56
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Showing 7 changed files with 140 additions and 8 deletions.
8 changes: 8 additions & 0 deletions compiler/rustc_resolve/messages.ftl
Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,10 @@ resolve_added_macro_use =
resolve_ancestor_only =
visibilities can only be restricted to ancestor modules
resolve_anonymous_livetime_non_gat_report_error =
in the trait associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
.label = this lifetime must come from the implemented type
resolve_arguments_macro_use_not_allowed = arguments to `macro_use` are not allowed here
resolve_associated_const_with_similar_name_exists =
Expand Down Expand Up @@ -234,6 +238,10 @@ resolve_items_in_traits_are_not_importable =
resolve_label_with_similar_name_reachable =
a label with a similar name is reachable
resolve_lending_iterator_report_error =
associated type `Iterator::Item` is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
.note = you can't create an `Iterator` that borrows each `Item` from itself, but you can instead create a new type that borrows your existing type and implement `Iterator` for that new type.
resolve_lifetime_param_in_enum_discriminant =
lifetime parameters may not be used in enum discriminant values
Expand Down
17 changes: 17 additions & 0 deletions compiler/rustc_resolve/src/errors.rs
Original file line number Diff line number Diff line change
Expand Up @@ -882,6 +882,23 @@ pub(crate) struct ElidedAnonymousLivetimeReportError {
pub(crate) suggestion: Option<ElidedAnonymousLivetimeReportErrorSuggestion>,
}

#[derive(Diagnostic)]
#[diag(resolve_lending_iterator_report_error)]
pub(crate) struct LendingIteratorReportError {
#[primary_span]
pub(crate) lifetime: Span,
#[note]
pub(crate) ty: Span,
}

#[derive(Diagnostic)]
#[diag(resolve_anonymous_livetime_non_gat_report_error)]
pub(crate) struct AnonymousLivetimeNonGatReportError {
#[primary_span]
#[label]
pub(crate) lifetime: Span,
}

#[derive(Subdiagnostic)]
#[multipart_suggestion(
resolve_elided_anonymous_lifetime_report_error_suggestion,
Expand Down
52 changes: 47 additions & 5 deletions compiler/rustc_resolve/src/late.rs
Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,7 @@
use crate::{errors, path_names_to_string, rustdoc, BindingError, Finalize, LexicalScopeBinding};
use crate::{BindingKey, Used};
use crate::{Module, ModuleOrUniformRoot, NameBinding, ParentScope, PathResult};
use crate::{ResolutionError, Resolver, Segment, UseError};
use crate::{ResolutionError, Resolver, Segment, TyCtxt, UseError};

use rustc_ast::ptr::P;
use rustc_ast::visit::{visit_opt, walk_list, AssocCtxt, BoundKind, FnCtxt, FnKind, Visitor};
Expand Down Expand Up @@ -629,6 +629,9 @@ struct DiagMetadata<'ast> {
in_assignment: Option<&'ast Expr>,
is_assign_rhs: bool,

/// If we are setting an associated type in trait impl, is it a non-GAT type?
in_non_gat_assoc_type: Option<bool>,

/// Used to detect possible `.` -> `..` typo when calling methods.
in_range: Option<(&'ast Expr, &'ast Expr)>,

Expand Down Expand Up @@ -1703,10 +1706,35 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
break;
}
}
self.r.dcx().emit_err(errors::ElidedAnonymousLivetimeReportError {
span: lifetime.ident.span,
suggestion,
});

// are we trying to use an anonymous lifetime
// on a non GAT associated trait type?
if !self.in_func_body
&& let Some((module, _)) = &self.current_trait_ref
&& let Some(ty) = &self.diag_metadata.current_self_type
&& Some(true) == self.diag_metadata.in_non_gat_assoc_type
&& let crate::ModuleKind::Def(DefKind::Trait, trait_id, _) = module.kind
{
if def_id_matches_path(
self.r.tcx,
trait_id,
&["core", "iter", "traits", "iterator", "Iterator"],
) {
self.r.dcx().emit_err(errors::LendingIteratorReportError {
lifetime: lifetime.ident.span,
ty: ty.span(),
});
} else {
self.r.dcx().emit_err(errors::AnonymousLivetimeNonGatReportError {
lifetime: lifetime.ident.span,
});
}
} else {
self.r.dcx().emit_err(errors::ElidedAnonymousLivetimeReportError {
span: lifetime.ident.span,
suggestion,
});
}
} else {
self.r.dcx().emit_err(errors::ExplicitAnonymousLivetimeReportError {
span: lifetime.ident.span,
Expand Down Expand Up @@ -3058,6 +3086,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
);
}
AssocItemKind::Type(box TyAlias { generics, .. }) => {
self.diag_metadata.in_non_gat_assoc_type = Some(generics.params.is_empty());
debug!("resolve_implementation AssocItemKind::Type");
// We also need a new scope for the impl item type parameters.
self.with_generic_param_rib(
Expand Down Expand Up @@ -3086,6 +3115,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
});
},
);
self.diag_metadata.in_non_gat_assoc_type = None;
}
AssocItemKind::Delegation(box delegation) => {
debug!("resolve_implementation AssocItemKind::Delegation");
Expand Down Expand Up @@ -4829,3 +4859,15 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
}
}
}

/// Check if definition matches a path
fn def_id_matches_path(tcx: TyCtxt<'_>, mut def_id: DefId, expected_path: &[&str]) -> bool {
let mut path = expected_path.iter().rev();
while let (Some(parent), Some(next_step)) = (tcx.opt_parent(def_id), path.next()) {
if !tcx.opt_item_name(def_id).map_or(false, |n| n.as_str() == *next_step) {
return false;
}
def_id = parent;
}
return true;
}
2 changes: 1 addition & 1 deletion tests/ui/impl-header-lifetime-elision/assoc-type.rs
Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,7 @@ trait MyTrait {

impl MyTrait for &i32 {
type Output = &i32;
//~^ ERROR `&` without an explicit lifetime name cannot be used here
//~^ ERROR 11:19: 11:20: in the trait associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
}

impl MyTrait for &u32 {
Expand Down
4 changes: 2 additions & 2 deletions tests/ui/impl-header-lifetime-elision/assoc-type.stderr
Original file line number Diff line number Diff line change
@@ -1,8 +1,8 @@
error[E0637]: `&` without an explicit lifetime name cannot be used here
error: in the trait associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
--> $DIR/assoc-type.rs:11:19
|
LL | type Output = &i32;
| ^ explicit lifetime name needed here
| ^ this lifetime must come from the implemented type

error[E0637]: `'_` cannot be used here
--> $DIR/assoc-type.rs:16:20
Expand Down
35 changes: 35 additions & 0 deletions tests/ui/lifetimes/no_lending_iterators.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,35 @@
struct Data(String);

impl Iterator for Data {
type Item = &str;
//~^ ERROR 4:17: 4:18: associated type `Iterator::Item` is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type

fn next(&mut self) -> Option<Self::Item> {
Some(&self.0)
}
}

trait Bar {
type Item;
fn poke(&mut self, item: Self::Item);
}

impl Bar for usize {
type Item = &usize;
//~^ ERROR 18:17: 18:18: in the trait associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type

fn poke(&mut self, item: Self::Item) {
self += *item;
}
}

impl Bar for isize {
type Item<'a> = &'a isize;
//~^ ERROR 27:14: 27:18: lifetime parameters or bounds on type `Item` do not match the trait declaration [E0195]

fn poke(&mut self, item: Self::Item) {
self += *item;
}
}

fn main() {}
30 changes: 30 additions & 0 deletions tests/ui/lifetimes/no_lending_iterators.stderr
Original file line number Diff line number Diff line change
@@ -0,0 +1,30 @@
error: associated type `Iterator::Item` is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
--> $DIR/no_lending_iterators.rs:4:17
|
LL | type Item = &str;
| ^
|
note: you can't create an `Iterator` that borrows each `Item` from itself, but you can instead create a new type that borrows your existing type and implement `Iterator` for that new type.
--> $DIR/no_lending_iterators.rs:3:19
|
LL | impl Iterator for Data {
| ^^^^

error: in the trait associated type is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type
--> $DIR/no_lending_iterators.rs:18:17
|
LL | type Item = &usize;
| ^ this lifetime must come from the implemented type

error[E0195]: lifetime parameters or bounds on type `Item` do not match the trait declaration
--> $DIR/no_lending_iterators.rs:27:14
|
LL | type Item;
| - lifetimes in impl do not match this type in trait
...
LL | type Item<'a> = &'a isize;
| ^^^^ lifetimes do not match type in trait

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0195`.

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