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Hint for a lifetime binding of a type in a trait suggests invalid syntax [E0309] #26271
Description
Hint in the error message for this code appears to suggest a syntax that doesn't compile (playpen):
trait Tr {
type TrSubtype;
}
struct Bar<'a, Item: Tr> {
item: Item,
item_sub: &'a <Item as Tr>::TrSubtype,
}error: the associated type
<Item as Tr>::TrSubtypemay not live long enough
[…] consider adding an explicit lifetime bound<Item as Tr>::TrSubtype: 'a...
When I did what (I thought was) suggested (since the same syntax worked in simpler cases):
struct Bar<'a, Item: Tr, <Item as Tr>::TrSubtype: 'a> {it failed with a very unfriendly syntax error:
error: expected ident, found
<
After a lot of head scratching I've discovered that it meant:
struct Bar<'a, Item: Tr> where <Item as Tr>::TrSubtype: 'a {So I suggest either:
- Make the error hint suggest syntax with the
wherekeyword when necessary - Allow the syntax without
where:<'a, Item: Tr, <Item as Tr>::TrSubtype: 'a>> - or detect when
<is found instead of an ident and emit error along lines of "Move that to thewhereclause".
Activity
Anachron commentedon Jun 14, 2015
I would give 👍 to the last one.
steveklabnik commentedon Jan 3, 2017
Triage: same error as today.
estebank commentedon Sep 19, 2018
Funnily enough, this code now works in the latest beta.
Do not close: we still want to change the parser to accept
struct Bar<'a, Item: Tr, <Item as Tr>::TrSubtype: 'a> {and emit a tailored diagnostic.Rollup merge of rust-lang#64676 - estebank:assoc-type-bound-in-generi…
Rollup merge of rust-lang#64676 - estebank:assoc-type-bound-in-generi…