Hint for a lifetime binding of a type in a trait suggests invalid syntax [E0309]

[kornelski]

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>::TrSubtype may 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 where keyword 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 the where clause".

[Anachron]

I would give 👍 to the last one.

[steveklabnik]

Triage: same error as today.

[estebank]

Funnily enough, this code now works in the latest beta.

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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.