Attempt to set a default value for a lifetime generic parameter produces confusing diagnostic

[obi1kenobi]

Code

pub struct DefaultLifetime<'a, 'b = 'static> {
    _marker: std::marker::PhantomData<&'a &'b ()>,
}

Current output

error: expected one of `,`, `:`, or `>`, found `=`
  --> src/lib.rs:20:39
   |
20 |     pub struct DefaultLifetime<'a, 'b = 'static> {
   |                                    -- ^ expected one of `,`, `:`, or `>`
   |                                    |
   |                                    maybe try to close unmatched angle bracket

Desired output

error: expected one of `,`, `:`, or `>`, found `=`
  --> src/lib.rs:20:39
   |
20 |     pub struct DefaultLifetime<'a, 'b = 'static> {
   |                                       ^^^^^^^^^ lifetime parameters cannot have default values

Rationale and extra context

AFAIK, lifetime generic parameters are not currently allowed to have default values. Attempting to use the "obvious" syntax for setting a default lifetime is an opportunity to teach the user that this is not a supported feature, and certainly shouldn't claim that there's an unmatched angle bracket just because it didn't expect the = during parsing.

This is happening on rustc 1.69.0-nightly (d7948c843 2023-01-26) with the 2021 edition.

Other cases

No response

Anything else?

No response

[compiler-errors]

This shouldn't be too hard to do. Happy to mentor anyone working on this.

---

Implementor of this fix should look here:

rust/compiler/rustc_parse/src/parser/generics.rs

Lines 140 to 145 in 001a77f

	let param = if this.check_lifetime() {
	let lifetime = this.expect_lifetime();
	// Parse lifetime parameter.
	let (colon_span, bounds) = if this.eat(&token::Colon) {
	(Some(this.prev_token.span), this.parse_lt_param_bounds())
	} else {

The recovery should be only done if the thing after the = is a lifetime.

[lenko-d]

@rustbot claim