v0 mangled symbol doesn't match specification nor can it be demangled with rustfilt or c++filt

[tmiasko]

For program below, mangled symbol doesn't match specification nor can it be demangled with rustfilt or c++filt (with rust v0 support):

pub fn f<T>() {}

pub fn g() {
    f::<&dyn FnMut(&u8)>();
}

$  rustc --crate-type=lib -Zsymbol-mangling-version=v0 a.rs && nm ./liba.rlib | c++filt 
a.a.3a1fbbbh-cgu.0.rcgu.o:
0000000000000000 T _RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_hEEG_p6OutputuEL_EB2_
0000000000000000 T a[cdfa367c26312547]::g

Partial derivation

"" <symbol-name>
"_R" [<decimal-number>] <path> [<instantiating-crate>]
"_R" <path> [<instantiating-crate>]
"_RI" <path> {<generic-arg>} "E" [<instantiating-crate>]
"_RIN" <namespace> <path> <identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINv" <path> <identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvC" <identifier> <identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvC" [<disambiguator>] <undisambiguated-identifier> <identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCs" <base-62-number> <undisambiguated-identifier> <identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_" <undisambiguated-identifier> <identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_" <decimal-number> ["_"] <bytes> <identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1" ["_"] <bytes> <identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1" <bytes> <identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a" <identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a" [<disambiguator>] <undisambiguated-identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a" <undisambiguated-identifier> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a" <decimal-number> ["_"] <bytes> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1" ["_"] <bytes> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1" <bytes> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1f" {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1f" <type> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fR" [<lifetime>] <type> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fR" <type> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRD" <dyn-bounds> <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRD" <binder> {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG" <base-62-number> {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_" {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_" <path> {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_I" <path> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_IN" <namespace> <path> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INt" <path> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtN" <namespace> <path> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNt" <path> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtN" <namespace> <path> <identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNt" <path> <identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtC" <identifier> <identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtC" [<disambiguator>] <undisambiguated-identifier> <identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs" <base-62-number> <undisambiguated-identifier> <identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_" <undisambiguated-identifier> <identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_" <decimal-number> ["_"] <bytes> <identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4" ["_"] <bytes> <identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4" <bytes> <identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core" <identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core" [<disambiguator>] <undisambiguated-identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core" <undisambiguated-identifier> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core" <decimal-number> ["_"] <bytes> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3" ["_"] <bytes> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3" <bytes> <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops" <identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops" [<disambiguator>] <undisambiguated-identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops" <undisambiguated-identifier> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops" <decimal-number> ["_"] <bytes> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8" ["_"] <bytes> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8" <bytes> <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function" <identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function" [<disambiguator>] <undisambiguated-identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function" <undisambiguated-identifier> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function" <decimal-number> ["_"] <bytes> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5" ["_"] <bytes> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5" <bytes> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMut" {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMut" <type> {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutT" {<type>} "E" {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTR" [<lifetime>] <type> {<type>} "E" {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTR" <base-62-number> <type> {<type>} "E" {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_" <type> {<type>} "E" {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_" <basic-type> {<type>} "E" {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_h" {<type>} "E" {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_h" "E" {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_hE" {<generic-arg>} "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_hE" "E" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_hEE" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_hEE" {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_hEE" <path> {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_hEEG_p6OutputuEL_EB2_" ?

[tmiasko]

cc @michaelwoerister @eddyb

[tmiasko]

The mangling was changed in #79945 cc @jackh726

[jackh726]

🙄 So, I'm not familiar at all with this (in fact, I didn't even realize we had a spec for symbol mangling until I saw this). But let's see.

I'm just looking at

"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_hEE" {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_hEE" {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_hEE" <path> {<dyn-trait-assoc-binding>} {<dyn-trait>} "E" <lifetime> {<generic-arg>} "E" [<instantiating-crate>]
"_RINvCshGpAVYOtgW1_1a1fRDG_INtNtNtCs4RZURTQcbNt_4core3ops8function5FnMutTRL0_hEEG_p6OutputuEL_EB2_" ?

It seems like where we expect the {<dyn-trait-assoc-binding>} we instead next have G_ which is a Binder. #79945 didn't change that (essentially the mangling of ExistentialPredicate::Projection at all).

Did you actually bisect to #79945 or is that a guess?

[jackh726]

Oh, I see what's happening, I think

[jackh726]

I guess we're lowering dyn FnMut(&u8) to

[
  for<> ExistentialProjection::Trait(Self: FnMut),
  for<> ExistentialProjection::Projection(<Self as FnMut>::Output = &u8),
]

whereas it was

for<> [
  ExistentialProjection::Trait(Self: FnMut),
  ExistentialProjection::Projection(<Self as FnMut>::Output = &u8)
]

before.

This is a bit interesting, because the (slightly desugared) FnMut<Output = &u8> really is only one level of binder. But we could imagine something like &(dyn for<'a> Foo<'a, As=()> + for<'b> Send) (which is completely valid). This gets lowered into

[
  for<'a> ExistentialProjection::Trait(Self: Foo<'a>),
  for<'a> ExistentialProjection::Projection(<Self as Foo<'a>>::As = (),
  for<'b> ExistentialProjection::Trait(Self: Send)
]

I have to think about this. The mangling is very much of the non-lowered version.

There is an easy fix here: just don't print the binders of ExistentialPredicate::Projection here. This assumes that we always print the projections immediately after the relevant trait ref (but that assumption is already made).

I am actually curious if the for<'a> Send bound works out correctly. I'm assuming that when that gets lowered we just drop the for<'a>. But I'm guessing that in mangling we now have an extra G.... So maybe we need to skip printing binders there too.

I may or may not be able to get to this myself in the next couple days, but should be easy enough to fix for anyone interested; I can mentor as needed.

[camelid]

Which is the part of the code that prints the binder?

rust/compiler/rustc_symbol_mangling/src/v0.rs

Lines 497 to 502 in 4a20eb6

	ty::ExistentialPredicate::Projection(projection) => {
	let name = cx.tcx.associated_item(projection.item_def_id).ident;
	cx.push("p");
	cx.push_ident(&name.as_str());
	cx = projection.ty.print(cx)?;
	}

I am potentially interested in implementing this :)

@rustbot claim

[jackh726]

It's the in_binder call. So, it's not really clean, but you would have to only call that if the inner ExistentialPredicate is a Trait.

[eddyb]

There's a single type with multiple bounds, so I would prefer keeping a single binder IMO (tho I can see us having to change the mangling if dyn for<'a> Foo<'a, for<'b> Assoc<'b> = Bar<'a, 'b>> becomes a thing).

You can basically work around having a different implementation, by having the bound-lifetime anonymizing process work for you.

Any ExistentialProjection::Projection's substs would effectively be required to match the ones of ExistentialProjection::Trait, at least after anonymization, which I believe should be trivially true if replicating the binder happens as a desugaring.

cc @nikomatsakis

[camelid]

So does that mean I need a different approach from #83611 (comment)? I haven't interacted with the trait-solving-related code that much, so I would appreciate if you could give me more detail on how I would implement what you're suggesting :)

Alternatively, I could open a PR using @jackh726's approach since it seems like it should be very quick to implement, and then you can comment on what part you would like to be different.

[jackh726]

I think what @eddyb is saying roughly lines up with my comment. @camelid I think just open a PR with my approach.

There's a single type with multiple bounds, so I would prefer keeping a single binder IMO (tho I can see us having to change the mangling if dyn for<'a> Foo<'a, for<'b> Assoc<'b> = Bar<'a, 'b>> becomes a thing).

I imagine this is a "single binder" in the mangling, not in the predicate list.

You can basically work around having a different implementation, by having the bound-lifetime anonymizing process work for you.

Any ExistentialProjection::Projection's substs would effectively be required to match the ones of ExistentialProjection::Trait, at least after anonymization, which I believe should be trivially true if replicating the binder happens as a desugaring.

I don't quite follow this. Maybe this referring to multiple bound traits? Lifetimes bound in Projection should always be the same as those bound under Trait, since it's just a desugaring.