fold instead of obliterating args

[BoxyUwU]

Fixes #105608

we call const_eval_resolve on the following constant:

def: playground::{impl#0}::and::{constant#0},
substs: [
  ConstKind::Unevaluated {
    def: playground::{impl#0}::and::{constant#0},
    substs: [
      ConstKind::Value(0x0),
      _,
    ]
  }
  _,
],

when expanded out to ConstKind::Expr there are no infer vars so we attempt to evaluate it after replacing infer vars with garbage, however the current logic for replacing with garbage replaces the whole arg containing the infer var rather than just the infer var. This means that after garbage replacement has occured we attempt to evaluate:

def: playground::{impl#0}::and::{constant#0},
substs: [
  PLACEHOLDER,
  PLACEHOLDER,
],

Which then leads to ctfe being unable to evaluate the const. With this PR we attempt to evaluate:

def: playground::{impl#0}::and::{constant#0},
substs: [
  ConstKind::Unevaluated {
    def: playground::{impl#0}::and::{constant#0},
    substs: [
      ConstKind::Value(0x0),
      PLACEHOLDER,
    ]
  }
  PLACEHOLDER,
],

which ctfe can handle.

I am not entirely sure why this function is supposed to replace params with placeholders rather than just inference vars 🤔

[rustbot]

r? @wesleywiser

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[wesleywiser]

Thanks @BoxyUwU!

@bors r+

[bors]

📌 Commit 5573485 has been approved by wesleywiser

It is now in the queue for this repository.