Rollup merge of #105611 - BoxyUwU:more_granular_placeholderification, r=wesleywiser

fold instead of obliterating args

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 :thinking:

0 files changed, 0 insertions, 0 deletions