Rollup merge of #82789 - csmoe:issue-82772, r=estebank

Rollup merge of #82789 - csmoe:issue-82772, r=estebank

Get with field index from pattern slice instead of directly indexing

Closes #82772
r? ``@estebank``

https://github.com/rust-lang/rust/pull/82789#issuecomment-796921977
> ``@estebank`` So the real cause is we only generate single pattern for Box here
https://github.com/csmoe/rust/blob/615b03aeaa8ce9819de7828740ab3cd7def4fa76/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs#L1130-L1132
But in the replacing function, it tries to index on the 1-length pattern slice with field 1, thus out of bounds.
https://github.com/csmoe/rust/blob/615b03aeaa8ce9819de7828740ab3cd7def4fa76/compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs#L1346