struct_lockstep_tails_raw missing ty::Pat arm, breaks unsizing coercions for pattern types

[SebTardif]

Bug

In compiler/rustc_middle/src/ty/util.rs, struct_tail_raw (line 299) handles ty::Pat(inner, _) by peeling through to the inner type, but struct_lockstep_tails_raw (line 341) does not. This function is used for unsizing coercions. When both source and target are pattern types wrapping different inner types, the lockstep walk stops prematurely at the Pat layer instead of peeling through to compare inner types.

Impact

Unsizing coercions through pattern types fail or produce wrong tail types.

Pattern types are unstable (feature(pattern_types)), limiting impact to nightly users.

Fix

PR #475

[SebTardif]

Upstream context and cross-reference

struct_lockstep_tails_raw has existed since commit 5a95acb by @arielb1 (Ariel Ben-Yehuda, 2015). The ty::Pat variant was added to struct_tail_raw (the single-type version) by @oli-obk in commit 84acfe8 ("Actually create ranged int types in the type system", 2023-02-02), but struct_lockstep_tails_raw (the dual-type version used for unsizing coercions) was not updated.

This is part of the pattern types feature (feature(pattern_types)), which @oli-obk actively maintains. Related upstream PRs on pattern types include rust-lang#142339, rust-lang#138393, rust-lang#137715, rust-lang#136922, and rust-lang#136646.

The asymmetry between struct_tail_raw (has ty::Pat arm at line 299) and struct_lockstep_tails_raw (missing it at line 369) means unsizing coercions through pattern types stop at the Pat layer.

Fix: PR #475