deref_patterns: let string and byte string literal patterns peel references and smart pointers before matching

[dianne]

This follows up on #140028. Together, they allow using string and byte string literal patterns to match on smart pointers when deref_patterns is enabled. In particular, string literals can now match on String, subsuming the functionality of the string_deref_patterns feature.

More generally, this works by letting literals peel references (and smart pointers) before matching, similar to most other patterns, providing an answer to #44849. Though it's only partially implemented at this point: this doesn't yet let named const patterns peel before matching. The peeling logic is general enough to support named consts, but the typing rules for named const patterns would need adjustments to feel consistent (e.g. arrays would need rules to be usable as slices, and const STR: &'static str wouldn't be able to match on a String unless a rule was added to let it be used where a str is expected, similar to what #140028 did for literals).

This also allows string and byte string patterns to match on mutable references, following up on #140028 (comment). Rather than forward the mutability of the scrutinee to literal patterns, I've opted to peel &muts from the scrutinee. From a design point of view, this makes the behavior consistent with what would be expected from deref coercions using the methodology in the next paragraph. From a diagnostics point of view, this avoids labeling string and byte string patterns as "mutable references", which I think could be confusing. See byte-string-type-errors.rs for how the diagnostics look.

At a high level, the peeling logic implemented here tries to mimic how deref coercions work for expressions: we peel references (and smart pointers) from the scrutinee until the pattern can match against it, and no more. This is primarily tested by const-pats-do-not-mislead-inference.rs. To illustrate the connection, I wasn't sure if this made sense to include in the test file, but I've translated those tests to make sure they give the same inference results as deref coercions: (playground). In each case, a reference to the scrutinee is coerced to have the type of the pattern (under a reference).

Tracking issue for deref patterns: #87121

r? @oli-obk
cc @Nadrieril

[oli-obk]

@bors r+

[bors]

📌 Commit 7e4f6d3 has been approved by oli-obk

It is now in the queue for this repository.