Syntactically reject tuple index shorthands in struct patterns to fix a correctness regression

[fmease]

Split out of PR #154492. This fixes a correctness regression introduced in PR #81235 from 2021. Crater was run in my other PR and didn't report any real regressions (#154492 (comment)); a rerun has been issued for a few spurious builds (#154492 (comment)) but I'm certain it won't find anything either.

This is a theoretical breaking change that doesn't need any T-lang input IMHO since it's such a minute, niche and crystal clear bug that's not worth bothering them with (such a decision is not unprecedented). I'm adding it to the compatibility section of the release notes as is customary.

The Reference doesn't need updating since it didn't adopt this bug and thus accurately describes this part of the grammar as it used to be before 2021-02-23 and as it's meant to be.

The majority of the diff is doc comment additions & necessary UI test restructurings.

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[fmease]

This was the culprit.

Prior to the regressing PR #81235 this used to be let fieldname = self.parse_ident()?;.

Changing it (back) to parse_ident_common makes it consistent again with its expression counterpart, cf:

rust/compiler/rustc_parse/src/parser/expr.rs

Line 4046 in 36ba2c7

let ident = this.parse_ident_common(false)?;

View changes since the review

[fmease]

Merged into tests/ui/parser/struct-expr-pat-tuple-index-shorthand.rs.

This test exercises struct expressions while the new struct-expr-pat-tuple-index-shorthand.rs tests both expressions and patterns to make it less likely that they accidentally diverge again.

View changes since the review

[fmease]

Split into

1. tests/ui/structs/struct-pat-unmentioned-tuple-indices.rs
   • the if let E::S { 0: a } = x { //~ ERROR: pattern does not mention field `1` part to be precise
2. tests/ui/tuple/tuple-variant-written-as-empty-struct-variant.rs
   • the S { } => {} //~ ERROR: tuple variant `S` written as struct variant [E0769] part to be precise

The third test case, E::S { 0, 1 } => {} // //~^ ERROR tuple variant `E::S` written as struct variant [E0769] is superseded by the test case in tests/ui/parser/struct-expr-pat-tuple-index-shorthand.rs since that snippet is now a syntax error again and no longer a semantic error.

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[mu001999]

LGTM!

@bors r+ rollup=always

[rust-bors]

📌 Commit 07d015e has been approved by mu001999

It is now in the queue for this repository.