[RFC] rustc_parse: improve the error diagnostic for "missing let in let chain"

[Unique-Usman]

No description provided.

[rustbot]

r? @chenyukang

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[Unique-Usman]

My approach seems not to be perfect and I have been stucked on this for a while. This is the issue this is supposed to fix - >#117977

[Unique-Usman]

@estebank

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

fmt check
fmt: checked 6657 files
tidy check
tidy [rustdoc_json (src)]: `rustdoc-json-types` modified, checking format version
tidy [fluent_alphabetical (compiler)]: /checkout/compiler/rustc_parse/messages.ftl: message `parse_zero_chars` appears before `parse_let_chain_missing_let`, but is alphabetically later than it. Run `./x.py test tidy --bless` to sort the file correctly
tidy [fluent_alphabetical (compiler)]: /checkout/compiler/rustc_parse/messages.ftl: message `parse_let_chain_start` appears before `parse_expected_let_found_assign`, but is alphabetically later than it. Run `./x.py test tidy --bless` to sort the file correctly
tidy [fluent_alphabetical (compiler)]: /checkout/compiler/rustc_parse/messages.ftl: message `parse_expected_let_found_assign` appears before `parse_add_let`, but is alphabetically later than it. Run `./x.py test tidy --bless` to sort the file correctly
tidy [fluent_alphabetical (compiler)]: FAIL
tidy: Skipping binary file check, read-only filesystem
tidy: The following check failed: fluent_alphabetical (compiler)
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 yarn` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1358:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:00:50
  local time: Thu Jan 22 15:10:18 UTC 2026
  network time: Thu, 22 Jan 2026 15:10:19 GMT
##[error]Process completed with exit code 1.

[rust-bors]

☔ The latest upstream changes (presumably #151701) made this pull request unmergeable. Please resolve the merge conflicts.