Allow .. to be parsed as let initializer

[RedDocMD]

.. and ..= are valid expressions, however when used in a let statement
it is not parsed.
Fixes #105634

[rustbot]

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[Noratrieb]

Thank you for your pull request! A small tip: you can write "Fixes #105634" in your description to clarify that you're fixing the issue and also because GitHub will then automatically close the issue when the PR is merged.

[lcnr]

r? compiler

[CAD97]

My immediate expectation would be that parse_prefix_expr includes parse_prefix_range_expr. Are there any cases where we use parse_prefix_expr and we want prefix range expressions to be an error? If yes, make sure there's a test for that; if no, then it's probably better to move the range separator check/parse inside parse_prefix_expr.

I've not looked at this code much, but the reason it might matter is for other operator precedence reasons; e.g. what's the correct result of parsing +..1 or 1+..1.

[RedDocMD]

I think this separation of prefix range parsing from other prefix expr parsing is done to prevent expressions such as -..1 or !..1 from being parsed. Other unary operators can be stacked (may not be typeable), but you cannot have .. or ..= after a unary operator.

[cjgillot]

Thanks!
@bors r+

[bors]

📌 Commit 4af9e6a has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4af9e6a with merge 300aa90...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 300aa90 to master...

[rust-timer]

Finished benchmarking commit (300aa90): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-0.5%	[-1.0%, -0.4%]	12
All ❌✅ (primary)	-0.4%	[-0.4%, -0.4%]	1

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.3%	[-1.3%, -1.3%]	1