What is considered a "read"?

[ehuss]

#2067 added a chapter on divergence with the rule expr.block.diverging that says:

A block is considered to be diverging if all reachable control flow paths contain a diverging expression, unless that expression is a place expression that is not read from.

But I don't think the reference has a clear definition of what it means to "read" from a place expression. What are the rules for what is considered a "read"? Is this unique to divergence checking?

Closure capture has has a bunch of rules of what it considers a "read" (or more often, what is not considered a read). I believe there are slight differences for various reasons. For example, a reference does not seem to be read for the purpose of divergence, but for the purpose of closure capture it does seem to be considered a read and is captured. There are various things around fake reads that I have forgotten how they work. I don't know if the rules are fundamentally different, or if there is some commonality.

I'll take a wild guess as to what is not considered a read based on expr_guaranteed_to_constitute_read_for_never:

• A borrow expression.
• The left-hand side of an assignment.
• The base of a field projection that doesn't itself constitute a read.
• The scrutinee of a match expression where at least one arm's pattern does not force a read.
• The initializer of a let statement where the pattern does not force a read.
• The scrutinee of a let condition where the pattern does not force a read.

(Of course non-place is always "read".)

I don't know if there are other considerations or how things like discriminants get involved.

It could also include details on which patterns force a read, which I think is is_guaranteed_to_constitute_read_for_never, which is more or less the following aren't reads:

• _ wildcard
• | or patterns where at least one alternative doesn't force a read

See also some recent things:

• Document how closure capturing interacts with discriminant reads #1837 --- Changed what a closure capture considers a "read", particularly around discriminants.
• Make operational semantics of pattern matching independent of crate and module rust#150681 --- Change for enum discriminant read.

cc @jackh726