Don't lex the entire input immediately #1110

This is in preparation for an upcoming change where lexing will be performed lazily rather than eagerly. In this situation lexing is possibly done during a `peek` operation which means that errors can happen at that time. This commit purely updates the related methods to return `Result<Option<T>>` instead of `Option<T>`, but it doesn't actually introduce any functional change other than that. The return value from these methods is always `Ok(...)` at this time, and a future commit will change that to possibly being an error. This is split out from the future change since it's a fair amount of churn with lots of new `?` operators popping up everywhere.

This commit fixes a few issues with annotation-related support in the wasm text format: * The `@producers` section is not parsed and recognized for components where previously it was ignored. * Similarly the `@name` annotation is not properly recognized for components instead of being ignored in a few places. * Registration of annotations has been moved to top-level parsers to avoid adding and re-adding entries to the known annotations map.

This commit is a change to the internals of parsing for wasm text files. Previously the entire input would be lex'd and stored into an array for parsing to access. This is, however, the largest contributing factor to the peak memory usage reported in bytecodealliance#1095. Tokens are only used a handful of times so buffering the entire file in-memory is quite wasteful. This change was tricky to apply, however, because the original rationale for lexing in this manner was performance related. The recursive-descent style `Parser` trait encourages `peek`-ing tokens and will often involve attempting a parse only to later unwind and try something else instead. This means that each individual token, especially whitespace and comments, would otherwise naively be lexed many times. For example if the buffer of all tokens is "simply" removed some instrumented analysis showed that over half of all tokens in the input file were lexed more than once. This means that simply removing the buffer resulted in a performance regression. This performance regression is in some sense inherently not addressable with a lazy-lexing strategy. I implemented a fixed-width cache of the latest tokens lex'd but it still didn't perform as well as caching all tokens. I think this is because lexing is quite fast and adding the layer of the cache spent a lot of time in checking and managing the cache. While this performance regression may not be 100% fixable though I've settled on a strategy that's a bit more of a half-measure. The general idea for the `Parser` is now that it stores the current position in the file plus the next "significant" token at the same time. Here "significant" means not-whitespace and not-comments for example. This enables the parser to always know the next token having pre-skipped whitespace and comments. Thus any single-token "peek" operations don't actually need to do any lexing, they can instead look at the current token and decide what to do based on that. This is enabled with a few new `Cursor::peek_*` methods to avoid generating the next `Cursor` which would otherwise require a lexing operation. Overall this means that the majority of tokens in the case of bytecodealliance#1095 are lexed only once. There's still ~10% of tokens that are lexed 2+ times, but the performance numbers are before this commit parsing that file took 7.6s with 4G of memory, and after this commit it takes 7.9s with 2G of memory. This means that for a 4% regression in time we're reducing memory usage by half.

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Don't lex the entire input immediately #1110

Don't lex the entire input immediately #1110

Commits on Jul 8, 2023