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Add get_or_default method to HashMap. - #8384
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Works like get except it doesn't fail if the key is not present. Instead it returns the passed default element.
erickt
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Aug 8, 2013
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@miloshadzic: Thanks for this! Unfortunately I'm going to vote against this. Instead we can get this same behavior by doing |
miloshadzic
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Aug 8, 2013
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@erickt Hm. I didn't know about |
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Aug 21, 2026
8384: add is quadratic test r=matklad a=matklad bors r+ 🤖 Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>
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Aug 21, 2026
This story begins in rust-lang#8384, where we added a smart test for our syntax highting, which run the algorithm on synthetic files of varying length in order to guesstimate if the complexity is O(N^2) or O(N)-ish. The test turned out to be pretty effective, and flagged rust-lang#9031 as a change that makes syntax highlighting accidentally quadratic. There was much rejoicing, for the time being. Then, lnicola asked an ominous question[1]: "Are we sure that the time is linear right now?" Of course it turned out that our sophisticated non-linearity detector *was* broken, and that our syntax highlighting *was* quadratic. Investigating that, many brave hearts dug deeper and deeper into the guts of rust-analyzer, only to get lost in a maze of traits delegating to traits delegating to macros. Eventually, matklad managed to peel off all layers of abstraction one by one, until almost nothing was left. In fact, the issue was discovered in the very foundation of the rust-analyzer -- in the syntax trees. Worse, it was not a new problem, but rather a well-know, well-understood and event (almost) well-fixed (!) performance bug. The problem lies within `SyntaxNodePtr` type -- a light-weight "address" of a node in a syntax tree [3]. Such pointers are used by rust-analyzer all other the place to record relationships between IR nodes and the original syntax. Internally, the pointer to a syntax node is represented by node's range. To "dereference" the pointer, you traverse the syntax tree from the root, looking for the node with the right range. The inner loop of this search is finding a node's child whose range contains the specified range. This inner loop was implemented by naive linear search over all the children. For wide trees, dereferencing a single `SyntaxNodePtr` was linear. The problem with wide trees though is that they contain a lot of nodes! And dereferencing pointers to all the nodes is quadratic in the size of the file! The solution to this problem is to speed up the children search -- rather than doing a linear lookup, we can use binary search to locate the child with the desired interval. Doing this optimization was one of the motivations (or rather, side effects) of rust-lang#6857. That's why `rowan` grew the useful `child_or_token_at_range` method which does exactly this binary search. But looks like we've never actually switch to this method? Oups. Lesson learned: do not leave broken windows in the fundamental infra. Otherwise, you'll have to repeatedly re-investigate the issue, by digging from the top of the Everest down to the foundation! [1]: https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/.60syntax_highlighting_not_quadratic.60.20failure/near/240811501 [2]: https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/Syntax.20highlighting.20is.20quadratic [3]: https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/Syntax.20highlighting.20is.20quadratic/near/243412392
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Works like get except it doesn't fail if the key is not present. Instead it returns the passed default element.
Please review.