Repro of huge spike tsc memory usage from 5.5.4 -> 5.6.2

[AlexeiDarmin-SA]

🔎 Search Terms

TSC Memory bump from 5.5.4 to 5.6.2 - one issue closed #60166

🕗 Version & Regression Information

• This changed between versions 5.5.4 and 5.6.2
• This changed in commit or PR N/A
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about N/A
• I was unable to test this on prior versions because N/A

⏯ Playground Link

https://github.com/AlexeiDarmin-SA/ts-memory-repro

💻 Code

Reproduce the error The inferred type of this node exceeds the maximum length the compiler will serialize. An explicit type annotation is needed with the following snippet:

function wrap<T>(x: T) { return { top: x, right: x, bottom: x, left: x }; }

// 9–10 wraps is usually enough. If it doesn't fail at 9, try 10 or 11.
export const box =
  wrap(wrap(wrap(wrap(wrap(wrap(wrap(wrap(wrap("x"))))))))); // 9 wraps

Approach taken from Medium article, it's behind a paywall :(

Core idea:

Discover through a simple 51-character example how and why a TypeScript type explodes to 2,000,000 and breaks your build

🙁 Actual behavior

TSC memory usage doubled to 540kb when running the following command in Typescript v5.6.2 in the repro repo.

yarn tsc -p tsconfig.app.json --declaration --emitDeclarationOnly --extendedDiagnostics`

In a real world example for a private repo I saw memory go 5x from 4GB to 20GB.

🙂 Expected behavior

Expected memory usage to remain unaffected when running the command at 270KB

Additional information about the issue

One other thread discussed this and was closed due to lacking repro steps: #60166

[RyanCavanaugh]

This is a quadratic function in terms of the size of anonymous type it's making, the .d.ts file from the example is 4.4 MB. A few more wrap calls and tsc is not going to have enough disk space in the universe to write to.

What are you expecting to happen? "TypeScript can always produce a file no matter how big" isn't a reasonable invariant

[AlexeiDarmin-SA]

TypeScript can always produce a file no matter how big" isn't a reasonable invariant

Agreed, the goal isn't to be able to produce a file of any size.

What are you expecting to happen?

I'm expecting Typescript to not use 2 to 5 times more memory to accomplish the same task with the same type definition between versions 5.5.4 and 5.6.2. The mega type provided is for illustrative purposes.

For example, even when I remove two instances of wrap from the repro such that the maximum compiler error no longer shows.

v5.6.2 still uses more memory than v5.5.4 to generate the main.d.ts

v5.5.4 -> 124145K
v5.6.2 -> 183652K

[RyanCavanaugh]

If you're saying something has gone up 2x-5x, I need a repro that goes up 2x-5x, not 50%. An increase during the printback of an enormous anonymous type may or may not be related to whatever is causing the 2x-5x issue.

[RyanCavanaugh]

The discontinuity identified there bisects to #59647, which was needed for correctness of comment emit. There's not much we can do with memory required to do things correctly.

[AlexeiDarmin-SA]

Appreciate the bisect to #59647. I can't share the repo with the 5x memory spike since it's private.

From a quick read, the added traversal in setTextRange:

let original = range.original;
while (original && original !== location) {
    original = original.original;

seems like it could lead to multiplicative work with codebases that have wide unions or big anonymous shapes which aligns with what we're seeing in this repro.

Curious if there might be a way to shortcut that traversal or provide an opt-in flag for the stricter comment emit behavior, so projects that don’t need it can trade correctness for performance?

[RyanCavanaugh]

If you're printing an enormous anonymous type, maybe just don't? Adding a type annotation should immediately bring down the number to a reasonable level, e.g. if you write

type Wrapped<T> = { top: T, right: T, bottom: T, left: T };

function wrap<T>(x: T): Wrapped<T> { return { top: x, right: x, bottom: x, left: x }; }

then the memory usage drops to 60 MB and the .d.ts file goes from 4.4 MB to 225 bytes.

[AlexeiDarmin-SA]

I've started doing that for the large private repo.

There, unlike in this repro, are many medium & large sized anonymous types so it will take me some time to identify and annotate each one.

@RyanCavanaugh thank you for the quick assist. I'll circle back here in a few days after I've taken a crack at it.