perf(core): reduce snapshot deserialization overhead

[bartlomieju]

Summary

Reduces V8 snapshot sidecar deserialization overhead at startup:

• Eliminate ~2500 Url::parse calls: Module requests (ModuleRequest containing ModuleSpecifier / url::Url) were serialized into the snapshot but never used at runtime — snapshotted modules are already instantiated and linked. Now cleared before serialization.

• Zero-copy string deserialization: Introduced snapshot-specific types (ModuleInfoSnapshot, SymbolicModuleSnapshot) that use Cow<'a, str> instead of FastString. Bincode borrows strings directly from the &'static snapshot buffer. cow_to_fast_string converts Cow::Borrowed to FastString::Static via transmute (safe because snapshot buffers are always Box::leak'd to 'static). Eliminates ~560 heap allocations.

• Skip redundant ASCII validation: Uses create_external_onebyte_const_unchecked in externalize_sources for snapshot and extension sources that were already validated during snapshot creation / transpilation.

Context

Profiling showed Url::parse consuming ~12% of sidecar deserialization time during startup. The sidecar is 18KB vs 815KB for the V8 snapshot itself, but every byte of it was being actively processed (parsing URLs, allocating strings). These changes make sidecar deserialization nearly zero-cost.

Test plan

• deno run, deno eval work correctly
• Node.js compat (node:path, Buffer, process) works
• cargo test -p deno_core --lib passes (402 passed, 1 pre-existing SIGSEGV in es_snapshot test unrelated to this change)

🤖 Generated with Claude Code

[nathanwhit]

Does this actually improve startup time?

Also this segfault

cargo test -p deno_core --lib passes (402 passed, 1 pre-existing SIGSEGV in es_snapshot test unrelated to this change)

is because you need to use cargo nextest for running the deno core tests.

[bartlomieju]

Does this actually improve startup time?

TBH I'm not sure anymore; the first few times I captured flamegraphs I saw quite a few percent spent in Url::parse and other op setup stuff but I can't reproduce it anymore. We can just close this one for now.