fix: add streaming DMMF binary command to bypass V8 and WASM limits

[chris-tophers]

Summary

Alternative approach to #5757 for bypassing V8's string length limit on large DMMF.

Adds a get-dmmf subcommand to the prisma-fmt CLI binary that streams DMMF JSON directly to stdout via serde_json::to_writer(). Unlike the buffered WASM approach, this has no memory ceiling — the native binary streams incrementally with zero intermediate allocation.

Root Cause

dmmf_json_from_validated_schema() uses serde_json::to_string(), producing a single Rust String. When wasm-bindgen converts this to a JS string, V8 rejects it if the string exceeds 0x1fffffe8 characters (~536MB). The WASM buffered approach (#5757) works around this by returning Vec<u8> chunks, but WASM32 linear memory is capped at ~4GB, giving a practical ceiling of ~1.5-2GB DMMF.

The binary streaming approach eliminates both limits: serde_json::to_writer() writes JSON tokens incrementally to stdout with only 1x peak memory (the in-memory DMMF struct — no serialized buffer).

Changes

File	Change
query-compiler/dmmf/src/lib.rs	Add dmmf_json_to_writer() using serde_json::to_writer()
prisma-fmt/src/get_dmmf.rs	Add get_dmmf_to_writer() that validates schema + streams
prisma-fmt/src/lib.rs	Export get_dmmf_to_writer()
prisma-fmt/src/main.rs	Add GetDmmf CLI variant: reads JSON params from stdin, streams DMMF to stdout

+47 lines, 4 files changed. No existing behavior modified. No new dependencies.

Comparison: WASM Buffered vs Binary Streaming

Aspect	WASM Buffered (#5757)	Binary Streaming (this PR)
Memory ceiling	~1.5-2GB (WASM32 limit)	Unlimited
Peak memory	2x DMMF (struct + Vec<u8>)	1x (struct only, streams)
New binary shipped	No	Yes (prisma-fmt)
Rust complexity	Medium (Mutex, chunks)	Low (to_writer + CLI cmd)
Lines added (Rust)	~100	~47
Works today	Yes (WASM-only path)	Needs prisma-fmt in npm or PRISMA_FMT_BINARY env var

Usage

# Pipe schema params to stdin, get DMMF JSON on stdout
echo '{"prismaSchema":"datasource db { ... }","noColor":true}' | prisma-fmt get-dmmf

The TypeScript companion PR detects the V8 error and spawns this binary as a fallback, stream-parsing stdout with @streamparser/json.

Companion PR

TypeScript fallback: prisma/prisma#29149 (adds binary streaming fallback in TypeScript)

Notes

• The Prisma team can choose either approach (fix: add buffered DMMF API to bypass V8 string length limit #5757 or this), or combine both (WASM buffered for moderate sizes, binary streaming for extreme sizes)
• prisma-fmt is already compiled as a native binary in CI — it just isn't shipped via npm today
• The PRISMA_FMT_BINARY env var allows users to point to a custom-built binary immediately

Fixes: prisma/prisma#29111