Phase 14. Fetch + json_decode

Field	Value
MEP	MEP-53 §Phases
Status	LANDED
Started	2026-05-29 (GMT+7)
Landed	2026-05-29 (GMT+7)
Tracking issue	#22609
Tracking PR	#22499
Commit	0e74a0deea

Gate

TestPhase14Fetch walks tests/transpiler3/rust/fixtures/phase14-fetch/ (17 fixtures), starts a local httptest.Server to back the fetch calls, substitutes the test server URL into each fixture's source (replacing the HTTPTEST_URL token), builds the Rust crate, runs the binary, and diffs stdout. Coverage: simple GET, JSON-returning GET piped through json_decode, status-code error handling (4xx and 5xx panic with code 98), chunked-encoding response, large response body.

Lowering decisions

fetch <url> and httpGet(url) both lower to:

let body: String = mochi_runtime::fetch::get(url);

mochi_runtime::fetch::get:

1. Parses the URL (strict: must start with http://, no TLS).
2. Connects via std::net::TcpStream::connect(addr).
3. Sends an HTTP/1.1 GET with Connection: close, User-Agent: mochi-rust/0.1, and a Host header.
4. Reads the response to EOF.
5. Splits at \r\n\r\n to separate headers from body.
6. If status >= 400, raises panic code 98.
7. If Transfer-Encoding: chunked, decodes the chunked body.
8. Returns the body as a String.

No TLS support means no https:// URLs. This is a deliberate choice to keep the runtime small enough to compile under the embedded feature (when std is re-enabled): rustls + ring would add ~50K LOC of crypto code. Mochi programs needing HTTPS are pointed at FFI'ing into libcurl or rolling their own TLS.

json_decode(s) lowers to:

let obj: std::collections::HashMap<String, String> = mochi_runtime::json::decode(s);

mochi_runtime::json::decode is a 90-LOC object decoder. It parses only top-level objects ({...}), coerces all values to strings:

• String values: literal string contents (with \n, \r, \t, \\, \", \/, \b, \f escapes decoded).
• Integer / float values: their lexical representation.
• true / false: "true" / "false".
• null: "".

This is intentionally narrow: Mochi's json_decode contract says "decode top-level object, all values are strings." The 90-LOC decoder matches this contract exactly with zero allocation past the output HashMap.

Files changed

File	Purpose
transpiler3/rust/lower/fetch.go	fetch / httpGet lowering
transpiler3/rust/lower/json.go	json_decode lowering
runtime3/rust/mochi-runtime/src/lib.rs	Add fetch and json modules
transpiler3/rust/build/phase14_test.go	17-fixture gate with httptest.Server
tests/transpiler3/rust/fixtures/phase14-fetch/*.mochi + .out	17 fixtures

Test set

• TestPhase14Fetch/<fixture> for each .mochi in the fixture directory (17 fixtures).

Closeout notes

HTTPTEST_URL substitution was the cleanest way to avoid baking a fixed port into fixtures. The test server picks a random port; the test harness reads the .mochi source as a string, replaces HTTPTEST_URL with the actual server URL, writes the substituted source to a temp file, and builds from there. This keeps fixtures portable across runs and across machines.

Chunked-encoding support landed in the same commit. Several test-server responses use chunked encoding for stream-style emit; the 30-line decode_chunked function handles the standard format ({hex_size}\r\n{data}\r\n repeating until 0\r\n).