Convert Buffers and DataViews to BigInt through their own text

BigInt of byte storage joined the elements with "," for every kind, so a
Buffer, whose text is its UTF-8 decoding, and a DataView, whose text is
"[object DataView]", converted as typed arrays. BigInt now parses the
value's ToString, which the runtime value decides, as String does. This also
covers a Buffer held in a Uint8Array slot.

Corpus 7403 adds Buffers, a DataView, and a Buffer in a typed array slot.

Refs #1647

packages/compiler/src/frontend/lowering/lower-calls.ts

@@ -2400,15 +2400,15 @@ function lowerBigIntConstructorValue(lowerer: Lowerer, arg: IrExpr, loc: SrcLoc)
     return { kind: "libCall", fn: "bigint.parse", args: [arg], type: BIGINT_T, loc };
   if (arg.type.kind === "f64")
     return { kind: "libCall", fn: "bigint.fromF64", args: [arg], type: BIGINT_T, loc };
-  // A typed array converts through ToPrimitive to its elements joined with
-  // ",", which BigInt then parses: one element reads as that element, none
-  // as 0n, and several throw the SyntaxError for the joined text.
+  // Byte storage converts through ToPrimitive, which BigInt then parses: a
+  // typed array's text is its elements joined with "," (one element reads
+  // as that element, none as 0n, several throw the SyntaxError for the
+  // joined text), a Buffer's is its UTF-8 decoding and a DataView's is
+  // "[object DataView]". The runtime value tells them apart, as for String.
   if (arg.type.kind === "bytes") {
     const text: IrExpr = {
-      kind: "bytesIntrinsic",
-      method: "join",
-      receiver: arg,
-      args: [{ kind: "strLit", value: ",", type: STRING, loc }],
+      kind: "toString",
+      operand: { kind: "dynFrom", value: arg, type: DYN, loc },
       type: STRING,
       loc,
     };

tests/corpus/7403-bigint-of-typed-arrays.mjs

@@ -2,7 +2,9 @@
 // "," and parses that text, as in Node: one element reads as its value,
 // an empty array as 0n, and anything else throws the SyntaxError for the
 // joined text. Every element kind, negative and clamped values, float
-// formatting, views and the results in arithmetic.
+// formatting, views and the results in arithmetic. A Buffer converts
+// through its UTF-8 text instead, also when a typed array slot holds it,
+// and a DataView through "[object DataView]".
 
 const one = [
   new Int8Array([-5]),
@@ -44,3 +46,17 @@ for (let i = 0; i < 100; i++) {
   total += BigInt(window) * 3n;
 }
 console.log("window", BigInt(window), total, BigInt(bytes.subarray(2)) ** 2n);
+
+function convert(label, make) {
+  try {
+    console.log(label, BigInt(make()));
+  } catch (e) {
+    console.log(label, e instanceof SyntaxError, e.message);
+  }
+}
+convert("buffer", () => Buffer.from("12"));
+convert("spaced buffer", () => Buffer.from(" -7 "));
+convert("empty buffer", () => Buffer.alloc(0));
+convert("fraction buffer", () => Buffer.from("1.5"));
+convert("data view", () => new DataView(new ArrayBuffer(2)));
+for (const view of [new Uint8Array([5]), Buffer.from("64")]) console.log("slot", BigInt(view));