fibers B1.2: Io capability + context.io + blocking impl + Future/async/await/cancel

Threads an `Io` capability onto `Context` exactly like `Allocator`: a
`protocol #inline` whose process-wide default is a stateless `CBlockingIo`
(the mirror of `CAllocator`), installed in `__sx_default_context`.

Library (library/modules/std):
- core.sx: `Io` protocol (spawn_raw / suspend_raw / ready / poll / now_ms /
  arm_timer) + `SpawnOpts` / `PinTarget` / `ParkToken`; `Context` gains an
  `io: Io` field LAST (allocator stays index 0, data stays index 1).
- io.sx (new): `CBlockingIo` + `impl Io` (blocking M:1 semantics — now_ms is
  a real monotonic clock, the rest are no-ops/0; suspend never called);
  `Future($R)` { value; state: FutureState; err: IoErr; park; task; canceled:
  Atomic(bool) } with `Value :: R`; the async ergonomic layer
  `async` / `async_void` / `await` (value-carrying `(R, !IoErr)`) / `cancel`.
  Built with the verified `= ---` + field-assign + `Closure(..$args) -> $R` +
  `..$args` idiom (NON-void $R only — Future(void) is deferred per issue 0150).
- std.sx: re-export the Io surface + the io.sx tail.

Compiler (src/ir):
- protocol.zig `emitDefaultContextGlobal` + comptime_vm.zig
  `materializeDefaultContext`: both materializers of `__sx_default_context`
  now build the inline CBlockingIo->Io vtable (7 words) at the new field.
- stmt.zig `lowerPush`: `push Context.{...}` now INHERITS omitted fields from
  the ambient context (seed the slot from current_ctx_ref, overwrite only the
  literal's named fields) — correct capability-bag semantics, so the partial
  `push Context.{ allocator = X }` sites don't zero a null `io` vtable.
- protocols.zig + lower.zig + error_analysis.zig: record protocol-impl method
  names so the "declared `!` but never errors" lint skips a conforming impl
  whose `!` is dictated by the protocol contract (e.g. Io.suspend_raw).

37 `.ir` snapshots regenerated: layout-only (the Context type now carries the
Io field, shifting type-table numbering); no stdout/stderr/exit changes.

The blocking Io + now_ms + Future/async work when `async` is called with the
receiver passed explicitly; the user-facing UFCS form `context.io.async(...)`
is blocked on a separate UFCS generic-inference bug (filed next).

Suite: 726 ran, 0 failed.

src/ir/comptime_vm.zig

@@ -455,6 +455,24 @@ pub const Vm = struct {
             try self.machine.writeWord(addr + ps, ps, funcRefWord(fid)); // allocator.alloc_fn @ +ptr_size
         if (self.findFuncByName(module, "__thunk_CAllocator_Allocator_dealloc_bytes")) |fid|
             try self.machine.writeWord(addr + 2 * ps, ps, funcRefWord(fid)); // allocator.dealloc_fn @ +2*ptr_size
+        // Context layout: { allocator(3 words), data(1 word), io }. The inline
+        // `Io` value starts at +4*ptr_size: { ctx, fn0..fn5 }, receiver ctx is
+        // null (CBlockingIo stateless), the 6 method func-refs follow in the
+        // protocol's declaration order. Mirrors the `emitDefaultContextGlobal`
+        // global path; absent thunks (std not imported) leave the field zeroed.
+        const io_base: Addr = addr + 4 * ps;
+        const io_methods = [_][]const u8{
+            "__thunk_CBlockingIo_Io_spawn_raw",
+            "__thunk_CBlockingIo_Io_suspend_raw",
+            "__thunk_CBlockingIo_Io_ready",
+            "__thunk_CBlockingIo_Io_poll",
+            "__thunk_CBlockingIo_Io_now_ms",
+            "__thunk_CBlockingIo_Io_arm_timer",
+        };
+        for (io_methods, 0..) |mname, i| {
+            if (self.findFuncByName(module, mname)) |fid|
+                try self.machine.writeWord(io_base + (@as(Addr, @intCast(i)) + 1) * ps, ps, funcRefWord(fid));
+        }
         return addr;
     }
 

src/ir/error_analysis.zig

@@ -174,7 +174,11 @@ pub const ErrorAnalysis = struct {
             const sorted = self.l.alloc.dupe(u32, se.value_ptr.tags.items) catch continue;
             std.mem.sort(u32, sorted, {}, std.sort.asc(u32));
             self.l.inferred_error_sets.put(se.key_ptr.*, sorted) catch {};
-            if (sorted.len == 0 and !std.mem.eql(u8, se.key_ptr.*, "main")) {
+            // Skip `main` (its `!` is the program's top error channel) and any
+            // protocol-impl method (its `!` is dictated by the protocol
+            // contract — e.g. `Io.suspend_raw` — so a non-raising impl body
+            // is not a "drop the `!`" case; see `impl_method_names`).
+            if (sorted.len == 0 and !std.mem.eql(u8, se.key_ptr.*, "main") and !self.l.impl_method_names.contains(se.key_ptr.*)) {
                 if (self.l.diagnostics) |diags| {
                     if (se.value_ptr.rt) |rt| {
                         diags.addFmt(.warn, rt.span, "function '{s}' is declared `!` but never errors — drop the `!`", .{se.key_ptr.*});

src/ir/lower.zig

@@ -391,6 +391,14 @@ pub const Lowering = struct {
     /// escape tags in. Read by `checkEscapeWidening` when a `try` operand is a
     /// closure/fn-type SLOT call (no static fn name). Key = `closureShapeKey`.
     shape_inferred_sets: std.StringHashMap([]const u32),
+    /// Qualified names (`Type.method`) of every explicitly-written protocol
+    /// impl method. A protocol method may be declared `!` (the error channel
+    /// is part of the contract — e.g. `Io.suspend_raw`); a conforming impl
+    /// MUST keep the `!` even when its concrete body never raises, so the
+    /// "declared `!` but never errors — drop the `!`" warning (a free-fn
+    /// linting hint) is a false positive for these. The empty-inferred-set
+    /// warning in `error_analysis.zig` skips names in this set.
+    impl_method_names: std.StringHashMap(void),
 
     pub const ComptimeValue = union(enum) {
         int_val: i64,
@@ -536,6 +544,7 @@ pub const Lowering = struct {
             .comptime_constants = std.StringHashMap(ComptimeValue).init(module.alloc),
             .xx_reentrancy = std.AutoHashMap(u64, void).init(module.alloc),
             .inferred_error_sets = std.StringHashMap([]const u32).init(module.alloc),
+            .impl_method_names = std.StringHashMap(void).init(module.alloc),
             .shape_inferred_sets = std.StringHashMap([]const u32).init(module.alloc),
             .program_index = ProgramIndex.init(module.alloc),
         };

src/ir/lower/protocol.zig

@@ -254,6 +254,8 @@ pub fn emitDefaultContextGlobal(self: *Lowering) void {
     const ctx_ty = tbl.findByName(ctx_name_id) orelse return;
     if (tbl.findByName(tbl.internString("Allocator")) == null) return;
     if (tbl.findByName(tbl.internString("CAllocator")) == null) return;
+    if (tbl.findByName(tbl.internString("Io")) == null) return;
+    if (tbl.findByName(tbl.internString("CBlockingIo")) == null) return;
 
     // Force the CAllocator → Allocator thunks to exist so we can
     // reference them by FuncId in the static initializer.
@@ -267,10 +269,20 @@ pub fn emitDefaultContextGlobal(self: *Lowering) void {
     alloc_fields[1] = .{ .func_ref = thunks[0] };
     alloc_fields[2] = .{ .func_ref = thunks[1] };
 
-    // Context value: { allocator: Allocator, data: *void }
-    const ctx_fields = self.alloc.alloc(inst_mod.ConstantValue, 2) catch return;
+    // Force the CBlockingIo → Io thunks to exist. The Io protocol has 6
+    // methods, so the inline value is { ctx, fn0..fn5 } — 7 pointer words.
+    const io_thunks = self.getOrCreateThunks("Io", "CBlockingIo");
+    if (io_thunks.len < 6) return;
+    const io_fields = self.alloc.alloc(inst_mod.ConstantValue, io_thunks.len + 1) catch return;
+    io_fields[0] = .null_val; // CBlockingIo is stateless → null receiver.
+    for (io_thunks, 0..) |fid, i| io_fields[i + 1] = .{ .func_ref = fid };
+
+    // Context value: { allocator: Allocator, data: *void, io: Io }.
+    // `data` keeps index 1; `io` is appended last.
+    const ctx_fields = self.alloc.alloc(inst_mod.ConstantValue, 3) catch return;
     ctx_fields[0] = .{ .aggregate = alloc_fields };
     ctx_fields[1] = .null_val;
+    ctx_fields[2] = .{ .aggregate = io_fields };
 
     const global_name = "__sx_default_context";
     const global_name_id = tbl.internString(global_name);

src/ir/lower/stmt.zig

@@ -1255,14 +1255,65 @@ pub fn lowerPush(self: *Lowering, ps: *const ast.PushStmt) void {
     const saved_ctx_ref = self.current_ctx_ref;
     defer self.current_ctx_ref = saved_ctx_ref;
 
-    const saved_target = self.target_type;
-    self.target_type = ctx_ty;
-    const ctx_val = self.lowerExpr(ps.context_expr);
-    self.target_type = saved_target;
-
     const slot = self.builder.alloca(ctx_ty);
-    self.builder.store(slot, ctx_val);
-    self.current_ctx_ref = slot;
+
+    // Inherit-omitted semantics: a `push Context.{ ... }` is a CAPABILITY
+    // bag — fields the literal does NOT name are inherited from the ambient
+    // context, not zero-inited. Zero-init would install a NULL `io`/
+    // `allocator` vtable (a latent crash if the field is later used inside
+    // the pushed scope). So seed the new slot from the ambient context,
+    // then overwrite only the fields the literal explicitly names.
+    //
+    // This applies only to a `Context.{...}` struct-literal context-expr;
+    // any other form (e.g. `push some_ctx_value`) keeps the whole-value
+    // store (no field-level merge to do).
+    const lit: ?*const ast.StructLiteral = switch (ps.context_expr.data) {
+        .struct_literal => |*sl| sl,
+        else => null,
+    };
+
+    if (lit != null and self.current_ctx_ref != Ref.none) {
+        // 1. Copy the ambient context into the fresh slot (load + store the
+        //    whole struct), so every omitted field carries its current value.
+        const ambient = self.builder.load(self.current_ctx_ref, ctx_ty);
+        self.builder.store(slot, ambient);
+
+        // 2. Overwrite only the named fields. `push Context.{...}` always
+        //    uses named field-inits (it is a Context literal); a positional
+        //    init has no field name to target, so it is rejected loudly
+        //    rather than silently writing the wrong field.
+        self.current_ctx_ref = slot; // body + field values see the new slot
+        for (lit.?.field_inits) |fi| {
+            const fname = fi.name orelse {
+                if (self.diagnostics) |d|
+                    d.addFmt(.err, ps.context_expr.span, "`push Context.{{...}}` requires named fields (positional init not supported)", .{});
+                continue;
+            };
+            const fl = self.fieldLvaluePtr(slot, ctx_ty, fname) orelse {
+                _ = self.emitFieldError(ctx_ty, fname, ps.context_expr.span);
+                continue;
+            };
+            const saved_target_f = self.target_type;
+            self.target_type = fl.ty;
+            const fval = self.lowerExpr(fi.value);
+            self.target_type = saved_target_f;
+            const fval_ty = self.builder.getRefType(fval);
+            const store_val = if (fval_ty != fl.ty and fval_ty != .void and fl.ty != .void)
+                self.coerceToType(fval, fval_ty, fl.ty)
+            else
+                fval;
+            self.builder.store(fl.ptr, store_val);
+        }
+    } else {
+        // Non-literal context-expr, or no ambient context to inherit from:
+        // lower the whole value and store it (the original behaviour).
+        const saved_target = self.target_type;
+        self.target_type = ctx_ty;
+        const ctx_val = self.lowerExpr(ps.context_expr);
+        self.target_type = saved_target;
+        self.builder.store(slot, ctx_val);
+        self.current_ctx_ref = slot;
+    }
 
     self.lowerBlock(ps.body);
 }

src/ir/protocols.zig

@@ -367,6 +367,11 @@ pub const ProtocolResolver = struct {
                 self.l.program_index.fn_ast_map.put(qualified, method_fd) catch {};
                 self.l.program_index.import_flags.put(qualified, is_imported) catch {};
                 self.l.declareFunction(method_fd, qualified);
+                // Record it as a protocol-impl method so the "declared `!`
+                // but never errors" warning skips it: a `!` on a protocol
+                // method is part of the contract (e.g. `Io.suspend_raw`), so
+                // a conforming impl can't drop it even if its body never raises.
+                self.l.impl_method_names.put(qualified, {}) catch {};
                 impl_methods.put(method_fd.name, {}) catch {};
             }
         }