diff --git a/doc/langref.html.in b/doc/langref.html.in
index 2b09ca81bd..9123b1df74 100644
--- a/doc/langref.html.in
+++ b/doc/langref.html.in
@@ -5663,7 +5663,7 @@ ErrorSetExpr = (PrefixOpExpression "!" PrefixOpExpression) | PrefixOpExpression
 
 BlockOrExpression = Block | Expression
 
-Expression = TryExpression | ReturnExpression | BreakExpression | AssignmentExpression
+Expression = TryExpression | ReturnExpression | BreakExpression | AssignmentExpression | CancelExpression
 
 AsmExpression = "asm" option("volatile") "(" String option(AsmOutput) ")"
 
@@ -5707,6 +5707,8 @@ TryExpression = "try" Expression
 
 BreakExpression = "break" option(":" Symbol) option(Expression)
 
+CancelExpression = "cancel" Expression;
+
 Defer(body) = ("defer" | "deferror") body
 
 IfExpression(body) = "if" "(" Expression ")" body option("else" BlockExpression(body))
@@ -5745,7 +5747,7 @@ MultiplyOperator = "||" | "*" | "/" | "%" | "**" | "*%"
 
 PrefixOpExpression = PrefixOp ErrorSetExpr | SuffixOpExpression
 
-SuffixOpExpression = PrimaryExpression option(FnCallExpression | ArrayAccessExpression | FieldAccessExpression | SliceExpression)
+SuffixOpExpression = ("async" option("(" Expression ")") PrimaryExpression FnCallExpression) | PrimaryExpression option(FnCallExpression | ArrayAccessExpression | FieldAccessExpression | SliceExpression)
 
 FieldAccessExpression = "." Symbol
 
diff --git a/src/all_types.hpp b/src/all_types.hpp
index 04b781a598..694a2f64d9 100644
--- a/src/all_types.hpp
+++ b/src/all_types.hpp
@@ -393,6 +393,7 @@ enum NodeType {
     NodeTypeIfErrorExpr,
     NodeTypeTestExpr,
     NodeTypeErrorSetDecl,
+    NodeTypeCancel,
 };
 
 struct AstNodeRoot {
@@ -567,6 +568,8 @@ struct AstNodeFnCallExpr {
     AstNode *fn_ref_expr;
     ZigList<AstNode *> params;
     bool is_builtin;
+    bool is_async;
+    AstNode *async_allocator;
 };
 
 struct AstNodeArrayAccessExpr {
@@ -829,6 +832,10 @@ struct AstNodeBreakExpr {
     AstNode *expr; // may be null
 };
 
+struct AstNodeCancelExpr {
+    AstNode *expr;
+};
+
 struct AstNodeContinueExpr {
     Buf *name;
 };
@@ -900,6 +907,7 @@ struct AstNode {
         AstNodeErrorType error_type;
         AstNodeVarLiteral var_literal;
         AstNodeErrorSetDecl err_set_decl;
+        AstNodeCancelExpr cancel_expr;
     } data;
 };
 
@@ -1495,6 +1503,7 @@ struct CodeGen {
         TypeTableEntry *entry_var;
         TypeTableEntry *entry_global_error_set;
         TypeTableEntry *entry_arg_tuple;
+        TypeTableEntry *entry_promise;
     } builtin_types;
 
     EmitFileType emit_file_type;
@@ -1939,6 +1948,7 @@ enum IrInstructionId {
     IrInstructionIdExport,
     IrInstructionIdErrorReturnTrace,
     IrInstructionIdErrorUnion,
+    IrInstructionIdCancel,
 };
 
 struct IrInstruction {
@@ -2776,6 +2786,12 @@ struct IrInstructionErrorUnion {
     IrInstruction *payload;
 };
 
+struct IrInstructionCancel {
+    IrInstruction base;
+
+    IrInstruction *target;
+};
+
 static const size_t slice_ptr_index = 0;
 static const size_t slice_len_index = 1;
 
diff --git a/src/analyze.cpp b/src/analyze.cpp
index c16a5d462a..4545db9d78 100644
--- a/src/analyze.cpp
+++ b/src/analyze.cpp
@@ -3117,6 +3117,7 @@ void scan_decls(CodeGen *g, ScopeDecls *decls_scope, AstNode *node) {
         case NodeTypeIfErrorExpr:
         case NodeTypeTestExpr:
         case NodeTypeErrorSetDecl:
+        case NodeTypeCancel:
             zig_unreachable();
     }
 }
diff --git a/src/ast_render.cpp b/src/ast_render.cpp
index aed4b3e6db..eec4b996a0 100644
--- a/src/ast_render.cpp
+++ b/src/ast_render.cpp
@@ -244,6 +244,8 @@ static const char *node_type_str(NodeType node_type) {
             return "TestExpr";
         case NodeTypeErrorSetDecl:
             return "ErrorSetDecl";
+        case NodeTypeCancel:
+            return "Cancel";
     }
     zig_unreachable();
 }
@@ -1037,6 +1039,12 @@ static void render_node_extra(AstRender *ar, AstNode *node, bool grouped) {
                 fprintf(ar->f, "}");
                 break;
             }
+        case NodeTypeCancel:
+            {
+                fprintf(ar->f, "cancel ");
+                render_node_grouped(ar, node->data.cancel_expr.expr);
+                break;
+            }
         case NodeTypeFnDecl:
         case NodeTypeParamDecl:
         case NodeTypeTestDecl:
diff --git a/src/codegen.cpp b/src/codegen.cpp
index 15648cbdec..4d9b0279d0 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -3088,6 +3088,10 @@ static LLVMValueRef ir_render_error_return_trace(CodeGen *g, IrExecutable *execu
     return g->cur_err_ret_trace_val;
 }
 
+static LLVMValueRef ir_render_cancel(CodeGen *g, IrExecutable *executable, IrInstructionCancel *instruction) {
+    zig_panic("TODO ir_render_cancel");
+}
+
 static LLVMAtomicOrdering to_LLVMAtomicOrdering(AtomicOrder atomic_order) {
     switch (atomic_order) {
         case AtomicOrderUnordered: return LLVMAtomicOrderingUnordered;
@@ -3862,6 +3866,8 @@ static LLVMValueRef ir_render_instruction(CodeGen *g, IrExecutable *executable,
             return ir_render_align_cast(g, executable, (IrInstructionAlignCast *)instruction);
         case IrInstructionIdErrorReturnTrace:
             return ir_render_error_return_trace(g, executable, (IrInstructionErrorReturnTrace *)instruction);
+        case IrInstructionIdCancel:
+            return ir_render_cancel(g, executable, (IrInstructionCancel *)instruction);
     }
     zig_unreachable();
 }
@@ -5271,6 +5277,16 @@ static void define_builtin_types(CodeGen *g) {
 
         g->primitive_type_table.put(&entry->name, entry);
     }
+    {
+        TypeTableEntry *u8_ptr_type = get_pointer_to_type(g, g->builtin_types.entry_u8, false);
+        TypeTableEntry *entry = new_type_table_entry(TypeTableEntryIdVoid);
+        entry->type_ref = u8_ptr_type->type_ref;
+        entry->zero_bits = false;
+        buf_init_from_str(&entry->name, "promise");
+        entry->di_type = u8_ptr_type->di_type;
+        g->builtin_types.entry_promise = entry;
+        g->primitive_type_table.put(&entry->name, entry);
+    }
 
 }
 
diff --git a/src/ir.cpp b/src/ir.cpp
index 7eac9e4d23..d220efde79 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -637,6 +637,10 @@ static constexpr IrInstructionId ir_instruction_id(IrInstructionErrorUnion *) {
     return IrInstructionIdErrorUnion;
 }
 
+static constexpr IrInstructionId ir_instruction_id(IrInstructionCancel *) {
+    return IrInstructionIdCancel;
+}
+
 template<typename T>
 static T *ir_create_instruction(IrBuilder *irb, Scope *scope, AstNode *source_node) {
     T *special_instruction = allocate<T>(1);
@@ -2396,6 +2400,17 @@ static IrInstruction *ir_build_error_union(IrBuilder *irb, Scope *scope, AstNode
     return &instruction->base;
 }
 
+static IrInstruction *ir_build_cancel(IrBuilder *irb, Scope *scope, AstNode *source_node,
+        IrInstruction *target)
+{
+    IrInstructionCancel *instruction = ir_build_instruction<IrInstructionCancel>(irb, scope, source_node);
+    instruction->target = target;
+
+    ir_ref_instruction(target, irb->current_basic_block);
+
+    return &instruction->base;
+}
+
 static void ir_count_defers(IrBuilder *irb, Scope *inner_scope, Scope *outer_scope, size_t *results) {
     results[ReturnKindUnconditional] = 0;
     results[ReturnKindError] = 0;
@@ -3873,6 +3888,10 @@ static IrInstruction *ir_gen_fn_call(IrBuilder *irb, Scope *scope, AstNode *node
             return args[i];
     }
 
+    if (node->data.fn_call_expr.is_async) {
+        zig_panic("TODO ir_gen_fn_call for async fn calls");
+    }
+
     return ir_build_call(irb, scope, node, nullptr, fn_ref, arg_count, args, false, FnInlineAuto);
 }
 
@@ -5598,6 +5617,16 @@ static IrInstruction *ir_gen_fn_proto(IrBuilder *irb, Scope *parent_scope, AstNo
     return ir_build_fn_proto(irb, parent_scope, node, param_types, align_value, return_type, is_var_args);
 }
 
+static IrInstruction *ir_gen_cancel(IrBuilder *irb, Scope *parent_scope, AstNode *node) {
+    assert(node->type == NodeTypeCancel);
+
+    IrInstruction *target_inst = ir_gen_node(irb, node->data.cancel_expr.expr, parent_scope);
+    if (target_inst == irb->codegen->invalid_instruction)
+        return irb->codegen->invalid_instruction;
+
+    return ir_build_cancel(irb, parent_scope, node, target_inst);
+}
+
 static IrInstruction *ir_gen_node_raw(IrBuilder *irb, AstNode *node, Scope *scope,
         LVal lval)
 {
@@ -5694,6 +5723,8 @@ static IrInstruction *ir_gen_node_raw(IrBuilder *irb, AstNode *node, Scope *scop
             return ir_lval_wrap(irb, scope, ir_gen_fn_proto(irb, scope, node), lval);
         case NodeTypeErrorSetDecl:
             return ir_lval_wrap(irb, scope, ir_gen_err_set_decl(irb, scope, node), lval);
+        case NodeTypeCancel:
+            return ir_lval_wrap(irb, scope, ir_gen_cancel(irb, scope, node), lval);
     }
     zig_unreachable();
 }
@@ -16459,6 +16490,17 @@ static TypeTableEntry *ir_analyze_instruction_tag_type(IrAnalyze *ira, IrInstruc
     }
 }
 
+static TypeTableEntry *ir_analyze_instruction_cancel(IrAnalyze *ira, IrInstructionCancel *instruction) {
+    IrInstruction *casted_target = ir_implicit_cast(ira, instruction->target->other, ira->codegen->builtin_types.entry_promise);
+    if (type_is_invalid(casted_target->value.type))
+        return ira->codegen->builtin_types.entry_invalid;
+
+    IrInstruction *result = ir_build_cancel(&ira->new_irb, instruction->base.scope, instruction->base.source_node, casted_target);
+    result->value.type = casted_target->value.type;
+    ir_link_new_instruction(result, &instruction->base);
+    return result->value.type;
+}
+
 static TypeTableEntry *ir_analyze_instruction_nocast(IrAnalyze *ira, IrInstruction *instruction) {
     switch (instruction->id) {
         case IrInstructionIdInvalid:
@@ -16661,6 +16703,8 @@ static TypeTableEntry *ir_analyze_instruction_nocast(IrAnalyze *ira, IrInstructi
             return ir_analyze_instruction_error_return_trace(ira, (IrInstructionErrorReturnTrace *)instruction);
         case IrInstructionIdErrorUnion:
             return ir_analyze_instruction_error_union(ira, (IrInstructionErrorUnion *)instruction);
+        case IrInstructionIdCancel:
+            return ir_analyze_instruction_cancel(ira, (IrInstructionCancel *)instruction);
     }
     zig_unreachable();
 }
@@ -16774,7 +16818,9 @@ bool ir_has_side_effects(IrInstruction *instruction) {
         case IrInstructionIdPtrTypeOf:
         case IrInstructionIdSetAlignStack:
         case IrInstructionIdExport:
+        case IrInstructionIdCancel:
             return true;
+
         case IrInstructionIdPhi:
         case IrInstructionIdUnOp:
         case IrInstructionIdBinOp:
diff --git a/src/ir_print.cpp b/src/ir_print.cpp
index f2c0d6a5b4..5c0c3bab0d 100644
--- a/src/ir_print.cpp
+++ b/src/ir_print.cpp
@@ -1010,6 +1010,11 @@ static void ir_print_error_union(IrPrint *irp, IrInstructionErrorUnion *instruct
     ir_print_other_instruction(irp, instruction->payload);
 }
 
+static void ir_print_cancel(IrPrint *irp, IrInstructionCancel *instruction) {
+    fprintf(irp->f, "cancel ");
+    ir_print_other_instruction(irp, instruction->target);
+}
+
 static void ir_print_instruction(IrPrint *irp, IrInstruction *instruction) {
     ir_print_prefix(irp, instruction);
     switch (instruction->id) {
@@ -1330,6 +1335,9 @@ static void ir_print_instruction(IrPrint *irp, IrInstruction *instruction) {
         case IrInstructionIdErrorUnion:
             ir_print_error_union(irp, (IrInstructionErrorUnion *)instruction);
             break;
+        case IrInstructionIdCancel:
+            ir_print_cancel(irp, (IrInstructionCancel *)instruction);
+            break;
     }
     fprintf(irp->f, "\n");
 }
diff --git a/src/parser.cpp b/src/parser.cpp
index 6ce9e25221..fc682ee62a 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -920,7 +920,7 @@ static AstNode *ast_parse_curly_suffix_expr(ParseContext *pc, size_t *token_inde
 }
 
 /*
-SuffixOpExpression = PrimaryExpression option(FnCallExpression | ArrayAccessExpression | FieldAccessExpression | SliceExpression)
+SuffixOpExpression = ("async" option("(" Expression ")") PrimaryExpression FnCallExpression) | PrimaryExpression option(FnCallExpression | ArrayAccessExpression | FieldAccessExpression | SliceExpression)
 FnCallExpression : token(LParen) list(Expression, token(Comma)) token(RParen)
 ArrayAccessExpression : token(LBracket) Expression token(RBracket)
 SliceExpression = "[" Expression ".." option(Expression) "]"
@@ -928,9 +928,34 @@ FieldAccessExpression : token(Dot) token(Symbol)
 StructLiteralField : token(Dot) token(Symbol) token(Eq) Expression
 */
 static AstNode *ast_parse_suffix_op_expr(ParseContext *pc, size_t *token_index, bool mandatory) {
-    AstNode *primary_expr = ast_parse_primary_expr(pc, token_index, mandatory);
-    if (!primary_expr)
-        return nullptr;
+    AstNode *primary_expr;
+
+    Token *async_token = &pc->tokens->at(*token_index);
+    if (async_token->id == TokenIdKeywordAsync) {
+        *token_index += 1;
+
+        AstNode *allocator_expr_node = nullptr;
+        Token *async_lparen_tok = &pc->tokens->at(*token_index);
+        if (async_lparen_tok->id == TokenIdLParen) {
+            *token_index += 1;
+            allocator_expr_node = ast_parse_expression(pc, token_index, true);
+            ast_eat_token(pc, token_index, TokenIdRParen);
+        }
+
+        AstNode *fn_ref_expr_node = ast_parse_primary_expr(pc, token_index, true);
+        Token *lparen_tok = ast_eat_token(pc, token_index, TokenIdLParen);
+        AstNode *node = ast_create_node(pc, NodeTypeFnCallExpr, lparen_tok);
+        node->data.fn_call_expr.is_async = true;
+        node->data.fn_call_expr.async_allocator = allocator_expr_node;
+        node->data.fn_call_expr.fn_ref_expr = fn_ref_expr_node;
+        ast_parse_fn_call_param_list(pc, token_index, &node->data.fn_call_expr.params);
+
+        primary_expr = node;
+    } else {
+        primary_expr = ast_parse_primary_expr(pc, token_index, mandatory);
+        if (!primary_expr)
+            return nullptr;
+    }
 
     while (true) {
         Token *first_token = &pc->tokens->at(*token_index);
@@ -1535,6 +1560,24 @@ static AstNode *ast_parse_break_expr(ParseContext *pc, size_t *token_index) {
     return node;
 }
 
+/*
+CancelExpression = "cancel" Expression;
+*/
+static AstNode *ast_parse_cancel_expr(ParseContext *pc, size_t *token_index) {
+    Token *token = &pc->tokens->at(*token_index);
+
+    if (token->id != TokenIdKeywordCancel) {
+        return nullptr;
+    }
+    *token_index += 1;
+
+    AstNode *node = ast_create_node(pc, NodeTypeCancel, token);
+
+    node->data.cancel_expr.expr = ast_parse_expression(pc, token_index, false);
+
+    return node;
+}
+
 /*
 Defer(body) = ("defer" | "errdefer") body
 */
@@ -2159,7 +2202,7 @@ static AstNode *ast_parse_block_or_expression(ParseContext *pc, size_t *token_in
 }
 
 /*
-Expression = TryExpression | ReturnExpression | BreakExpression | AssignmentExpression
+Expression = TryExpression | ReturnExpression | BreakExpression | AssignmentExpression | CancelExpression
 */
 static AstNode *ast_parse_expression(ParseContext *pc, size_t *token_index, bool mandatory) {
     Token *token = &pc->tokens->at(*token_index);
@@ -2176,6 +2219,10 @@ static AstNode *ast_parse_expression(ParseContext *pc, size_t *token_index, bool
     if (break_expr)
         return break_expr;
 
+    AstNode *cancel_expr = ast_parse_cancel_expr(pc, token_index);
+    if (cancel_expr)
+        return cancel_expr;
+
     AstNode *ass_expr = ast_parse_ass_expr(pc, token_index, false);
     if (ass_expr)
         return ass_expr;
@@ -2809,6 +2856,7 @@ void ast_visit_node_children(AstNode *node, void (*visit)(AstNode **, void *cont
         case NodeTypeFnCallExpr:
             visit_field(&node->data.fn_call_expr.fn_ref_expr, visit, context);
             visit_node_list(&node->data.fn_call_expr.params, visit, context);
+            visit_field(&node->data.fn_call_expr.async_allocator, visit, context);
             break;
         case NodeTypeArrayAccessExpr:
             visit_field(&node->data.array_access_expr.array_ref_expr, visit, context);
@@ -2931,5 +2979,8 @@ void ast_visit_node_children(AstNode *node, void (*visit)(AstNode **, void *cont
         case NodeTypeErrorSetDecl:
             visit_node_list(&node->data.err_set_decl.decls, visit, context);
             break;
+        case NodeTypeCancel:
+            visit_field(&node->data.cancel_expr.expr, visit, context);
+            break;
     }
 }
diff --git a/std/mem.zig b/std/mem.zig
index 07521bfcb8..1dfef86a8f 100644
--- a/std/mem.zig
+++ b/std/mem.zig
@@ -116,6 +116,22 @@ pub const Allocator = struct {
         const non_const_ptr = @intToPtr(&u8, @ptrToInt(bytes.ptr));
         self.freeFn(self, non_const_ptr[0..bytes.len]);
     }
+
+    pub const AsyncAllocator = struct {
+        allocator: &Allocator,
+
+        fn alloc(self: &const AsyncAllocator, byte_count: usize, alignment: u29) Error![]u8 {
+            return self.allocator.allocFn(self.allocator, byte_count, alignment);
+        }
+
+        fn free(self: &const AsyncAllocator, old_mem: []u8) {
+            return self.allocator.freeFn(self.allocator, old_mem);
+        }
+    };
+
+    fn toAsync(self: &Allocator) AsyncAllocator {
+        return AsyncAllocator { .allocator = self };
+    }
 };
 
 /// Copy all of source into dest at position 0.