diff --git a/src/all_types.hpp b/src/all_types.hpp
index 461270357b..39e6033988 100644
--- a/src/all_types.hpp
+++ b/src/all_types.hpp
@@ -1252,6 +1252,7 @@ enum BuiltinFnId {
     BuiltinFnIdShlExact,
     BuiltinFnIdShrExact,
     BuiltinFnIdSetEvalBranchQuota,
+    BuiltinFnIdAlignCast,
 };
 
 struct BuiltinFnEntry {
@@ -1274,6 +1275,7 @@ enum PanicMsgId {
     PanicMsgIdSliceWidenRemainder,
     PanicMsgIdUnwrapMaybeFail,
     PanicMsgIdInvalidErrorCode,
+    PanicMsgIdIncorrectAlignment,
 
     PanicMsgIdCount,
 };
@@ -1856,6 +1858,7 @@ enum IrInstructionId {
     IrInstructionIdTypeId,
     IrInstructionIdSetEvalBranchQuota,
     IrInstructionIdPtrTypeOf,
+    IrInstructionIdAlignCast,
 };
 
 struct IrInstruction {
@@ -2462,6 +2465,7 @@ struct IrInstructionFnProto {
     IrInstruction base;
 
     IrInstruction **param_types;
+    IrInstruction *align_value;
     IrInstruction *return_type;
     bool is_var_args;
 };
@@ -2638,6 +2642,13 @@ struct IrInstructionPtrTypeOf {
     bool is_volatile;
 };
 
+struct IrInstructionAlignCast {
+    IrInstruction base;
+
+    IrInstruction *align_bytes;
+    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 35ccd3b4d3..6adf4b3d51 100644
--- a/src/analyze.cpp
+++ b/src/analyze.cpp
@@ -884,6 +884,9 @@ TypeTableEntry *get_fn_type(CodeGen *g, FnTypeId *fn_type_id) {
         buf_appendf(&fn_type->name, "%s...", comma);
     }
     buf_appendf(&fn_type->name, ")");
+    if (fn_type_id->alignment != 0) {
+        buf_appendf(&fn_type->name, " align %" PRIu32, fn_type_id->alignment);
+    }
     if (fn_type_id->return_type->id != TypeTableEntryIdVoid) {
         buf_appendf(&fn_type->name, " -> %s", buf_ptr(&fn_type_id->return_type->name));
     }
@@ -1058,12 +1061,13 @@ void init_fn_type_id(FnTypeId *fn_type_id, AstNode *proto_node, size_t param_cou
     fn_type_id->is_var_args = fn_proto->is_var_args;
 }
 
-static TypeTableEntry *analyze_fn_type(CodeGen *g, AstNode *proto_node, Scope *child_scope) {
+static TypeTableEntry *analyze_fn_type(CodeGen *g, AstNode *proto_node, Scope *child_scope, uint32_t alignment) {
     assert(proto_node->type == NodeTypeFnProto);
     AstNodeFnProto *fn_proto = &proto_node->data.fn_proto;
 
     FnTypeId fn_type_id = {0};
     init_fn_type_id(&fn_type_id, proto_node, proto_node->data.fn_proto.params.length);
+    fn_type_id.alignment = alignment;
 
     for (; fn_type_id.next_param_index < fn_type_id.param_count; fn_type_id.next_param_index += 1) {
         AstNode *param_node = fn_proto->params.at(fn_type_id.next_param_index);
@@ -2056,23 +2060,23 @@ static void resolve_decl_fn(CodeGen *g, TldFn *tld_fn) {
         }
 
         Scope *child_scope = fn_table_entry->fndef_scope ? &fn_table_entry->fndef_scope->base : tld_fn->base.parent_scope;
-        fn_table_entry->type_entry = analyze_fn_type(g, source_node, child_scope);
-
-        if (fn_table_entry->type_entry->id == TypeTableEntryIdInvalid) {
-            tld_fn->base.resolution = TldResolutionInvalid;
-            return;
-        }
 
+        uint32_t alignment = 0;
         if (fn_proto->align_expr != nullptr) {
-            if (!analyze_const_align(g, tld_fn->base.parent_scope, fn_proto->align_expr,
-                        &fn_table_entry->align_bytes))
-            {
+            if (!analyze_const_align(g, child_scope, fn_proto->align_expr, &alignment)) {
                 fn_table_entry->type_entry = g->builtin_types.entry_invalid;
                 tld_fn->base.resolution = TldResolutionInvalid;
                 return;
             }
         }
 
+        fn_table_entry->type_entry = analyze_fn_type(g, source_node, child_scope, alignment);
+
+        if (fn_table_entry->type_entry->id == TypeTableEntryIdInvalid) {
+            tld_fn->base.resolution = TldResolutionInvalid;
+            return;
+        }
+
         if (!fn_table_entry->type_entry->data.fn.is_generic) {
             g->fn_protos.append(fn_table_entry);
 
@@ -2663,6 +2667,9 @@ bool types_match_const_cast_only(TypeTableEntry *expected_type, TypeTableEntry *
     if (expected_type->id == TypeTableEntryIdFn &&
         actual_type->id == TypeTableEntryIdFn)
     {
+        if (expected_type->data.fn.fn_type_id.alignment > actual_type->data.fn.fn_type_id.alignment) {
+            return false;
+        }
         if (expected_type->data.fn.fn_type_id.cc != actual_type->data.fn.fn_type_id.cc) {
             return false;
         }
@@ -3384,6 +3391,7 @@ uint32_t fn_type_id_hash(FnTypeId *id) {
     result += ((uint32_t)(id->cc)) * (uint32_t)3349388391;
     result += id->is_var_args ? (uint32_t)1931444534 : 0;
     result += hash_ptr(id->return_type);
+    result += id->alignment * 0xd3b3f3e2;
     for (size_t i = 0; i < id->param_count; i += 1) {
         FnTypeParamInfo *info = &id->param_info[i];
         result += info->is_noalias ? (uint32_t)892356923 : 0;
@@ -3396,7 +3404,8 @@ bool fn_type_id_eql(FnTypeId *a, FnTypeId *b) {
     if (a->cc != b->cc ||
         a->return_type != b->return_type ||
         a->is_var_args != b->is_var_args ||
-        a->param_count != b->param_count)
+        a->param_count != b->param_count ||
+        a->alignment != b->alignment)
     {
         return false;
     }
diff --git a/src/ast_render.cpp b/src/ast_render.cpp
index b053a4e8af..d245863216 100644
--- a/src/ast_render.cpp
+++ b/src/ast_render.cpp
@@ -953,7 +953,7 @@ static void render_node_extra(AstRender *ar, AstNode *node, bool grouped) {
                 render_node_ungrouped(ar, node->data.slice_expr.array_ref_expr);
                 fprintf(ar->f, "[");
                 render_node_grouped(ar, node->data.slice_expr.start);
-                fprintf(ar->f, "...");
+                fprintf(ar->f, "..");
                 if (node->data.slice_expr.end)
                     render_node_grouped(ar, node->data.slice_expr.end);
                 fprintf(ar->f, "]");
diff --git a/src/codegen.cpp b/src/codegen.cpp
index cd0dc60f00..3954f310b6 100644
--- a/src/codegen.cpp
+++ b/src/codegen.cpp
@@ -688,6 +688,8 @@ static Buf *panic_msg_buf(PanicMsgId msg_id) {
             return buf_create_from_str("reached unreachable code");
         case PanicMsgIdInvalidErrorCode:
             return buf_create_from_str("invalid error code");
+        case PanicMsgIdIncorrectAlignment:
+            return buf_create_from_str("incorrect alignment");
     }
     zig_unreachable();
 }
@@ -2605,6 +2607,72 @@ static LLVMValueRef ir_render_field_parent_ptr(CodeGen *g, IrExecutable *executa
     }
 }
 
+static LLVMValueRef get_default_aligned_load(CodeGen *g, LLVMValueRef ptr) {
+    LLVMValueRef result = LLVMBuildLoad(g->builder, ptr, "");
+    LLVMSetAlignment(result, LLVMABIAlignmentOfType(g->target_data_ref, LLVMGetElementType(LLVMTypeOf(ptr))));
+    return result;
+}
+
+static LLVMValueRef ir_render_align_cast(CodeGen *g, IrExecutable *executable, IrInstructionAlignCast *instruction) {
+    LLVMValueRef target_val = ir_llvm_value(g, instruction->target);
+    assert(target_val);
+
+    bool want_debug_safety = ir_want_debug_safety(g, &instruction->base);
+    if (!want_debug_safety) {
+        return target_val;
+    }
+
+    TypeTableEntry *target_type = instruction->base.value.type;
+    uint32_t align_bytes;
+    LLVMValueRef ptr_val;
+
+    if (target_type->id == TypeTableEntryIdPointer) {
+        align_bytes = target_type->data.pointer.alignment;
+        ptr_val = target_val;
+    } else if (target_type->id == TypeTableEntryIdFn) {
+        align_bytes = target_type->data.fn.fn_type_id.alignment;
+        ptr_val = target_val;
+    } else if (target_type->id == TypeTableEntryIdMaybe &&
+            target_type->data.maybe.child_type->id == TypeTableEntryIdPointer)
+    {
+        align_bytes = target_type->data.maybe.child_type->data.pointer.alignment;
+        ptr_val = target_val;
+    } else if (target_type->id == TypeTableEntryIdMaybe &&
+            target_type->data.maybe.child_type->id == TypeTableEntryIdFn)
+    {
+        align_bytes = target_type->data.maybe.child_type->data.fn.fn_type_id.alignment;
+        ptr_val = target_val;
+    } else if (target_type->id == TypeTableEntryIdStruct && target_type->data.structure.is_slice) {
+        TypeTableEntry *slice_ptr_type = target_type->data.structure.fields[slice_ptr_index].type_entry;
+        align_bytes = slice_ptr_type->data.pointer.alignment;
+
+        size_t ptr_index = target_type->data.structure.fields[slice_ptr_index].gen_index;
+        LLVMValueRef ptr_val_ptr = LLVMBuildStructGEP(g->builder, target_val, (unsigned)ptr_index, "");
+        ptr_val = get_default_aligned_load(g, ptr_val_ptr);
+    } else {
+        zig_unreachable();
+    }
+
+    assert(align_bytes != 1);
+
+    TypeTableEntry *usize = g->builtin_types.entry_usize;
+    LLVMValueRef ptr_as_int_val = LLVMBuildPtrToInt(g->builder, ptr_val, usize->type_ref, "");
+    LLVMValueRef alignment_minus_1 = LLVMConstInt(usize->type_ref, align_bytes - 1, false);
+    LLVMValueRef anded_val = LLVMBuildAnd(g->builder, ptr_as_int_val, alignment_minus_1, "");
+    LLVMValueRef ok_bit = LLVMBuildICmp(g->builder, LLVMIntEQ, anded_val, LLVMConstNull(usize->type_ref), "");
+
+    LLVMBasicBlockRef ok_block = LLVMAppendBasicBlock(g->cur_fn_val, "AlignCastOk");
+    LLVMBasicBlockRef fail_block = LLVMAppendBasicBlock(g->cur_fn_val, "AlignCastFail");
+
+    LLVMBuildCondBr(g->builder, ok_bit, ok_block, fail_block);
+
+    LLVMPositionBuilderAtEnd(g->builder, fail_block);
+    gen_debug_safety_crash(g, PanicMsgIdIncorrectAlignment);
+
+    LLVMPositionBuilderAtEnd(g->builder, ok_block);
+
+    return target_val;
+}
 
 static LLVMAtomicOrdering to_LLVMAtomicOrdering(AtomicOrder atomic_order) {
     switch (atomic_order) {
@@ -3350,6 +3418,8 @@ static LLVMValueRef ir_render_instruction(CodeGen *g, IrExecutable *executable,
             return ir_render_enum_tag_name(g, executable, (IrInstructionEnumTagName *)instruction);
         case IrInstructionIdFieldParentPtr:
             return ir_render_field_parent_ptr(g, executable, (IrInstructionFieldParentPtr *)instruction);
+        case IrInstructionIdAlignCast:
+            return ir_render_align_cast(g, executable, (IrInstructionAlignCast *)instruction);
     }
     zig_unreachable();
 }
@@ -4633,6 +4703,7 @@ static void define_builtin_fns(CodeGen *g) {
     create_builtin_fn(g, BuiltinFnIdShlExact, "shlExact", 2);
     create_builtin_fn(g, BuiltinFnIdShrExact, "shrExact", 2);
     create_builtin_fn(g, BuiltinFnIdSetEvalBranchQuota, "setEvalBranchQuota", 1);
+    create_builtin_fn(g, BuiltinFnIdAlignCast, "alignCast", 2);
 }
 
 static const char *bool_to_str(bool b) {
diff --git a/src/ir.cpp b/src/ir.cpp
index 251c597224..243a8a2389 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -555,6 +555,10 @@ static constexpr IrInstructionId ir_instruction_id(IrInstructionPtrTypeOf *) {
     return IrInstructionIdPtrTypeOf;
 }
 
+static constexpr IrInstructionId ir_instruction_id(IrInstructionAlignCast *) {
+    return IrInstructionIdAlignCast;
+}
+
 template<typename T>
 static T *ir_create_instruction(IrBuilder *irb, Scope *scope, AstNode *source_node) {
     T *special_instruction = allocate<T>(1);
@@ -1899,10 +1903,11 @@ static IrInstruction *ir_build_unwrap_err_payload_from(IrBuilder *irb, IrInstruc
 }
 
 static IrInstruction *ir_build_fn_proto(IrBuilder *irb, Scope *scope, AstNode *source_node,
-    IrInstruction **param_types, IrInstruction *return_type, bool is_var_args)
+    IrInstruction **param_types, IrInstruction *align_value, IrInstruction *return_type, bool is_var_args)
 {
     IrInstructionFnProto *instruction = ir_build_instruction<IrInstructionFnProto>(irb, scope, source_node);
     instruction->param_types = param_types;
+    instruction->align_value = align_value;
     instruction->return_type = return_type;
     instruction->is_var_args = is_var_args;
 
@@ -1912,6 +1917,7 @@ static IrInstruction *ir_build_fn_proto(IrBuilder *irb, Scope *scope, AstNode *s
     for (size_t i = 0; i < param_count; i += 1) {
         ir_ref_instruction(param_types[i], irb->current_basic_block);
     }
+    if (align_value != nullptr) ir_ref_instruction(align_value, irb->current_basic_block);
     ir_ref_instruction(return_type, irb->current_basic_block);
 
     return &instruction->base;
@@ -2219,6 +2225,19 @@ static IrInstruction *ir_build_set_eval_branch_quota(IrBuilder *irb, Scope *scop
     return &instruction->base;
 }
 
+static IrInstruction *ir_build_align_cast(IrBuilder *irb, Scope *scope, AstNode *source_node,
+        IrInstruction *align_bytes, IrInstruction *target)
+{
+    IrInstructionAlignCast *instruction = ir_build_instruction<IrInstructionAlignCast>(irb, scope, source_node);
+    instruction->align_bytes = align_bytes;
+    instruction->target = target;
+
+    ir_ref_instruction(align_bytes, irb->current_basic_block);
+    ir_ref_instruction(target, irb->current_basic_block);
+
+    return &instruction->base;
+}
+
 static IrInstruction *ir_instruction_br_get_dep(IrInstructionBr *instruction, size_t index) {
     return nullptr;
 }
@@ -2738,6 +2757,10 @@ static IrInstruction *ir_instruction_fnproto_get_dep(IrInstructionFnProto *instr
     if (param_index < instruction->base.source_node->data.fn_proto.params.length) {
         return instruction->param_types[param_index];
     }
+    size_t next_index = param_index - instruction->base.source_node->data.fn_proto.params.length;
+    if (next_index == 0 && instruction->align_value != nullptr) {
+        return instruction->align_value;
+    }
     return nullptr;
 }
 
@@ -2925,6 +2948,14 @@ static IrInstruction *ir_instruction_ptrtypeof_get_dep(IrInstructionPtrTypeOf *i
     }
 }
 
+static IrInstruction *ir_instruction_aligncast_get_dep(IrInstructionAlignCast *instruction, size_t index) {
+    switch (index) {
+        case 0: return instruction->align_bytes;
+        case 1: return instruction->target;
+        default: return nullptr;
+    }
+}
+
 static IrInstruction *ir_instruction_get_dep(IrInstruction *instruction, size_t index) {
     switch (instruction->id) {
         case IrInstructionIdInvalid:
@@ -3121,6 +3152,8 @@ static IrInstruction *ir_instruction_get_dep(IrInstruction *instruction, size_t
             return ir_instruction_setevalbranchquota_get_dep((IrInstructionSetEvalBranchQuota *) instruction, index);
         case IrInstructionIdPtrTypeOf:
             return ir_instruction_ptrtypeof_get_dep((IrInstructionPtrTypeOf *) instruction, index);
+        case IrInstructionIdAlignCast:
+            return ir_instruction_aligncast_get_dep((IrInstructionAlignCast *) instruction, index);
     }
     zig_unreachable();
 }
@@ -4531,6 +4564,20 @@ static IrInstruction *ir_gen_builtin_fn_call(IrBuilder *irb, Scope *scope, AstNo
 
                 return ir_build_set_eval_branch_quota(irb, scope, node, arg0_value);
             }
+        case BuiltinFnIdAlignCast:
+            {
+                AstNode *arg0_node = node->data.fn_call_expr.params.at(0);
+                IrInstruction *arg0_value = ir_gen_node(irb, arg0_node, scope);
+                if (arg0_value == irb->codegen->invalid_instruction)
+                    return arg0_value;
+
+                AstNode *arg1_node = node->data.fn_call_expr.params.at(1);
+                IrInstruction *arg1_value = ir_gen_node(irb, arg1_node, scope);
+                if (arg1_value == irb->codegen->invalid_instruction)
+                    return arg1_value;
+
+                return ir_build_align_cast(irb, scope, node, arg0_value, arg1_value);
+            }
     }
     zig_unreachable();
 }
@@ -6060,11 +6107,18 @@ static IrInstruction *ir_gen_fn_proto(IrBuilder *irb, Scope *parent_scope, AstNo
         param_types[i] = type_value;
     }
 
+    IrInstruction *align_value = nullptr;
+    if (node->data.fn_proto.align_expr != nullptr) {
+        align_value = ir_gen_node(irb, node->data.fn_proto.align_expr, parent_scope);
+        if (align_value == irb->codegen->invalid_instruction)
+            return irb->codegen->invalid_instruction;
+    }
+
     IrInstruction *return_type = ir_gen_node(irb, node->data.fn_proto.return_type, parent_scope);
     if (return_type == irb->codegen->invalid_instruction)
         return irb->codegen->invalid_instruction;
 
-    return ir_build_fn_proto(irb, parent_scope, node, param_types, return_type, is_var_args);
+    return ir_build_fn_proto(irb, parent_scope, node, param_types, align_value, return_type, is_var_args);
 }
 
 static IrInstruction *ir_gen_node_raw(IrBuilder *irb, AstNode *node, Scope *scope,
@@ -8316,15 +8370,29 @@ static IrInstruction *ir_analyze_cast(IrAnalyze *ira, IrInstruction *source_inst
     }
 
     // explicit cast from []T to []u8 or []u8 to []T
-    if (is_slice(wanted_type) && is_slice(actual_type) &&
-        (is_u8(wanted_type->data.structure.fields[slice_ptr_index].type_entry->data.pointer.child_type) ||
-        is_u8(actual_type->data.structure.fields[slice_ptr_index].type_entry->data.pointer.child_type)) &&
-        (wanted_type->data.structure.fields[slice_ptr_index].type_entry->data.pointer.is_const ||
-         !actual_type->data.structure.fields[slice_ptr_index].type_entry->data.pointer.is_const))
-    {
-        if (!ir_emit_global_runtime_side_effect(ira, source_instr))
-            return ira->codegen->invalid_instruction;
-        return ir_resolve_cast(ira, source_instr, value, wanted_type, CastOpResizeSlice, true);
+    if (is_slice(wanted_type) && is_slice(actual_type)) {
+        TypeTableEntry *wanted_ptr_type = wanted_type->data.structure.fields[slice_ptr_index].type_entry;
+        TypeTableEntry *actual_ptr_type = actual_type->data.structure.fields[slice_ptr_index].type_entry;
+        if ((is_u8(wanted_ptr_type->data.pointer.child_type) || is_u8(actual_ptr_type->data.pointer.child_type)) &&
+            (wanted_ptr_type->data.pointer.is_const || !actual_ptr_type->data.pointer.is_const))
+        {
+            uint32_t src_align_bytes = get_ptr_align(actual_ptr_type);
+            uint32_t dest_align_bytes = get_ptr_align(wanted_ptr_type);
+
+            if (dest_align_bytes > src_align_bytes) {
+                ErrorMsg *msg = ir_add_error(ira, source_instr,
+                        buf_sprintf("cast increases pointer alignment"));
+                add_error_note(ira->codegen, msg, source_instr->source_node,
+                        buf_sprintf("'%s' has alignment %" PRIu32, buf_ptr(&actual_type->name), src_align_bytes));
+                add_error_note(ira->codegen, msg, source_instr->source_node,
+                        buf_sprintf("'%s' has alignment %" PRIu32, buf_ptr(&wanted_type->name), dest_align_bytes));
+                return ira->codegen->invalid_instruction;
+            }
+
+            if (!ir_emit_global_runtime_side_effect(ira, source_instr))
+                return ira->codegen->invalid_instruction;
+            return ir_resolve_cast(ira, source_instr, value, wanted_type, CastOpResizeSlice, true);
+        }
     }
 
     // explicit cast from [N]u8 to []const T
@@ -10226,7 +10294,10 @@ static TypeTableEntry *ir_analyze_fn_call(IrAnalyze *ira, IrInstructionCall *cal
                     ira->new_irb.exec->backward_branch_count, ira->new_irb.exec->backward_branch_quota,
                     nullptr, nullptr, fn_proto_node->data.fn_proto.align_expr, nullptr, ira->new_irb.exec);
 
-            ir_resolve_align(ira, align_result, &impl_fn->align_bytes);
+            uint32_t align_bytes = 0;
+            ir_resolve_align(ira, align_result, &align_bytes);
+            impl_fn->align_bytes = align_bytes;
+            inst_fn_type_id.alignment = align_bytes;
         }
 
         {
@@ -13728,11 +13799,9 @@ static TypeTableEntry *ir_analyze_instruction_slice(IrAnalyze *ira, IrInstructio
     TypeTableEntry *return_type;
 
     if (array_type->id == TypeTableEntryIdArray) {
-        uint32_t normal_array_alignment = get_abi_alignment(ira->codegen, array_type);
-        uint32_t align_bytes = (ptr_type->data.pointer.alignment >= normal_array_alignment) ?
-            normal_array_alignment : 1;
         TypeTableEntry *slice_ptr_type = get_pointer_to_type_extra(ira->codegen, array_type->data.array.child_type,
-            ptr_type->data.pointer.is_const, ptr_type->data.pointer.is_volatile, align_bytes, 0, 0);
+            ptr_type->data.pointer.is_const, ptr_type->data.pointer.is_volatile,
+            ptr_type->data.pointer.alignment, 0, 0);
         return_type = get_slice_type(ira->codegen, slice_ptr_type);
     } else if (array_type->id == TypeTableEntryIdPointer) {
         TypeTableEntry *slice_ptr_type = get_pointer_to_type_extra(ira->codegen, array_type->data.pointer.child_type,
@@ -14237,6 +14306,11 @@ static TypeTableEntry *ir_analyze_instruction_fn_proto(IrAnalyze *ira, IrInstruc
         }
     }
 
+    if (instruction->align_value != nullptr) {
+        if (!ir_resolve_align(ira, instruction->align_value->other, &fn_type_id.alignment))
+            return ira->codegen->builtin_types.entry_invalid;
+    }
+
     IrInstruction *return_type_value = instruction->return_type->other;
     fn_type_id.return_type = ir_resolve_type(ira, return_type_value);
     if (type_is_invalid(fn_type_id.return_type))
@@ -14866,6 +14940,90 @@ static TypeTableEntry *ir_analyze_instruction_ptr_type_of(IrAnalyze *ira, IrInst
     return ira->codegen->builtin_types.entry_type;
 }
 
+static TypeTableEntry *ir_analyze_instruction_align_cast(IrAnalyze *ira, IrInstructionAlignCast *instruction) {
+    uint32_t align_bytes;
+    IrInstruction *align_bytes_inst = instruction->align_bytes->other;
+    if (!ir_resolve_align(ira, align_bytes_inst, &align_bytes))
+        return ira->codegen->builtin_types.entry_invalid;
+
+    IrInstruction *target = instruction->target->other;
+    TypeTableEntry *target_type = target->value.type;
+    if (type_is_invalid(target_type))
+        return ira->codegen->builtin_types.entry_invalid;
+
+    TypeTableEntry *result_type;
+    uint32_t old_align_bytes;
+
+    if (target_type->id == TypeTableEntryIdPointer) {
+        result_type = get_pointer_to_type_extra(ira->codegen,
+                target_type->data.pointer.child_type,
+                target_type->data.pointer.is_const, target_type->data.pointer.is_volatile,
+                align_bytes,
+                target_type->data.pointer.bit_offset, target_type->data.pointer.unaligned_bit_count);
+    } else if (target_type->id == TypeTableEntryIdFn) {
+        FnTypeId fn_type_id = target_type->data.fn.fn_type_id;
+        old_align_bytes = fn_type_id.alignment;
+        fn_type_id.alignment = align_bytes;
+        result_type = get_fn_type(ira->codegen, &fn_type_id);
+    } else if (target_type->id == TypeTableEntryIdMaybe &&
+            target_type->data.maybe.child_type->id == TypeTableEntryIdPointer)
+    {
+        TypeTableEntry *ptr_type = target_type->data.maybe.child_type;
+        old_align_bytes = ptr_type->data.pointer.alignment;
+        TypeTableEntry *better_ptr_type = get_pointer_to_type_extra(ira->codegen,
+                ptr_type->data.pointer.child_type,
+                ptr_type->data.pointer.is_const, ptr_type->data.pointer.is_volatile,
+                align_bytes,
+                ptr_type->data.pointer.bit_offset, ptr_type->data.pointer.unaligned_bit_count);
+
+        result_type = get_maybe_type(ira->codegen, better_ptr_type);
+    } else if (target_type->id == TypeTableEntryIdMaybe &&
+            target_type->data.maybe.child_type->id == TypeTableEntryIdFn)
+    {
+        FnTypeId fn_type_id = target_type->data.maybe.child_type->data.fn.fn_type_id;
+        old_align_bytes = fn_type_id.alignment;
+        fn_type_id.alignment = align_bytes;
+        TypeTableEntry *fn_type = get_fn_type(ira->codegen, &fn_type_id);
+        result_type = get_maybe_type(ira->codegen, fn_type);
+    } else if (is_slice(target_type)) {
+        TypeTableEntry *slice_ptr_type = target_type->data.structure.fields[slice_ptr_index].type_entry;
+        old_align_bytes = slice_ptr_type->data.pointer.alignment;
+        TypeTableEntry *result_ptr_type = get_pointer_to_type_extra(ira->codegen,
+                slice_ptr_type->data.pointer.child_type,
+                slice_ptr_type->data.pointer.is_const, slice_ptr_type->data.pointer.is_volatile,
+                align_bytes,
+                slice_ptr_type->data.pointer.bit_offset, slice_ptr_type->data.pointer.unaligned_bit_count);
+        result_type = get_slice_type(ira->codegen, result_ptr_type);
+    } else {
+        ir_add_error(ira, target,
+                buf_sprintf("expected pointer or slice, found '%s'", buf_ptr(&target_type->name)));
+        return ira->codegen->builtin_types.entry_invalid;
+    }
+
+    if (instr_is_comptime(target)) {
+        ConstExprValue *val = ir_resolve_const(ira, target, UndefBad);
+        if (!val)
+            return ira->codegen->builtin_types.entry_invalid;
+
+        ConstExprValue *out_val = ir_build_const_from(ira, &instruction->base);
+        copy_const_val(out_val, val, false);
+        out_val->type = result_type;
+        return result_type;
+    }
+
+    IrInstruction *result;
+    if (align_bytes > old_align_bytes && align_bytes != 1) {
+        result = ir_build_align_cast(&ira->new_irb, instruction->base.scope, instruction->base.source_node,
+                align_bytes_inst, target);
+    } else {
+        result = ir_build_cast(&ira->new_irb, instruction->base.scope, instruction->base.source_node,
+                result_type, target, CastOpNoop);
+    }
+    ir_link_new_instruction(result, &instruction->base);
+    result->value.type = result_type;
+    return result_type;
+}
+
 static TypeTableEntry *ir_analyze_instruction_nocast(IrAnalyze *ira, IrInstruction *instruction) {
     switch (instruction->id) {
         case IrInstructionIdInvalid:
@@ -14877,6 +15035,10 @@ static TypeTableEntry *ir_analyze_instruction_nocast(IrAnalyze *ira, IrInstructi
         case IrInstructionIdStructFieldPtr:
         case IrInstructionIdEnumFieldPtr:
         case IrInstructionIdInitEnum:
+        case IrInstructionIdMaybeWrap:
+        case IrInstructionIdErrWrapCode:
+        case IrInstructionIdErrWrapPayload:
+        case IrInstructionIdCast:
             zig_unreachable();
         case IrInstructionIdReturn:
             return ir_analyze_instruction_return(ira, (IrInstructionReturn *)instruction);
@@ -15046,11 +15208,8 @@ static TypeTableEntry *ir_analyze_instruction_nocast(IrAnalyze *ira, IrInstructi
             return ir_analyze_instruction_set_eval_branch_quota(ira, (IrInstructionSetEvalBranchQuota *)instruction);
         case IrInstructionIdPtrTypeOf:
             return ir_analyze_instruction_ptr_type_of(ira, (IrInstructionPtrTypeOf *)instruction);
-        case IrInstructionIdMaybeWrap:
-        case IrInstructionIdErrWrapCode:
-        case IrInstructionIdErrWrapPayload:
-        case IrInstructionIdCast:
-            zig_panic("TODO analyze more instructions");
+        case IrInstructionIdAlignCast:
+            return ir_analyze_instruction_align_cast(ira, (IrInstructionAlignCast *)instruction);
     }
     zig_unreachable();
 }
@@ -15228,6 +15387,7 @@ bool ir_has_side_effects(IrInstruction *instruction) {
         case IrInstructionIdFieldParentPtr:
         case IrInstructionIdOffsetOf:
         case IrInstructionIdTypeId:
+        case IrInstructionIdAlignCast:
             return false;
         case IrInstructionIdAsm:
             {
diff --git a/src/ir_print.cpp b/src/ir_print.cpp
index bfaf45835b..3c6154b21d 100644
--- a/src/ir_print.cpp
+++ b/src/ir_print.cpp
@@ -174,10 +174,16 @@ static void ir_print_decl_var(IrPrint *irp, IrInstructionDeclVar *decl_var_instr
     if (decl_var_instruction->var_type) {
         fprintf(irp->f, "%s %s: ", var_or_const, name);
         ir_print_other_instruction(irp, decl_var_instruction->var_type);
-        fprintf(irp->f, " = ");
+        fprintf(irp->f, " ");
     } else {
-        fprintf(irp->f, "%s %s = ", var_or_const, name);
+        fprintf(irp->f, "%s %s ", var_or_const, name);
     }
+    if (decl_var_instruction->align_value) {
+        fprintf(irp->f, "align ");
+        ir_print_other_instruction(irp, decl_var_instruction->align_value);
+        fprintf(irp->f, " ");
+    }
+    fprintf(irp->f, "= ");
     ir_print_other_instruction(irp, decl_var_instruction->init_value);
     if (decl_var_instruction->var->is_comptime != nullptr) {
         fprintf(irp->f, " // comptime = ");
@@ -640,7 +646,7 @@ static void ir_print_slice(IrPrint *irp, IrInstructionSlice *instruction) {
     ir_print_other_instruction(irp, instruction->ptr);
     fprintf(irp->f, "[");
     ir_print_other_instruction(irp, instruction->start);
-    fprintf(irp->f, "...");
+    fprintf(irp->f, "..");
     if (instruction->end)
         ir_print_other_instruction(irp, instruction->end);
     fprintf(irp->f, "]");
@@ -745,7 +751,13 @@ static void ir_print_fn_proto(IrPrint *irp, IrInstructionFnProto *instruction) {
             ir_print_other_instruction(irp, instruction->param_types[i]);
         }
     }
-    fprintf(irp->f, ")->");
+    fprintf(irp->f, ")");
+    if (instruction->align_value != nullptr) {
+        fprintf(irp->f, " align ");
+        ir_print_other_instruction(irp, instruction->align_value);
+        fprintf(irp->f, " ");
+    }
+    fprintf(irp->f, "->");
     ir_print_other_instruction(irp, instruction->return_type);
 }
 
@@ -920,6 +932,14 @@ static void ir_print_set_eval_branch_quota(IrPrint *irp, IrInstructionSetEvalBra
     fprintf(irp->f, ")");
 }
 
+static void ir_print_align_cast(IrPrint *irp, IrInstructionAlignCast *instruction) {
+    fprintf(irp->f, "@alignCast(");
+    ir_print_other_instruction(irp, instruction->align_bytes);
+    fprintf(irp->f, ",");
+    ir_print_other_instruction(irp, instruction->target);
+    fprintf(irp->f, ")");
+}
+
 static void ir_print_instruction(IrPrint *irp, IrInstruction *instruction) {
     ir_print_prefix(irp, instruction);
     switch (instruction->id) {
@@ -1213,6 +1233,9 @@ static void ir_print_instruction(IrPrint *irp, IrInstruction *instruction) {
         case IrInstructionIdSetEvalBranchQuota:
             ir_print_set_eval_branch_quota(irp, (IrInstructionSetEvalBranchQuota *)instruction);
             break;
+        case IrInstructionIdAlignCast:
+            ir_print_align_cast(irp, (IrInstructionAlignCast *)instruction);
+            break;
     }
     fprintf(irp->f, "\n");
 }
diff --git a/std/debug.zig b/std/debug.zig
index 6d7138a551..0b80f4b104 100644
--- a/std/debug.zig
+++ b/std/debug.zig
@@ -957,16 +957,21 @@ pub var global_allocator = mem.Allocator {
 var some_mem: [100 * 1024]u8 = undefined;
 var some_mem_index: usize = 0;
 
-fn globalAlloc(self: &mem.Allocator, n: usize) -> %[]u8 {
-    const result = some_mem[some_mem_index .. some_mem_index + n];
-    some_mem_index += n;
+fn globalAlloc(self: &mem.Allocator, n: usize, alignment: usize) -> %[]u8 {
+    const addr = @ptrToInt(&some_mem[some_mem_index]);
+    const rem = @rem(addr, alignment);
+    const march_forward_bytes = if (rem == 0) 0 else (alignment - rem);
+    const adjusted_index = some_mem_index + march_forward_bytes;
+    const end_index = adjusted_index + n;
+    const result = some_mem[adjusted_index .. end_index];
+    some_mem_index = end_index;
     return result;
 }
 
-fn globalRealloc(self: &mem.Allocator, old_mem: []u8, new_size: usize) -> %[]u8 {
-    const result = %return globalAlloc(self, new_size);
+fn globalRealloc(self: &mem.Allocator, old_mem: []u8, new_size: usize, alignment: usize) -> %[]u8 {
+    const result = %return globalAlloc(self, new_size, alignment);
     @memcpy(result.ptr, old_mem.ptr, old_mem.len);
     return result;
 }
 
-fn globalFree(self: &mem.Allocator, old_mem: []u8) { }
+fn globalFree(self: &mem.Allocator, ptr: &u8) { }
diff --git a/std/mem.zig b/std/mem.zig
index 26fb2a3cd6..671ea8de87 100644
--- a/std/mem.zig
+++ b/std/mem.zig
@@ -11,21 +11,18 @@ pub const Cmp = math.Cmp;
 error NoMem;
 
 pub const Allocator = struct {
-    allocFn: fn (self: &Allocator, n: usize) -> %[]u8,
-    /// Note that old_mem may be a slice of length 0, in which case reallocFn
-    /// should simply call allocFn.
-    reallocFn: fn (self: &Allocator, old_mem: []u8, new_size: usize) -> %[]u8,
-    /// Note that mem may be a slice of length 0, in which case freeFn
-    /// should do nothing.
-    freeFn: fn (self: &Allocator, mem: []u8),
+    /// Allocate byte_count bytes and return them in a slice, with the
+    /// slicer's pointer aligned at least to alignment bytes.
+    allocFn: fn (self: &Allocator, byte_count: usize, alignment: usize) -> %[]u8,
 
-    /// Aborts the program if an allocation fails.
-    fn checkedAlloc(self: &Allocator, comptime T: type, n: usize) -> []T {
-        alloc(self, T, n) %% |err| debug.panic("allocation failure: {}", @errorName(err))
-    }
+    /// Guaranteed: old_mem.len > 0 and alignment >= alignment of old_mem.ptr
+    reallocFn: fn (self: &Allocator, old_mem: []u8, new_byte_count: usize, alignment: usize) -> %[]u8,
+
+    freeFn: fn (self: &Allocator, ptr: &u8),
 
     fn create(self: &Allocator, comptime T: type) -> %&T {
-        &(%return self.alloc(T, 1))[0]
+        const slice = %return self.alloc(T, 1);
+        &slice[0]
     }
 
     fn destroy(self: &Allocator, ptr: var) {
@@ -34,16 +31,29 @@ pub const Allocator = struct {
 
     fn alloc(self: &Allocator, comptime T: type, n: usize) -> %[]T {
         const byte_count = %return math.mul(usize, @sizeOf(T), n);
-        ([]T)(%return self.allocFn(self, byte_count))
+        const byte_slice = %return self.allocFn(self, byte_count, @alignOf(T));
+        ([]T)(@alignCast(@alignOf(T), byte_slice))
     }
 
     fn realloc(self: &Allocator, comptime T: type, old_mem: []T, n: usize) -> %[]T {
+        if (old_mem.len == 0) {
+            return self.alloc(T, n);
+        }
+
+        // Assert that old_mem.ptr is properly aligned.
+        _ = @alignCast(@alignOf(T), old_mem.ptr);
+
         const byte_count = %return math.mul(usize, @sizeOf(T), n);
-        ([]T)(%return self.reallocFn(self, ([]u8)(old_mem), byte_count))
+        const byte_slice = %return self.reallocFn(self, ([]u8)(old_mem), byte_count, @alignOf(T));
+        ([]T)(@alignCast(@alignOf(T), byte_slice))
     }
 
-    fn free(self: &Allocator, mem: var) {
-        self.freeFn(self, ([]u8)(mem));
+    fn free(self: &Allocator, memory: var) {
+        const const_slice = ([]const u8)(memory);
+        if (memory.len == 0)
+            return;
+        const ptr = @intToPtr(&u8, @ptrToInt(const_slice.ptr));
+        self.freeFn(self, ptr);
     }
 };
 
@@ -79,24 +89,28 @@ pub const IncrementingAllocator = struct {
         _ = os.posix.munmap(self.bytes.ptr, self.bytes.len);
     }
 
-    fn alloc(allocator: &Allocator, n: usize) -> %[]u8 {
+    fn alloc(allocator: &Allocator, n: usize, alignment: usize) -> %[]u8 {
         const self = @fieldParentPtr(IncrementingAllocator, "allocator", allocator);
-        const new_end_index = self.end_index + n;
+        const addr = @ptrToInt(&self.bytes[self.end_index]);
+        const rem = @rem(addr, alignment);
+        const march_forward_bytes = if (rem == 0) 0 else (alignment - rem);
+        const adjusted_index = self.end_index + march_forward_bytes;
+        const new_end_index = adjusted_index + n;
         if (new_end_index > self.bytes.len) {
             return error.NoMem;
         }
-        const result = self.bytes[self.end_index..new_end_index];
+        const result = self.bytes[adjusted_index .. new_end_index];
         self.end_index = new_end_index;
         return result;
     }
 
-    fn realloc(allocator: &Allocator, old_mem: []u8, new_size: usize) -> %[]u8 {
-        const result = %return alloc(allocator, new_size);
+    fn realloc(allocator: &Allocator, old_mem: []u8, new_size: usize, alignment: usize) -> %[]u8 {
+        const result = %return alloc(allocator, new_size, alignment);
         copy(u8, result, old_mem);
         return result;
     }
 
-    fn free(allocator: &Allocator, bytes: []u8) {
+    fn free(allocator: &Allocator, bytes: &u8) {
         // Do nothing. That's the point of an incrementing allocator.
     }
 };
diff --git a/test/cases/align.zig b/test/cases/align.zig
index ee68408226..33013732de 100644
--- a/test/cases/align.zig
+++ b/test/cases/align.zig
@@ -62,3 +62,24 @@ fn testBytesAlign(b: u8) {
     const ptr = @ptrCast(&u32, &bytes[0]);
     assert(*ptr == 0x33333333);
 }
+
+test "specifying alignment allows slice cast" {
+    testBytesAlignSlice(0x33);
+}
+fn testBytesAlignSlice(b: u8) {
+    var bytes align 4 = []u8{b, b, b, b};
+    const slice = ([]u32)(bytes[0..]);
+    assert(slice[0] == 0x33333333);
+}
+
+test "@alignCast" {
+    var x: u32 align 4 = 1;
+    expectsOnly1(&x);
+    assert(x == 2);
+}
+fn expectsOnly1(x: &align 1 u32) {
+    expects4(@alignCast(4, x));
+}
+fn expects4(x: &align 4 u32) {
+    *x += 1;
+}
diff --git a/test/cases/cast.zig b/test/cases/cast.zig
index 8c1df39f66..310d5e3d9b 100644
--- a/test/cases/cast.zig
+++ b/test/cases/cast.zig
@@ -277,7 +277,7 @@ fn cast128Float(x: u128) -> f128 {
 }
 
 test "const slice widen cast" {
-    const bytes = []u8{0x12, 0x12, 0x12, 0x12};
+    const bytes align 4 = []u8{0x12, 0x12, 0x12, 0x12};
 
     const u32_value = ([]const u32)(bytes[0..])[0];
     assert(u32_value == 0x12121212);
diff --git a/test/cases/misc.zig b/test/cases/misc.zig
index 46f86c4bd1..abe9d3871d 100644
--- a/test/cases/misc.zig
+++ b/test/cases/misc.zig
@@ -404,7 +404,7 @@ test "cast slice to u8 slice" {
     bytes[6] = 0;
     bytes[7] = 0;
     assert(big_thing_slice[1] == 0);
-    const big_thing_again = ([]i32)(bytes);
+    const big_thing_again = ([]align 1 i32)(bytes);
     assert(big_thing_again[2] == 3);
     big_thing_again[2] = -1;
     assert(bytes[8] == @maxValue(u8));
diff --git a/test/compile_errors.zig b/test/compile_errors.zig
index 21575bfc34..e21e56f4e8 100644
--- a/test/compile_errors.zig
+++ b/test/compile_errors.zig
@@ -2022,4 +2022,21 @@ pub fn addCases(cases: &tests.CompileErrorContext) {
         ".tmp_source.zig:3:17: error: cast increases pointer alignment",
         ".tmp_source.zig:3:38: note: '&u8' has alignment 1",
         ".tmp_source.zig:3:27: note: '&u32' has alignment 4");
+
+    cases.add("increase pointer alignment in slice resize",
+        \\export fn entry() -> u32 {
+        \\    var bytes = []u8{0x01, 0x02, 0x03, 0x04};
+        \\    return ([]u32)(bytes[0..])[0];
+        \\}
+    ,
+        ".tmp_source.zig:3:19: error: cast increases pointer alignment",
+        ".tmp_source.zig:3:19: note: '[]u8' has alignment 1",
+        ".tmp_source.zig:3:19: note: '[]u32' has alignment 4");
+
+    cases.add("@alignCast expects pointer or slice",
+        \\export fn entry() {
+        \\    @alignCast(4, u32(3))
+        \\}
+    ,
+        ".tmp_source.zig:2:22: error: expected pointer or slice, found 'u32'");
 }
diff --git a/test/debug_safety.zig b/test/debug_safety.zig
index 7767477b6e..4a60a881ed 100644
--- a/test/debug_safety.zig
+++ b/test/debug_safety.zig
@@ -200,8 +200,8 @@ pub fn addCases(cases: &tests.CompareOutputContext) {
         \\    const x = widenSlice([]u8{1, 2, 3, 4, 5});
         \\    if (x.len == 0) return error.Whatever;
         \\}
-        \\fn widenSlice(slice: []const u8) -> []const i32 {
-        \\    ([]const i32)(slice)
+        \\fn widenSlice(slice: []align 1 const u8) -> []align 1 const i32 {
+        \\    ([]align 1 const i32)(slice)
         \\}
     );
 
@@ -261,4 +261,22 @@ pub fn addCases(cases: &tests.CompareOutputContext) {
         \\    return error(x);
         \\}
     );
+
+    cases.addDebugSafety("@alignCast misaligned",
+        \\pub fn panic(message: []const u8) -> noreturn {
+        \\    @breakpoint();
+        \\    while (true) {}
+        \\}
+        \\error Wrong;
+        \\pub fn main() -> %void {
+        \\    var array align 4 = []u32{0x11111111, 0x11111111};
+        \\    const bytes = ([]u8)(array[0..]);
+        \\    if (foo(bytes) != 0x11111111) return error.Wrong;
+        \\}
+        \\fn foo(bytes: []u8) -> u32 {
+        \\    const slice4 = bytes[1..5];
+        \\    const int_slice = ([]u32)(@alignCast(4, slice4));
+        \\    return int_slice[0];
+        \\}
+    );
 }