add @precondition, like @Assert but thows ArgumentError, e.g.

    function split{T}(c::T, n::Int)
        @require n > 0
        return SplitIterator{T}(c, n)
    end

    split("xxx", 0)

    ERROR: ArgumentError: split(::ASCIIString, ::Int64) requires n > 0

base/contracts.jl

@@ -0,0 +1,22 @@
+# This file is a part of Julia. License is MIT: http://julialang.org/license
+
+
+@noinline function precondition_error(msg)
+    io = IOBuffer()
+    StackTraces.show_spec_linfo(io, StackTraces.lookup(backtrace()[3]))
+    throw(ArgumentError("$(takebuf_string(io)) requires $msg"))
+end
+
+
+"""
+    @require precondition [message]
+
+Throw `ArgumentError` if `precondition` is false.
+"""
+macro require(precondition, msgs...)
+    msg = isempty(msgs) ? string(precondition) : msgs[1]
+    :(if ! $(esc(precondition)) precondition_error($msg) end)
+end
+# FIXME
+# Should this have a branch-prediction hint? (same for @assert?)
+# http://llvm.org/docs/BranchWeightMetadata.html#built-in-expect-instructions

base/exports.jl

@@ -1436,6 +1436,7 @@ export
     @noinline,
 
     @assert,
+    @require,
     @enum,
     @label,
     @goto,

base/sysimg.jl

@@ -90,6 +90,8 @@ include("arraymath.jl")
 include("simdloop.jl")
 importall .SimdLoop
 
+include("contracts.jl")
+
 # map-reduce operators
 include("reduce.jl")
 

doc/stdlib/base.rst

@@ -1068,6 +1068,12 @@ Errors
 
    Throw an ``AssertionError`` if ``cond`` is ``false``\ . Preferred syntax for writing assertions. Message ``text`` is optionally displayed upon assertion failure.
 
+.. function:: @require precondition [message]
+
+   .. Docstring generated from Julia source
+
+   Throw ``ArgumentError`` if ``precondition`` is false.
+
 .. function:: ArgumentError(msg)
 
    .. Docstring generated from Julia source

test/misc.jl

@@ -101,6 +101,31 @@ let deepthought(x, y) = 42
     end
 end
 
+# test @require
+let
+    f1(n) = begin @require n > 0; n end
+    f2(n) = begin @require n != 0 "non-zero n"; n end
+
+    @test f1(1) == 1
+    @test f2(1) == 1
+
+    try
+        f1(0)
+        error("unexpected")
+    catch ex
+        @test isa(ex, ArgumentError)
+        @test ismatch(r"f1(.*) requires n > 0", ex.msg)
+    end
+
+    try
+        f2(0)
+        error("unexpected")
+    catch ex
+        @test isa(ex, ArgumentError)
+        @test ismatch(r"f2(.*) requires non-zero n", ex.msg)
+    end
+end
+
 let # test the process title functions, issue #9957
     oldtitle = Sys.get_process_title()
     Sys.set_process_title("julia0x1")