Why no warning for unused let bindings?

Question

C# warns for unused variables that are compile-time constants:

static void Main(string[] args)
{
    var unused = "hey"; //CS0219 The variable 'unused' is assigned but its value is never used
    Console.WriteLine("Hello World!");
}

But the F# compiler does not, even though the editor now does pick it up:

If it covered not just compile-time constants but all let bindings, this would have caught a real bug in production caused by a trivial mistake, something like

let callApiXyz connectionInfo = async {
    let fullUrl = sprintf "%s..." connectionInfo.Url
    ...
    let! result = httpGet connectionInfo // fail, didn't use the modified url
    // Should have been:
    // let! result = httpGet { connectionInfo with Url = fullUrl }
    ...
}

Is there any reason not to have this (other than "features are not free")? I feel this should be more important in a functional-first language where expressions tend not to have side-effects, than in C#.

Answer 1

For those of us that don't have Visual Studio and edit the fsproj by hand, the way to implement Tomas's answer is

<PropertyGroup>
    <OtherFlags>$(OtherFlags) --warnon:1182</OtherFlags>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
</PropertyGroup>

As an example, see the project file of FSharp.Core itself.