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Remove explicit use of let from Function Parameters

Introduction

Since function parameters are immutable by default, allowing function parameters to be explicitly labeled as let is a bit of a syntactic redundancy that would best be removed. Not allowing function parameters to be explicitly declared as let would permit a more simple and uniform function declaration syntax for swift. Furthermore proposal SE-0003​: "Removing var from Function Parameters" removes var from function parameters removing any possible ambiguity as to whether a function parameter is immutable or not.

Swift-evolution thread: Removing explicit use of let from Function Parameters

Motivation

Now that proposal SE-0003​: "Removing var from Function Parameters" has been accepted, it would make sense that the syntax for function parameters being explicitly declared as let would be removed as well. Since prepending let to an argument label does not modify behavior, leaving it as part of the language would only add redundancy and complexity to Swift's syntax at no added benefit. Furthermore SE-0001 allowed the use of all keywords as argument labels except for inout, var and let.
Proposal SE-0031 made inout a type modifier freeing inout to be used as an argument label and proposal SE-0003 prohibits declaring function parameters as var freeing var to be used as an argument label. The only keyword still in use that is preventing any keyword from being used as an argument label is let which if removed from function parameter syntax would permit SE-0001 to allow all keywords as argument labels with no exceptions.

Proposed solution

Make functions with parameters declared with an explicit let prohibited and a compile time error.

Basically make functions declared like this a compile time error:

func foo(let x: Int) { ... }

In favor of omitting let like this:

func foo(x: Int) { ... }

Impact on existing code

In code that is migrating to this newer proposed syntax the let keyword should be deleted if placed before a function parameter or else it will be treated as an external label. This should not be too disruptive since the common convention is already to not label function parameters as let.

Alternatives considered

Leave the redundant syntax in place, but I personally don't see any merit in that.