[Feature Request] [mojo-lang] Comptime if ternary at runtime

Review Mojo's priorities

• I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

Our current setup only allows us to do

var a: Int
comptime if condition:
  a = 1
else:
  a = 0

AFAIK if we do

var a = 1 if condition else 0

the bool gets materialized and the expression is evaluated at runtime, and for trivial cases like this LLVM should optimize it away.

Could we maybe get a way to express a ternary expression to be forcibly evaluated at compile time? It would avoid materialization for expressions that could be more tersely declared in a ternary than a full if-else:

comptime elems = [1, 2, 3]
var a = 3 comptime if len(elems) >= 3 else 0

What is your motivation for this change?

...

Any other details?

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Start with the request's existing comptime if examples and compare them with the ternary expressions shown in the issue. Review how compile-time conditions and runtime expressions are currently distinguished in Mojo. Done means a documented, agreed-upon way to express a ternary that is forcibly evaluated at compile time, including the len(elems) example.