Summary

Enable if and match during const evaluation and make them evaluate lazily. In short, this will allow if x < y { y - x } else { x - y } even though the else branch would emit an overflow error for unsigned types if x < y.

Motivation

Conditions in constants are important for making functions like NonZero::new const fn and interpreting assertions.

Guide-level explanation

If you write

let x: u32 = ...;
let y: u32 = ...;
let a = x - y;
let b = y - x;
if x > y {
    // do something with a
} else {
    // do something with b
}

The program will always panic (except if both x and y are 0) because either x - y will overflow or y - x will. To resolve this one must move the let a and let b into the if and else branch respectively.

let x: u32 = ...;
let y: u32 = ...;
if x > y {
    let a = x - y;
    // do something with a
} else {
    let b = y - x;
    // do something with b
}

When constants are involved, new issues arise:

const X: u32 = ...;
const Y: u32 = ...;
const FOO: SomeType = if X > Y {
    const A: u32 = X - Y;
    ...
} else {
    const B: u32 = Y - X;
    ...
};

A and B are evaluated before FOO, since constants are by definition constant, so their order of evaluation should not matter. This assumption breaks in the presence of errors, because errors are side effects, and thus not pure.

To resolve this issue, one needs to eliminate the intermediate constants and directly evaluate X - Y and Y - X.

const X: u32 = ...;
const Y: u32 = ...;
const FOO: SomeType = if X > Y {
    let a = X - Y;
    ...
} else {
    let b = Y - X;
    ...
};

Reference-level explanation

match on enums whose variants have no fields or if is translated during HIR -> MIR lowering to a switchInt terminator. Mir interpretation will now have to evaluate those terminators (which it already can).

match on enums with variants which have fields is translated to switch, which will check either the discriminant or compute the discriminant in the case of packed enums like Option<&T> (which has no special memory location for the discriminant, but encodes None as all zeros and treats everything else as a Some). When entering a match arm's branch, the matched on value is essentially transmuted to the enum variant's type, allowing further code to access its fields.

Drawbacks

This makes it easier to fail compilation on random "constant" values like size_of::<T>() or other platform specific constants.

Rationale and alternatives

Require intermediate const fns to break the eager const evaluation

Instead of writing

const X: u32 = ...;
const Y: u32 = ...;
const AB: u32 = if X > Y {
    X - Y
} else {
    Y - X
};

where either X - Y or Y - X would emit an error, add an intermediate const fn

const X: u32 = ...;
const Y: u32 = ...;
const fn foo(x: u32, y: u32) -> u32 {
    if x > y {
        x - y
    } else {
        y - x
    }
}
const AB: u32 = foo(X, Y);

Since the const fn's x and y arguments are unknown, they cannot be const evaluated. When the const fn is evaluated with given arguments, only the taken branch is evaluated.

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