Summary

If the constant evaluator encounters erroneous code during the evaluation of an expression that is not part of a true constant evaluation context a warning must be emitted and the expression needs to be translated normally.

Definition of constant evaluation context

There are exactly five places where an expression needs to be constant.

the initializer of a constant const foo: ty = EXPR or static foo: ty = EXPR
the size of an array [T; EXPR]
the length of a repeat expression [VAL; LEN_EXPR]
C-Like enum variant discriminant values
patterns

In the future the body of const fn might also be interpreted as a constant evaluation context.

Any other expression might still be constant evaluated, but it could just as well be compiled normally and executed at runtime.

Motivation

Expressions are const-evaluated even when they are not in a const environment.

For example

fn blub<T>(t: T) -> T { t }
let x = 5 << blub(42);

will not cause a compiler error currently, while 5 << 42 will. If the constant evaluator gets smart enough, it will be able to const evaluate the blub function. This would be a breaking change, since the code would not compile anymore. (this occurred in https://github.com/rust-lang/rust/pull/26848).

Detailed design

The PRs https://github.com/rust-lang/rust/pull/26848 and https://github.com/rust-lang/rust/pull/25570 will be setting a precedent for warning about such situations (WIP, not pushed yet).

When the constant evaluator fails while evaluating a normal expression, a warning will be emitted and normal translation needs to be resumed.

Drawbacks

None, if we don't do anything, the const evaluator cannot get much smarter.

Alternatives

allow breaking changes

Let the compiler error on things that will unconditionally panic at runtime.

insert an unconditional panic instead of generating regular code

GNAT (an Ada compiler) does this already:

procedure Hello is
  Var: Integer range 15 .. 20 := 21;
begin
  null;
end Hello;

The anonymous subtype Integer range 15 .. 20 only accepts values in [15, 20]. This knowledge is used by GNAT to emit the following warning during compilation:

warning: value not in range of subtype of "Standard.Integer" defined at line 2
warning: "Constraint_Error" will be raised at run time

I don't have a GNAT with -emit-llvm handy, but here's the asm with -O0:

.cfi_startproc
pushq   %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq    %rsp, %rbp
.cfi_def_cfa_register 6
movl    $2, %esi
movl    $.LC0, %edi
movl    $0, %eax
call    __gnat_rcheck_CE_Range_Check

Unresolved questions

Const-eval the body of `const fn` that are never used in a constant environment

Currently a const fn that is called in non-const code is treated just like a normal function.

In case there is a statically known erroneous situation in the body of the function, the compiler should raise an error, even if the function is never called.

The same applies to unused associated constants.

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