go/constant: add StringLen function

[griesemer]

The proposal is to add the following function to the go/constant package:

// StringLen returns the length of x if x is a [String].
// If x is [Unknown], the result is 0.
// In all other cases, the function panics.
func StringLen(x Value) int64

This allows the determination of the length of a String value w/o having to call StringVal.
(StringVal causes the construction and thus allocation of a new single string if the actual string value was created via addition of many substrings.)

[griesemer]

See also #78346.

[gopherbot]

Change https://go.dev/cl/772320 mentions this issue: go/constant: add StringLen function

[mateusz834]

/ StringLen returns the length of x if x is a [String].
// If x is [Unknown], the result is 0.
// In all other cases, the function panics.
func StringLen(x Value) int64

Any reason that it returns int64, instead of int? On 32b platforms you will never have > int32 string in memory.

EDIT: concatenation can cause that, see #78346.

[mateusz834]

FYI this can be a method instead, since Value iface has an unexported implementsValue().

[gabyhelp]

Related Issues

• go/constant: add Kind.String #46211 (closed)

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[mwriter]

concatenation can cause that

Do I understand correctly that it's possible to create strings longer than math.MaxInt?

[ianlancetaylor]

There can be a distinction between host and target here. If a program running on a 32-bit host is analyzing a program that is only able to run on a 64-bit host, then, yes, it is technically possible for the 64-bit program to include a constant string that is longer than the 32-bit math.MaxInt.

(Though I struggle to imagine an actual case where this might happen.)

[griesemer]

At least temporarily one could create a string longer than math.MaxInt. For instance, one could have a string constant s that is significantly shorter, and than use a sequence of (constant) doublings of the string. The resulting strings are represented as trees, where all the leaves are the same (short) string. Of course things would go haywire as soon as the string has to be materialized. But maybe it is not ever materialized.

Also, as a more practical matter, returning an int64 simplifies the use of the length in some places.

Since all the other functionality in the constant package is operating on abstract values, and with functions, it makes sense to use a function here and not a method.

[ianlancetaylor]

Oh, right. https://go.dev/play/p/l3Rl0bWzfwk . Printing len(s8) works, printing len(s9) times out on the playground (on Go 1.26).