1661476 - Prevent Result<const V, E> or Result<V, const E> from being instantiated

Status: RESOLVED FIXED

(Core :: MFBT, task)

Description

[Simon Giesecke [:sg] [he/him]]

Result<const V, E> is not meaningful to express that the success value must not be changed within a Result, because Result provides this guarantee anyway. OTOH, when unwrapping the value, const has no effect, it is still possible to unwrap into a non-const V.

However, Result<const V, E> has the negative side-effect of preventing moving instances of that type. To prevent this, and to avoid confusion on the effect of const V, this should be prevented via a static_assert.

Also, documentation should be extended to make the non-modifiability of the success value clear(er) to developers using and evolving Result.

Note that Result<const V*, E> remains possible and useful, just Result<const V* const, E> will no longer be possible.

Comment 1

[Simon Giesecke [:sg] [he/him]]

Same goes for const E.

Comment 2

[Simon Giesecke [:sg] [he/him]]

Attachment: Bug 1661476 - Prevent Result<const V, E> or Result<V, const E> from being instantiated. r=froydnj

Also, remove the existing uses of Result<const nsCString, nsresult> in URLPreloader
and Result<CryptoScheme, const nsCString> in SampleIterator.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/52ef06111f20
Prevent Result<const V, E> or Result<V, const E> from being instantiated. r=froydnj

Comment 4

[Cosmin Sabou [:CosminS]]

https://hg.mozilla.org/mozilla-central/rev/52ef06111f20