Always disallow assignments to constants #57567
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We have historically allowed the following without warning or error: ``` const x = 1 x = 1 ``` As well as ``` const x = 1 x = 2 ``` with a UB warning. In 1.12, we made the second case error, as part of the general binding partition changes, but did not touch the first case, because it did not have the warning. I think this made a reasonable amount of sense, because we essentially treated constants as special kinds of mutable globals (to which assignment happened to be UB). However, in the 1.12+ design, constants and globals are quite sepearate beasts, and I think it makes sense to extend the error to the egal case also, even if it is technically more breaking. In fact, I already thought that's what I did when I implemented the new effect model for global assignment, causing #57566. I can't think of a legitimate reason to keep this special case. For those who want to do binding replacement, the `const` keyword is mandatory, so the assignment is now literally always a semantic no-op or an error. Fixes #57566.
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Having heard no complaints in informal survey and since it currently segfaults, I'm just gonna go ahead and merge it. Happy to still entertain arguments in favor of the previous 1.12 behavior if anybody has them. |
KristofferC
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Mar 3, 2025
We have historically allowed the following without warning or error: ``` const x = 1 x = 1 ``` As well as ``` const x = 1 x = 2 ``` with a UB warning. In 1.12, we made the second case error, as part of the general binding partition changes, but did not touch the first case, because it did not have the warning. I think this made a reasonable amount of sense, because we essentially treated constants as special kinds of mutable globals (to which assignment happened to be UB). However, in the 1.12+ design, constants and globals are quite sepearate beasts, and I think it makes sense to extend the error to the egal case also, even if it is technically more breaking. In fact, I already thought that's what I did when I implemented the new effect model for global assignment, causing #57566. I can't think of a legitimate reason to keep this special case. For those who want to do binding replacement, the `const` keyword is mandatory, so the assignment is now literally always a semantic no-op or an error. Fixes #57566. (cherry picked from commit 8f00a51)
KristofferC
removed
the
backport 1.12
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We have historically allowed the following without warning or error:
As well as
with a UB warning.
In 1.12, we made the second case error, as part of the general binding partition changes, but did not touch the first case, because it did not have the warning.
I think this made a reasonable amount of sense, because we essentially treated constants as special kinds of mutable globals (to which assignment happened to be UB).
However, in the 1.12+ design, constants and globals are quite sepearate beasts, and I think it makes sense to extend the error to the egal case also, even if it is technically more breaking.
In fact, I already thought that's what I did when I implemented the new effect model for global assignment, causing #57566. I can't think of a legitimate reason to keep this special case. For those who want to do binding replacement, the
constkeyword is mandatory, so the assignment is now literally always a semantic no-op or an error.Fixes #57566.