Restrictions on moves into and out-from fixed-length arrays. #21971
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r? @pcwalton (rust_highfive has picked a reviewer for you, use r? to override) |
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@pnkfelix r+ modulo nits. |
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⌛ Testing commit 0e60c27 with merge d347b18... |
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💔 Test failed - auto-win-32-nopt-t |
No longer legal: `fn foo(a: [D; 5]) { drop(a); a[2] = D::new(); }`;
one must first initialize the entirety of `a` before assigning to its
individual elements.
No longer legal: `fn foo(arr: [D; 5]) -> D { arr[2] }`, unless `D`
implements `Copy`. This "move out-from" restriction only affects
`expr[i]`, and not destructuring (e.g. `f([a, b, c]: Array) { ... }`).
uses mem_categorization to distinguish destructuring-bind from array
indexing.
See discussion on RFC PR 533.
[breaking-change]
Note that the change to the error message in borrowck-use-in-index-lvalue.rs, where we report that `*w` is uninitialized rather than `w`, was unintended fallout from the implementation strategy used here. The change appears harmless to me, but I welcome advice on how to bring back the old message, which was slightly cleaner (i.e. less unintelligible). ---- drive-by: revise compile-fail/borrowck-vec-pattern-move-tail to make it really clear that there is a conflict that must be signaled. (A hypothetical future version of Rust might be able to accept the prior version of the code, since the previously updated index was not actually aliased.)
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Feb 7, 2015
…komatsakis Revised version of PR #21930. Restrictions on moves into and out-from fixed-length arrays. (There was only one use of this "feature" in the compiler source.) Note 1: the change to the error message in tests/compile-fail/borrowck-use-in-index-lvalue.rs, where we now report that *w is uninitialized (rather than w), was unintended fallout from the implementation strategy used here. The change appears harmless to me, but I welcome advice on how to bring back the old message, which was slightly cleaner (i.e. less unintelligible) since that the syntactic form *w does not actually appear in the source text. Note 2: the move out-from restriction to only apply to expr[i], and not destructuring bind (e.g. f([a, b, c]: Array) { ... }) since the latter is compatible with nonzeroing drop, AFAICT. [breaking-change]
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Revised version of PR #21930.
Restrictions on moves into and out-from fixed-length arrays.
(There was only one use of this "feature" in the compiler source.)
Note 1: the change to the error message in
tests/compile-fail/borrowck-use-in-index-lvalue.rs, where we now report that*wis uninitialized (rather thanw), was unintended fallout from the implementation strategy used here. The change appears harmless to me, but I welcome advice on how to bring back the old message, which was slightly cleaner (i.e. less unintelligible) since that the syntactic form*wdoes not actually appear in the source text.Note 2: the move out-from restriction to only apply to
expr[i], and not destructuring bind (e.g.f([a, b, c]: Array) { ... })since the latter is compatible with nonzeroing drop, AFAICT.[breaking-change]