make for PAT in ITER_EXPR { ... } a terminating-scope for ITER_EXPR.
#21984
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In effect, temporary anonymous values created during the evaluation of
ITER_EXPR no longer not live for the entirety of the block surrounding
the for-loop; instead they only live for the extent of the for-loop
itself, and no longer.
----
There is one case I know of that this breaks, demonstrated to me by
niko (but it is also a corner-case that is useless in practice). Here
is that case:
```
fn main() {
let mut foo: Vec<&i8> = Vec::new();
for i in &[1, 2, 3] { foo.push(i) }
}
```
Note that if you add any code following the for-loop above, or even a
semicolon to the end of it, then the code will stop compiling (i.e.,
it gathers a vector of references but the gathered vector cannot
actually be used.)
(The above code, despite being useless, did occur in one run-pass test
by accident; that test is updated here to accommodate the new
striction.)
----
So, technically this is a:
[breaking-change]
|
Note that this landing this is blocked on PR #21980. That is, on my machine with my other commits, if you do not have a robust codemap, then you end up ICE'ing (for me, its while compiling librand with (It is possible that some different way of manipulating the spans than what I did here would resolve the above problem. But I think its more important that we first land this change to the terminating scopes before we worry too much about span handling.) |
|
(oh, and this was spawned off of #21972) |
|
🙀 You have the wrong number! Please try again with |
|
@bors don't be so nosy |
bors
added a commit
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Feb 7, 2015
…=nikomatsakis
make `for PAT in ITER_EXPR { ... }` a terminating-scope for ITER_EXPR.
In effect, temporary anonymous values created during the evaluation of ITER_EXPR no longer not live for the entirety of the block surrounding the for-loop; instead they only live for the extent of the for-loop itself, and no longer.
----
There is one case I know of that this breaks, demonstrated to me by @nikomatsakis (but it is also a corner-case that is useless in practice). Here is that case:
```
fn main() {
let mut foo: Vec<&i8> = Vec::new();
for i in &[1, 2, 3] { foo.push(i) }
}
```
Note that if you add any code following the for-loop above, or even a semicolon to the end of it, then the code will stop compiling (i.e., it gathers a vector of references but the gathered vector cannot actually be used.)
(The above code, despite being useless, did occur in one run-pass test by accident; that test is updated here to accommodate the new striction.)
----
So, technically this is a:
[breaking-change]
bors
added a commit
that referenced
this pull request
Apr 24, 2019
Introduce hir::ExprKind::Use and employ in for loop desugaring.
In the `for $pat in $expr $block` desugaring we end with a `{ let _result = $match_expr; _result }` construct which makes `for` loops into a terminating scope and affects drop order. The construct was introduced in year 2015 by @pnkfelix in #21984.
This PR replaces the construct with `hir::ExprKind::Use(P<hir::Expr>)` which is equivalent semantically but should hopefully be less costly in terms of compile time performance (to be determined).
This is extracted out of 91b0abd from #59288 for easier review and so that the perf implications wrt. `for`-loops can be measured.
r? @oli-obk
Centril
added a commit
to Centril/rust
that referenced
this pull request
Apr 25, 2019
…ps, r=oli-obk
Introduce hir::ExprKind::Use and employ in for loop desugaring.
In the `for $pat in $expr $block` desugaring we end with a `{ let _result = $match_expr; _result }` construct which makes `for` loops into a terminating scope and affects drop order. The construct was introduced in year 2015 by @pnkfelix in rust-lang#21984.
This PR replaces the construct with `hir::ExprKind::Use(P<hir::Expr>)` which is equivalent semantically but should hopefully be less costly in terms of compile time performance (to be determined).
This is extracted out of rust-lang@91b0abd from rust-lang#59288 for easier review and so that the perf implications wrt. `for`-loops can be measured.
r? @oli-obk
Centril
added a commit
to Centril/rust
that referenced
this pull request
Apr 25, 2019
…ps, r=oli-obk
Introduce hir::ExprKind::Use and employ in for loop desugaring.
In the `for $pat in $expr $block` desugaring we end with a `{ let _result = $match_expr; _result }` construct which makes `for` loops into a terminating scope and affects drop order. The construct was introduced in year 2015 by @pnkfelix in rust-lang#21984.
This PR replaces the construct with `hir::ExprKind::Use(P<hir::Expr>)` which is equivalent semantically but should hopefully be less costly in terms of compile time performance (to be determined).
This is extracted out of rust-lang@91b0abd from rust-lang#59288 for easier review and so that the perf implications wrt. `for`-loops can be measured.
r? @oli-obk
Centril
added a commit
to Centril/rust
that referenced
this pull request
Apr 26, 2019
…ps, r=oli-obk
Introduce hir::ExprKind::Use and employ in for loop desugaring.
In the `for $pat in $expr $block` desugaring we end with a `{ let _result = $match_expr; _result }` construct which makes `for` loops into a terminating scope and affects drop order. The construct was introduced in year 2015 by @pnkfelix in rust-lang#21984.
This PR replaces the construct with `hir::ExprKind::Use(P<hir::Expr>)` which is equivalent semantically but should hopefully be less costly in terms of compile time performance (to be determined).
This is extracted out of rust-lang@91b0abd from rust-lang#59288 for easier review and so that the perf implications wrt. `for`-loops can be measured.
r? @oli-obk
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make
for PAT in ITER_EXPR { ... }a terminating-scope for ITER_EXPR.In effect, temporary anonymous values created during the evaluation of ITER_EXPR no longer not live for the entirety of the block surrounding the for-loop; instead they only live for the extent of the for-loop itself, and no longer.
There is one case I know of that this breaks, demonstrated to me by @nikomatsakis (but it is also a corner-case that is useless in practice). Here is that case:
Note that if you add any code following the for-loop above, or even a semicolon to the end of it, then the code will stop compiling (i.e., it gathers a vector of references but the gathered vector cannot actually be used.)
(The above code, despite being useless, did occur in one run-pass test by accident; that test is updated here to accommodate the new striction.)
So, technically this is a:
[breaking-change]