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Auto merge of #21984 - pnkfelix:make-for-loops-a-terminating-scope, r=nikomatsakis

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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:

```
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]
This commit is contained in:
bors 2015-02-07 18:20:55 +00:00
commit 61626b3063
2 changed files with 21 additions and 9 deletions
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28
src/libsyntax/ext/expand.rs
View File

@ -230,15 +230,18 @@ pub fn expand_expr(e: P<ast::Expr>, fld: &mut MacroExpander) -> P<ast::Expr> {
ast::ExprForLoop(pat, head, body, opt_ident) => {
// to:
//
// match ::std::iter::IntoIterator::into_iter(<head>) {
// mut iter => {
// [opt_ident]: loop {
// match ::std::iter::Iterator::next(&mut iter) {
// ::std::option::Option::Some(<pat>) => <body>,
// ::std::option::Option::None => break
// {
// let result = match ::std::iter::IntoIterator::into_iter(<head>) {
// mut iter => {
// [opt_ident]: loop {
// match ::std::iter::Iterator::next(&mut iter) {
// ::std::option::Option::Some(<pat>) => <body>,
// ::std::option::Option::None => break
// }
// }
// }
// }
// };
// result
// }
// expand <head>
@ -319,7 +322,16 @@ pub fn expand_expr(e: P<ast::Expr>, fld: &mut MacroExpander) -> P<ast::Expr> {
fld.cx.expr_call(span, fld.cx.expr_path(into_iter_path), vec![head])
};
fld.cx.expr_match(span, into_iter_expr, vec![iter_arm])
let match_expr = fld.cx.expr_match(span, into_iter_expr, vec![iter_arm]);
// `{ let result = ...; result }`
let result_ident = token::gensym_ident("result");
fld.cx.expr_block(
fld.cx.block_all(
span,
vec![fld.cx.stmt_let(span, false, result_ident, match_expr)],
Some(fld.cx.expr_ident(span, result_ident))))
}
ast::ExprClosure(capture_clause, fn_decl, block) => {

2
src/test/run-pass/loop-label-shadowing.rs
View File

@ -13,7 +13,7 @@
fn main() {
let mut foo = Vec::new();
'foo: for i in &[1, 2, 3] {
foo.push(i);
foo.push(*i);
}
}