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improve lower_pat_slice docs + while -> for

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This commit is contained in:
Mazdak Farrokhzad 2019-12-15 13:17:39 +01:00
parent 66bb978ab6
commit d9d1f2376f
1 changed files with 26 additions and 12 deletions
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38
src/librustc/hir/lowering.rs
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@ -2852,19 +2852,23 @@ fn lower_pat_tuple(
let mut rest = None;
let mut iter = pats.iter().enumerate();
while let Some((idx, pat)) = iter.next() {
// Interpret the first `..` pattern as a subtuple pattern.
for (idx, pat) in iter.by_ref() {
// Interpret the first `..` pattern as a sub-tuple pattern.
// Note that unlike for slice patterns,
// where `xs @ ..` is a legal sub-slice pattern,
// it is not a legal sub-tuple pattern.
if pat.is_rest() {
rest = Some((idx, pat.span));
break;
}
// It was not a subslice pattern so lower it normally.
// It was not a sub-tuple pattern so lower it normally.
elems.push(self.lower_pat(pat));
}
while let Some((_, pat)) = iter.next() {
// There was a previous subtuple pattern; make sure we don't allow more.
for (_, pat) in iter {
// There was a previous sub-tuple pattern; make sure we don't allow more...
if pat.is_rest() {
// ...but there was one again, so error.
self.ban_extra_rest_pat(pat.span, rest.unwrap().1, ctx);
} else {
elems.push(self.lower_pat(pat));
@ -2874,6 +2878,12 @@ fn lower_pat_tuple(
(elems.into(), rest.map(|(ddpos, _)| ddpos))
}
/// Lower a slice pattern of form `[pat_0, ..., pat_n]` into
/// `hir::PatKind::Slice(before, slice, after)`.
///
/// When encountering `($binding_mode $ident @)? ..` (`slice`),
/// this is interpreted as a sub-slice pattern semantically.
/// Patterns that follow, which are not like `slice` -- or an error occurs, are in `after`.
fn lower_pat_slice(&mut self, pats: &[AstP<Pat>]) -> hir::PatKind {
let mut before = Vec::new();
let mut after = Vec::new();
@ -2881,14 +2891,17 @@ fn lower_pat_slice(&mut self, pats: &[AstP<Pat>]) -> hir::PatKind {
let mut prev_rest_span = None;
let mut iter = pats.iter();
while let Some(pat) = iter.next() {
// Interpret the first `((ref mut?)? x @)? ..` pattern as a subslice pattern.
// Lower all the patterns until the first occurence of a sub-slice pattern.
for pat in iter.by_ref() {
match pat.kind {
// Found a sub-slice pattern `..`. Record, lower it to `_`, and stop here.
PatKind::Rest => {
prev_rest_span = Some(pat.span);
slice = Some(self.pat_wild_with_node_id_of(pat));
break;
},
// Found a sub-slice pattern `$binding_mode $ident @ ..`.
// Record, lower it to `$binding_mode $ident @ _`, and stop here.
PatKind::Ident(ref bm, ident, Some(ref sub)) if sub.is_rest() => {
prev_rest_span = Some(sub.span);
let lower_sub = |this: &mut Self| Some(this.pat_wild_with_node_id_of(sub));
@ -2896,14 +2909,13 @@ fn lower_pat_slice(&mut self, pats: &[AstP<Pat>]) -> hir::PatKind {
slice = Some(self.pat_with_node_id_of(pat, node));
break;
},
_ => {}
// It was not a subslice pattern so lower it normally.
_ => before.push(self.lower_pat(pat)),
}
// It was not a subslice pattern so lower it normally.
before.push(self.lower_pat(pat));
}
while let Some(pat) = iter.next() {
// Lower all the patterns after the first sub-slice pattern.
for pat in iter {
// There was a previous subslice pattern; make sure we don't allow more.
let rest_span = match pat.kind {
PatKind::Rest => Some(pat.span),
@ -2915,8 +2927,10 @@ fn lower_pat_slice(&mut self, pats: &[AstP<Pat>]) -> hir::PatKind {
_ => None,
};
if let Some(rest_span) = rest_span {
// We have e.g., `[a, .., b, ..]`. That's no good, error!
self.ban_extra_rest_pat(rest_span, prev_rest_span.unwrap(), "slice");
} else {
// Lower the pattern normally.
after.push(self.lower_pat(pat));
}
}