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Rollup merge of rust-lang#67318 - Centril:spdocs, r=matthewjasper
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Improve typeck & lowering docs for slice patterns

cc rust-lang#62254

r? @matthewjasper
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Centril authored Dec 16, 2019
2 parents f9e1d3b + d9d1f23 commit eb14f91
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Showing 4 changed files with 63 additions and 17 deletions.
38 changes: 26 additions & 12 deletions src/librustc/hir/lowering.rs
Original file line number Diff line number Diff line change
Expand Up @@ -2852,19 +2852,23 @@ impl<'a> LoweringContext<'a> {
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));
Expand All @@ -2874,36 +2878,44 @@ impl<'a> LoweringContext<'a> {
(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();
let mut slice = None;
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));
let node = self.lower_pat_ident(pat, bm, ident, lower_sub);
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),
Expand All @@ -2915,8 +2927,10 @@ impl<'a> LoweringContext<'a> {
_ => 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));
}
}
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11 changes: 9 additions & 2 deletions src/librustc/hir/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1048,8 +1048,15 @@ pub enum PatKind {
/// A range pattern (e.g., `1..=2` or `1..2`).
Range(P<Expr>, P<Expr>, RangeEnd),

/// `[a, b, ..i, y, z]` is represented as:
/// `PatKind::Slice(box [a, b], Some(i), box [y, z])`.
/// A slice pattern, `[before_0, ..., before_n, (slice, after_0, ..., after_n)?]`.
///
/// Here, `slice` is lowered from the syntax `($binding_mode $ident @)? ..`.
/// If `slice` exists, then `after` can be non-empty.
///
/// The representation for e.g., `[a, b, .., c, d]` is:
/// ```
/// PatKind::Slice([Binding(a), Binding(b)], Some(Wild), [Binding(c), Binding(d)])
/// ```
Slice(HirVec<P<Pat>>, Option<P<Pat>>, HirVec<P<Pat>>),
}

Expand Down
6 changes: 3 additions & 3 deletions src/librustc_typeck/check/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -5351,9 +5351,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
directly, not through a function pointer");
}

// Resolves `typ` by a single level if `typ` is a type variable.
// If no resolution is possible, then an error is reported.
// Numeric inference variables may be left unresolved.
/// Resolves `typ` by a single level if `typ` is a type variable.
/// If no resolution is possible, then an error is reported.
/// Numeric inference variables may be left unresolved.
pub fn structurally_resolved_type(&self, sp: Span, ty: Ty<'tcx>) -> Ty<'tcx> {
let ty = self.resolve_vars_with_obligations(ty);
if !ty.is_ty_var() {
Expand Down
25 changes: 25 additions & 0 deletions src/librustc_typeck/check/pat.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1154,6 +1154,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
self.tcx.mk_ref(region, mt)
}

/// Type check a slice pattern.
///
/// Syntactically, these look like `[pat_0, ..., pat_n]`.
/// Semantically, we are type checking a pattern with structure:
/// ```
/// [before_0, ..., before_n, (slice, after_0, ... after_n)?]
/// ```
/// The type of `slice`, if it is present, depends on the `expected` type.
/// If `slice` is missing, then so is `after_i`.
/// If `slice` is present, it can still represent 0 elements.
fn check_pat_slice(
&self,
span: Span,
Expand All @@ -1167,27 +1177,39 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
let tcx = self.tcx;
let expected_ty = self.structurally_resolved_type(span, expected);
let (inner_ty, slice_ty) = match expected_ty.kind {
// An array, so we might have something like `let [a, b, c] = [0, 1, 2];`.
ty::Array(inner_ty, size) => {
let slice_ty = if let Some(size) = size.try_eval_usize(tcx, self.param_env) {
// Now we know the length...
let min_len = before.len() as u64 + after.len() as u64;
if slice.is_none() {
// ...and since there is no variable-length pattern,
// we require an exact match between the number of elements
// in the array pattern and as provided by the matched type.
if min_len != size {
self.error_scrutinee_inconsistent_length(span, min_len, size)
}
tcx.types.err
} else if let Some(rest) = size.checked_sub(min_len) {
// The variable-length pattern was there,
// so it has an array type with the remaining elements left as its size...
tcx.mk_array(inner_ty, rest)
} else {
// ...however, in this case, there were no remaining elements.
// That is, the slice pattern requires more than the array type offers.
self.error_scrutinee_with_rest_inconsistent_length(span, min_len, size);
tcx.types.err
}
} else {
// No idea what the length is, which happens if we have e.g.,
// `let [a, b] = arr` where `arr: [T; N]` where `const N: usize`.
self.error_scrutinee_unfixed_length(span);
tcx.types.err
};
(inner_ty, slice_ty)
}
ty::Slice(inner_ty) => (inner_ty, expected_ty),
// The expected type must be an array or slice, but was neither, so error.
_ => {
if !expected_ty.references_error() {
self.error_expected_array_or_slice(span, expected_ty);
Expand All @@ -1196,12 +1218,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
}
};

// Type check all the patterns before `slice`.
for elt in before {
self.check_pat(&elt, inner_ty, def_bm, discrim_span);
}
// Type check the `slice`, if present, against its expected type.
if let Some(slice) = slice {
self.check_pat(&slice, slice_ty, def_bm, discrim_span);
}
// Type check the elements after `slice`, if present.
for elt in after {
self.check_pat(&elt, inner_ty, def_bm, discrim_span);
}
Expand Down

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