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match ergonomics: replace peel_off_references with a recursive call #139449

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Merged
merged 6 commits into from
Apr 10, 2025
Merged

match ergonomics: replace peel_off_references with a recursive call #139449

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a0d9c87
Inline `calc_default_binding_mode`
Nadrieril Apr 6, 2025
9f57903
Insert adjustments incrementally
Nadrieril Apr 6, 2025
19950b5
Turn the peeling loop into a recursive call
Nadrieril Apr 6, 2025
38adb99
Reorganize `check_pat_inner`
Nadrieril Apr 6, 2025
6588018
Return a type from `check_pat_inner`
Nadrieril Apr 6, 2025
f458151
Remove redundant assignment
Nadrieril Apr 6, 2025
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260 changes: 115 additions & 145 deletions compiler/rustc_hir_typeck/src/pat.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -163,9 +163,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
enum AdjustMode {
/// Peel off all immediate reference types.
Peel,
/// Reset binding mode to the initial mode.
/// Used for destructuring assignment, where we don't want any match ergonomics.
Reset,
/// Pass on the input binding mode and expected type.
Pass,
}
Expand Down Expand Up @@ -321,77 +318,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
/// Conversely, inside this module, `check_pat_top` should never be used.
#[instrument(level = "debug", skip(self, pat_info))]
fn check_pat(&self, pat: &'tcx Pat<'tcx>, expected: Ty<'tcx>, pat_info: PatInfo<'tcx>) {
let PatInfo { binding_mode, max_ref_mutbl, top_info: ti, current_depth, .. } = pat_info;

let path_res = match pat.kind {
let opt_path_res = match pat.kind {
PatKind::Expr(PatExpr { kind: PatExprKind::Path(qpath), hir_id, span }) => {
Some(self.resolve_ty_and_res_fully_qualified_call(qpath, *hir_id, *span))
}
_ => None,
};
let adjust_mode = self.calc_adjust_mode(pat, path_res.map(|(res, ..)| res));
let (expected, binding_mode, max_ref_mutbl) =
self.calc_default_binding_mode(pat, expected, binding_mode, adjust_mode, max_ref_mutbl);
let pat_info = PatInfo {
binding_mode,
max_ref_mutbl,
top_info: ti,
decl_origin: pat_info.decl_origin,
current_depth: current_depth + 1,
};

let ty = match pat.kind {
PatKind::Missing | PatKind::Wild | PatKind::Err(_) => expected,
// We allow any type here; we ensure that the type is uninhabited during match checking.
PatKind::Never => expected,
PatKind::Expr(PatExpr { kind: PatExprKind::Path(qpath), hir_id, span }) => {
let ty = self.check_pat_path(
*hir_id,
pat.hir_id,
*span,
qpath,
path_res.unwrap(),
expected,
&pat_info.top_info,
);
self.write_ty(*hir_id, ty);
ty
}
PatKind::Expr(lt) => self.check_pat_lit(pat.span, lt, expected, &pat_info.top_info),
PatKind::Range(lhs, rhs, _) => {
self.check_pat_range(pat.span, lhs, rhs, expected, &pat_info.top_info)
}
PatKind::Binding(ba, var_id, ident, sub) => {
self.check_pat_ident(pat, ba, var_id, ident, sub, expected, pat_info)
}
PatKind::TupleStruct(ref qpath, subpats, ddpos) => {
self.check_pat_tuple_struct(pat, qpath, subpats, ddpos, expected, pat_info)
}
PatKind::Struct(ref qpath, fields, has_rest_pat) => {
self.check_pat_struct(pat, qpath, fields, has_rest_pat, expected, pat_info)
}
PatKind::Guard(pat, cond) => {
self.check_pat(pat, expected, pat_info);
self.check_expr_has_type_or_error(cond, self.tcx.types.bool, |_| {});
expected
}
PatKind::Or(pats) => {
for pat in pats {
self.check_pat(pat, expected, pat_info);
}
expected
}
PatKind::Tuple(elements, ddpos) => {
self.check_pat_tuple(pat.span, elements, ddpos, expected, pat_info)
}
PatKind::Box(inner) => self.check_pat_box(pat.span, inner, expected, pat_info),
PatKind::Deref(inner) => self.check_pat_deref(pat.span, inner, expected, pat_info),
PatKind::Ref(inner, mutbl) => self.check_pat_ref(pat, inner, mutbl, expected, pat_info),
PatKind::Slice(before, slice, after) => {
self.check_pat_slice(pat.span, before, slice, after, expected, pat_info)
}
};

let adjust_mode = self.calc_adjust_mode(pat, opt_path_res.map(|(res, ..)| res));
let ty = self.check_pat_inner(pat, opt_path_res, adjust_mode, expected, pat_info);
self.write_ty(pat.hir_id, ty);

// (note_1): In most of the cases where (note_1) is referenced
Expand Down Expand Up @@ -437,39 +371,133 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
// `regions-relate-bound-regions-on-closures-to-inference-variables.rs`,
}

/// Compute the new expected type and default binding mode from the old ones
/// as well as the pattern form we are currently checking.
fn calc_default_binding_mode(
// Helper to avoid resolving the same path pattern several times.
fn check_pat_inner(
&self,
pat: &'tcx Pat<'tcx>,
expected: Ty<'tcx>,
def_br: ByRef,
opt_path_res: Option<(Res, Option<LoweredTy<'tcx>>, &'tcx [hir::PathSegment<'tcx>])>,
adjust_mode: AdjustMode,
max_ref_mutbl: MutblCap,
) -> (Ty<'tcx>, ByRef, MutblCap) {
expected: Ty<'tcx>,
pat_info: PatInfo<'tcx>,
) -> Ty<'tcx> {
#[cfg(debug_assertions)]
if def_br == ByRef::Yes(Mutability::Mut)
&& max_ref_mutbl != MutblCap::Mut
if pat_info.binding_mode == ByRef::Yes(Mutability::Mut)
&& pat_info.max_ref_mutbl != MutblCap::Mut
&& self.downgrade_mut_inside_shared()
{
span_bug!(pat.span, "Pattern mutability cap violated!");
}
match adjust_mode {
AdjustMode::Pass => (expected, def_br, max_ref_mutbl),
AdjustMode::Reset => (expected, ByRef::No, MutblCap::Mut),
AdjustMode::Peel => self.peel_off_references(pat, expected, def_br, max_ref_mutbl),

// Resolve type if needed.
let expected = if let AdjustMode::Peel = adjust_mode
&& pat.default_binding_modes
{
self.try_structurally_resolve_type(pat.span, expected)
} else {
expected
};
let old_pat_info = pat_info;
let pat_info = PatInfo { current_depth: old_pat_info.current_depth + 1, ..old_pat_info };

match pat.kind {
// Peel off a `&` or `&mut` from the scrutinee type. See the examples in
// `tests/ui/rfcs/rfc-2005-default-binding-mode`.
_ if let AdjustMode::Peel = adjust_mode
&& pat.default_binding_modes
&& let ty::Ref(_, inner_ty, inner_mutability) = *expected.kind() =>
{
debug!("inspecting {:?}", expected);

debug!("current discriminant is Ref, inserting implicit deref");
// Preserve the reference type. We'll need it later during THIR lowering.
self.typeck_results
.borrow_mut()
.pat_adjustments_mut()
.entry(pat.hir_id)
.or_default()
.push(expected);

let mut binding_mode = ByRef::Yes(match pat_info.binding_mode {
// If default binding mode is by value, make it `ref` or `ref mut`
// (depending on whether we observe `&` or `&mut`).
ByRef::No |
// When `ref mut`, stay a `ref mut` (on `&mut`) or downgrade to `ref` (on `&`).
ByRef::Yes(Mutability::Mut) => inner_mutability,
// Once a `ref`, always a `ref`.
// This is because a `& &mut` cannot mutate the underlying value.
ByRef::Yes(Mutability::Not) => Mutability::Not,
});

let mut max_ref_mutbl = pat_info.max_ref_mutbl;
if self.downgrade_mut_inside_shared() {
binding_mode = binding_mode.cap_ref_mutability(max_ref_mutbl.as_mutbl());
}
if binding_mode == ByRef::Yes(Mutability::Not) {
max_ref_mutbl = MutblCap::Not;
}
debug!("default binding mode is now {:?}", binding_mode);

// Use the old pat info to keep `current_depth` to its old value.
let new_pat_info = PatInfo { binding_mode, max_ref_mutbl, ..old_pat_info };
// Recurse with the new expected type.
self.check_pat_inner(pat, opt_path_res, adjust_mode, inner_ty, new_pat_info)
}
PatKind::Missing | PatKind::Wild | PatKind::Err(_) => expected,
// We allow any type here; we ensure that the type is uninhabited during match checking.
PatKind::Never => expected,
PatKind::Expr(PatExpr { kind: PatExprKind::Path(qpath), hir_id, span }) => {
let ty = self.check_pat_path(
*hir_id,
pat.hir_id,
*span,
qpath,
opt_path_res.unwrap(),
expected,
&pat_info.top_info,
);
self.write_ty(*hir_id, ty);
ty
}
PatKind::Expr(lt) => self.check_pat_lit(pat.span, lt, expected, &pat_info.top_info),
PatKind::Range(lhs, rhs, _) => {
self.check_pat_range(pat.span, lhs, rhs, expected, &pat_info.top_info)
}
PatKind::Binding(ba, var_id, ident, sub) => {
self.check_pat_ident(pat, ba, var_id, ident, sub, expected, pat_info)
}
PatKind::TupleStruct(ref qpath, subpats, ddpos) => {
self.check_pat_tuple_struct(pat, qpath, subpats, ddpos, expected, pat_info)
}
PatKind::Struct(ref qpath, fields, has_rest_pat) => {
self.check_pat_struct(pat, qpath, fields, has_rest_pat, expected, pat_info)
}
PatKind::Guard(pat, cond) => {
self.check_pat(pat, expected, pat_info);
self.check_expr_has_type_or_error(cond, self.tcx.types.bool, |_| {});
expected
}
PatKind::Or(pats) => {
for pat in pats {
self.check_pat(pat, expected, pat_info);
}
expected
}
PatKind::Tuple(elements, ddpos) => {
self.check_pat_tuple(pat.span, elements, ddpos, expected, pat_info)
}
PatKind::Box(inner) => self.check_pat_box(pat.span, inner, expected, pat_info),
PatKind::Deref(inner) => self.check_pat_deref(pat.span, inner, expected, pat_info),
PatKind::Ref(inner, mutbl) => self.check_pat_ref(pat, inner, mutbl, expected, pat_info),
PatKind::Slice(before, slice, after) => {
self.check_pat_slice(pat.span, before, slice, after, expected, pat_info)
}
}
}

/// How should the binding mode and expected type be adjusted?
///
/// When the pattern is a path pattern, `opt_path_res` must be `Some(res)`.
fn calc_adjust_mode(&self, pat: &'tcx Pat<'tcx>, opt_path_res: Option<Res>) -> AdjustMode {
// When we perform destructuring assignment, we disable default match bindings, which are
// unintuitive in this context.
if !pat.default_binding_modes {
return AdjustMode::Reset;
}
match &pat.kind {
// Type checking these product-like types successfully always require
// that the expected type be of those types and not reference types.
Expand Down Expand Up @@ -526,64 +554,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
}
}

/// Peel off as many immediately nested `& mut?` from the expected type as possible
/// and return the new expected type and binding default binding mode.
/// The adjustments vector, if non-empty is stored in a table.
fn peel_off_references(
&self,
pat: &'tcx Pat<'tcx>,
expected: Ty<'tcx>,
mut def_br: ByRef,
mut max_ref_mutbl: MutblCap,
) -> (Ty<'tcx>, ByRef, MutblCap) {
let mut expected = self.try_structurally_resolve_type(pat.span, expected);
// Peel off as many `&` or `&mut` from the scrutinee type as possible. For example,
// for `match &&&mut Some(5)` the loop runs three times, aborting when it reaches
// the `Some(5)` which is not of type Ref.
//
// For each ampersand peeled off, update the binding mode and push the original
// type into the adjustments vector.
//
// See the examples in `ui/match-defbm*.rs`.
let mut pat_adjustments = vec![];
while let ty::Ref(_, inner_ty, inner_mutability) = *expected.kind() {
debug!("inspecting {:?}", expected);

debug!("current discriminant is Ref, inserting implicit deref");
// Preserve the reference type. We'll need it later during THIR lowering.
pat_adjustments.push(expected);

expected = self.try_structurally_resolve_type(pat.span, inner_ty);
def_br = ByRef::Yes(match def_br {
// If default binding mode is by value, make it `ref` or `ref mut`
// (depending on whether we observe `&` or `&mut`).
ByRef::No |
// When `ref mut`, stay a `ref mut` (on `&mut`) or downgrade to `ref` (on `&`).
ByRef::Yes(Mutability::Mut) => inner_mutability,
// Once a `ref`, always a `ref`.
// This is because a `& &mut` cannot mutate the underlying value.
ByRef::Yes(Mutability::Not) => Mutability::Not,
});
}

if self.downgrade_mut_inside_shared() {
def_br = def_br.cap_ref_mutability(max_ref_mutbl.as_mutbl());
}
if def_br == ByRef::Yes(Mutability::Not) {
max_ref_mutbl = MutblCap::Not;
}

if !pat_adjustments.is_empty() {
debug!("default binding mode is now {:?}", def_br);
self.typeck_results
.borrow_mut()
.pat_adjustments_mut()
.insert(pat.hir_id, pat_adjustments);
}

(expected, def_br, max_ref_mutbl)
}

fn check_pat_expr_unadjusted(&self, lt: &'tcx hir::PatExpr<'tcx>) -> Ty<'tcx> {
let ty = match &lt.kind {
rustc_hir::PatExprKind::Lit { lit, negated } => {
Expand Down
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