Auto merge of #119581 - Nadrieril:detangle-arena, r=compiler-errors

Exhaustiveness: remove the need for arena-allocation within the algorithm

After #119688, exhaustiveness checking doesn't need access to the arena anymore. This simplifies the lifetime story and makes it compile on stable without the extra dependency.

r? `@compiler-errors`

Author: bors

Cargo.lock

@@ -4366,7 +4366,6 @@ dependencies = [
  "rustc_target",
  "smallvec",
  "tracing",
- "typed-arena",
 ]
 
 [[package]]
@@ -5705,12 +5704,6 @@ dependencies = [
  "rustc-hash",
 ]
 
-[[package]]
-name = "typed-arena"
-version = "2.0.2"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6af6ae20167a9ece4bcb41af5b80f8a1f1df981f6391189ce00fd257af04126a"
-
 [[package]]
 name = "typenum"
 version = "1.16.0"

compiler/rustc_pattern_analysis/Cargo.toml

@@ -20,13 +20,10 @@ rustc_span = { path = "../rustc_span", optional = true }
 rustc_target = { path = "../rustc_target", optional = true }
 smallvec = { version = "1.8.1", features = ["union"] }
 tracing = "0.1"
-typed-arena = { version = "2.0.2", optional = true }
 # tidy-alphabetical-end
 
 [features]
 default = ["rustc"]
-# It's not possible to only enable the `typed_arena` dependency when the `rustc` feature is off, so
-# we use another feature instead. The crate won't compile if one of these isn't enabled.
 rustc = [
     "dep:rustc_arena",
     "dep:rustc_data_structures",
@@ -41,6 +38,3 @@ rustc = [
     "smallvec/may_dangle",
     "rustc_index/nightly",
 ]
-stable = [
-    "dep:typed-arena",
-]

compiler/rustc_pattern_analysis/src/constructor.rs

@@ -718,7 +718,7 @@ impl<Cx: TypeCx> Constructor<Cx> {
 
     /// The number of fields for this constructor. This must be kept in sync with
     /// `Fields::wildcards`.
-    pub(crate) fn arity(&self, pcx: &PlaceCtxt<'_, '_, Cx>) -> usize {
+    pub(crate) fn arity(&self, pcx: &PlaceCtxt<'_, Cx>) -> usize {
         pcx.ctor_arity(self)
     }
 
@@ -727,7 +727,7 @@ impl<Cx: TypeCx> Constructor<Cx> {
     /// this checks for inclusion.
     // We inline because this has a single call site in `Matrix::specialize_constructor`.
     #[inline]
-    pub(crate) fn is_covered_by<'p>(&self, pcx: &PlaceCtxt<'_, 'p, Cx>, other: &Self) -> bool {
+    pub(crate) fn is_covered_by(&self, pcx: &PlaceCtxt<'_, Cx>, other: &Self) -> bool {
         match (self, other) {
             (Wildcard, _) => pcx
                 .mcx
@@ -861,7 +861,7 @@ impl<Cx: TypeCx> ConstructorSet<Cx> {
     #[instrument(level = "debug", skip(self, pcx, ctors), ret)]
     pub(crate) fn split<'a>(
         &self,
-        pcx: &PlaceCtxt<'a, '_, Cx>,
+        pcx: &PlaceCtxt<'a, Cx>,
         ctors: impl Iterator<Item = &'a Constructor<Cx>> + Clone,
     ) -> SplitConstructorSet<Cx> {
         let mut present: SmallVec<[_; 1]> = SmallVec::new();

compiler/rustc_pattern_analysis/src/lib.rs

@@ -38,13 +38,6 @@ use crate::rustc::RustcMatchCheckCtxt;
 #[cfg(feature = "rustc")]
 use crate::usefulness::{compute_match_usefulness, ValidityConstraint};
 
-// It's not possible to only enable the `typed_arena` dependency when the `rustc` feature is off, so
-// we use another feature instead. The crate won't compile if one of these isn't enabled.
-#[cfg(feature = "rustc")]
-pub(crate) use rustc_arena::TypedArena;
-#[cfg(feature = "stable")]
-pub(crate) use typed_arena::Arena as TypedArena;
-
 pub trait Captures<'a> {}
 impl<'a, T: ?Sized> Captures<'a> for T {}
 
@@ -89,11 +82,9 @@ pub trait TypeCx: Sized + fmt::Debug {
 /// Context that provides information global to a match.
 #[derive(derivative::Derivative)]
 #[derivative(Clone(bound = ""), Copy(bound = ""))]
-pub struct MatchCtxt<'a, 'p, Cx: TypeCx> {
+pub struct MatchCtxt<'a, Cx: TypeCx> {
     /// The context for type information.
     pub tycx: &'a Cx,
-    /// An arena to store the wildcards we produce during analysis.
-    pub wildcard_arena: &'p TypedArena<DeconstructedPat<'p, Cx>>,
 }
 
 /// The arm of a match expression.
@@ -114,11 +105,9 @@ pub fn analyze_match<'p, 'tcx>(
     arms: &[rustc::MatchArm<'p, 'tcx>],
     scrut_ty: Ty<'tcx>,
 ) -> Result<rustc::UsefulnessReport<'p, 'tcx>, ErrorGuaranteed> {
-    // Arena to store the extra wildcards we construct during analysis.
-    let wildcard_arena = tycx.pattern_arena;
     let scrut_ty = tycx.reveal_opaque_ty(scrut_ty);
     let scrut_validity = ValidityConstraint::from_bool(tycx.known_valid_scrutinee);
-    let cx = MatchCtxt { tycx, wildcard_arena };
+    let cx = MatchCtxt { tycx };
 
     let report = compute_match_usefulness(cx, arms, scrut_ty, scrut_validity)?;
 

compiler/rustc_pattern_analysis/src/pat.rs

@@ -240,7 +240,7 @@ impl<Cx: TypeCx> WitnessPat<Cx> {
     /// Construct a pattern that matches everything that starts with this constructor.
     /// For example, if `ctor` is a `Constructor::Variant` for `Option::Some`, we get the pattern
     /// `Some(_)`.
-    pub(crate) fn wild_from_ctor(pcx: &PlaceCtxt<'_, '_, Cx>, ctor: Constructor<Cx>) -> Self {
+    pub(crate) fn wild_from_ctor(pcx: &PlaceCtxt<'_, Cx>, ctor: Constructor<Cx>) -> Self {
         let field_tys = pcx.ctor_sub_tys(&ctor);
         let fields = field_tys.iter().map(|ty| Self::wildcard(*ty)).collect();
         Self::new(ctor, fields, pcx.ty)

compiler/rustc_pattern_analysis/src/rustc.rs

@@ -33,11 +33,11 @@ pub type ConstructorSet<'p, 'tcx> =
 pub type DeconstructedPat<'p, 'tcx> =
     crate::pat::DeconstructedPat<'p, RustcMatchCheckCtxt<'p, 'tcx>>;
 pub type MatchArm<'p, 'tcx> = crate::MatchArm<'p, RustcMatchCheckCtxt<'p, 'tcx>>;
-pub type MatchCtxt<'a, 'p, 'tcx> = crate::MatchCtxt<'a, 'p, RustcMatchCheckCtxt<'p, 'tcx>>;
+pub type MatchCtxt<'a, 'p, 'tcx> = crate::MatchCtxt<'a, RustcMatchCheckCtxt<'p, 'tcx>>;
 pub type OverlappingRanges<'p, 'tcx> =
     crate::usefulness::OverlappingRanges<'p, RustcMatchCheckCtxt<'p, 'tcx>>;
 pub(crate) type PlaceCtxt<'a, 'p, 'tcx> =
-    crate::usefulness::PlaceCtxt<'a, 'p, RustcMatchCheckCtxt<'p, 'tcx>>;
+    crate::usefulness::PlaceCtxt<'a, RustcMatchCheckCtxt<'p, 'tcx>>;
 pub(crate) type SplitConstructorSet<'p, 'tcx> =
     crate::constructor::SplitConstructorSet<RustcMatchCheckCtxt<'p, 'tcx>>;
 pub type Usefulness<'p, 'tcx> = crate::usefulness::Usefulness<'p, RustcMatchCheckCtxt<'p, 'tcx>>;
@@ -80,7 +80,9 @@ pub struct RustcMatchCheckCtxt<'p, 'tcx> {
     /// outside its module and should not be matchable with an empty match statement.
     pub module: DefId,
     pub param_env: ty::ParamEnv<'tcx>,
+    /// To allocate lowered patterns
     pub pattern_arena: &'p TypedArena<DeconstructedPat<'p, 'tcx>>,
+    /// To allocate the result of `self.ctor_sub_tys()`
     pub dropless_arena: &'p DroplessArena,
     /// Lint level at the match.
     pub match_lint_level: HirId,

compiler/rustc_pattern_analysis/src/usefulness.rs

@@ -732,19 +732,19 @@ pub fn ensure_sufficient_stack<R>(f: impl FnOnce() -> R) -> R {
 /// Context that provides information local to a place under investigation.
 #[derive(derivative::Derivative)]
 #[derivative(Debug(bound = ""), Clone(bound = ""), Copy(bound = ""))]
-pub(crate) struct PlaceCtxt<'a, 'p, Cx: TypeCx> {
+pub(crate) struct PlaceCtxt<'a, Cx: TypeCx> {
     #[derivative(Debug = "ignore")]
-    pub(crate) mcx: MatchCtxt<'a, 'p, Cx>,
+    pub(crate) mcx: MatchCtxt<'a, Cx>,
     /// Type of the place under investigation.
     pub(crate) ty: Cx::Ty,
     /// Whether the place is the original scrutinee place, as opposed to a subplace of it.
     pub(crate) is_scrutinee: bool,
 }
 
-impl<'a, 'p, Cx: TypeCx> PlaceCtxt<'a, 'p, Cx> {
+impl<'a, Cx: TypeCx> PlaceCtxt<'a, Cx> {
     /// A `PlaceCtxt` when code other than `is_useful` needs one.
     #[cfg_attr(not(feature = "rustc"), allow(dead_code))]
-    pub(crate) fn new_dummy(mcx: MatchCtxt<'a, 'p, Cx>, ty: Cx::Ty) -> Self {
+    pub(crate) fn new_dummy(mcx: MatchCtxt<'a, Cx>, ty: Cx::Ty) -> Self {
         PlaceCtxt { mcx, ty, is_scrutinee: false }
     }
 
@@ -1067,7 +1067,7 @@ impl<'p, Cx: TypeCx> Matrix<'p, Cx> {
     /// This computes `specialize(ctor, self)`. See top of the file for explanations.
     fn specialize_constructor(
         &self,
-        pcx: &PlaceCtxt<'_, 'p, Cx>,
+        pcx: &PlaceCtxt<'_, Cx>,
         ctor: &Constructor<Cx>,
         ctor_is_relevant: bool,
     ) -> Matrix<'p, Cx> {
@@ -1226,7 +1226,7 @@ impl<Cx: TypeCx> WitnessStack<Cx> {
     /// pats: [(false, "foo"), _, true]
     /// result: [Enum::Variant { a: (false, "foo"), b: _ }, true]
     /// ```
-    fn apply_constructor(&mut self, pcx: &PlaceCtxt<'_, '_, Cx>, ctor: &Constructor<Cx>) {
+    fn apply_constructor(&mut self, pcx: &PlaceCtxt<'_, Cx>, ctor: &Constructor<Cx>) {
         let len = self.0.len();
         let arity = ctor.arity(pcx);
         let fields = self.0.drain((len - arity)..).rev().collect();
@@ -1277,7 +1277,7 @@ impl<Cx: TypeCx> WitnessMatrix<Cx> {
     /// Reverses specialization by `ctor`. See the section on `unspecialize` at the top of the file.
     fn apply_constructor(
         &mut self,
-        pcx: &PlaceCtxt<'_, '_, Cx>,
+        pcx: &PlaceCtxt<'_, Cx>,
         missing_ctors: &[Constructor<Cx>],
         ctor: &Constructor<Cx>,
         report_individual_missing_ctors: bool,
@@ -1421,7 +1421,7 @@ fn collect_overlapping_range_endpoints<'p, Cx: TypeCx>(
 /// This is all explained at the top of the file.
 #[instrument(level = "debug", skip(mcx, is_top_level), ret)]
 fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: TypeCx>(
-    mcx: MatchCtxt<'a, 'p, Cx>,
+    mcx: MatchCtxt<'a, Cx>,
     matrix: &mut Matrix<'p, Cx>,
     overlapping_range_endpoints: &mut Vec<OverlappingRanges<'p, Cx>>,
     is_top_level: bool,
@@ -1591,7 +1591,7 @@ pub struct UsefulnessReport<'p, Cx: TypeCx> {
 /// Computes whether a match is exhaustive and which of its arms are useful.
 #[instrument(skip(cx, arms), level = "debug")]
 pub fn compute_match_usefulness<'p, Cx: TypeCx>(
-    cx: MatchCtxt<'_, 'p, Cx>,
+    cx: MatchCtxt<'_, Cx>,
     arms: &[MatchArm<'p, Cx>],
     scrut_ty: Cx::Ty,
     scrut_validity: ValidityConstraint,

src/tools/tidy/src/deps.rs

@@ -359,7 +359,6 @@ const PERMITTED_RUSTC_DEPENDENCIES: &[&str] = &[
     "tracing-tree",
     "twox-hash",
     "type-map",
-    "typed-arena",
     "typenum",
     "unic-langid",
     "unic-langid-impl",