Auto merge of #17880 - lnicola:sync-from-rust, r=lnicola

minor: sync from downstream

8 files changed, 365 insertions, 150 deletions

diff --git a/compiler/rustc_pattern_analysis/src/constructor.rs b/compiler/rustc_pattern_analysis/src/constructor.rs
index f3e8e547066..3a2a75a638f 100644
--- a/compiler/rustc_pattern_analysis/src/constructor.rs
+++ b/compiler/rustc_pattern_analysis/src/constructor.rs
@@ -180,16 +180,14 @@ use std::cmp::{self, max, min, Ordering};
 use std::fmt;
 use std::iter::once;
 
-use smallvec::SmallVec;
-
 use rustc_apfloat::ieee::{DoubleS, HalfS, IeeeFloat, QuadS, SingleS};
 use rustc_index::bit_set::{BitSet, GrowableBitSet};
 use rustc_index::IndexVec;
+use smallvec::SmallVec;
 
 use self::Constructor::*;
 use self::MaybeInfiniteInt::*;
 use self::SliceKind::*;
-
 use crate::PatCx;
 
 /// Whether we have seen a constructor in the column or not.
diff --git a/compiler/rustc_pattern_analysis/src/errors.rs b/compiler/rustc_pattern_analysis/src/errors.rs
index 27f227e6d9c..1f7852e5190 100644
--- a/compiler/rustc_pattern_analysis/src/errors.rs
+++ b/compiler/rustc_pattern_analysis/src/errors.rs
@@ -1,6 +1,5 @@
 use rustc_errors::{Diag, EmissionGuarantee, SubdiagMessageOp, Subdiagnostic};
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
-use rustc_middle::thir::Pat;
 use rustc_middle::ty::Ty;
 use rustc_span::Span;
 
@@ -8,18 +7,18 @@ use crate::rustc::{RustcPatCtxt, WitnessPat};
 
 #[derive(Subdiagnostic)]
 #[label(pattern_analysis_uncovered)]
-pub struct Uncovered<'tcx> {
+pub struct Uncovered {
     #[primary_span]
     span: Span,
     count: usize,
-    witness_1: Pat<'tcx>,
-    witness_2: Pat<'tcx>,
-    witness_3: Pat<'tcx>,
+    witness_1: String, // a printed pattern
+    witness_2: String, // a printed pattern
+    witness_3: String, // a printed pattern
     remainder: usize,
 }
 
-impl<'tcx> Uncovered<'tcx> {
-    pub fn new<'p>(
+impl Uncovered {
+    pub fn new<'p, 'tcx>(
         span: Span,
         cx: &RustcPatCtxt<'p, 'tcx>,
         witnesses: Vec<WitnessPat<'p, 'tcx>>,
@@ -27,19 +26,13 @@ impl<'tcx> Uncovered<'tcx> {
     where
         'tcx: 'p,
     {
-        let witness_1 = cx.hoist_witness_pat(witnesses.get(0).unwrap());
+        let witness_1 = cx.print_witness_pat(witnesses.get(0).unwrap());
         Self {
             span,
             count: witnesses.len(),
             // Substitute dummy values if witnesses is smaller than 3. These will never be read.
-            witness_2: witnesses
-                .get(1)
-                .map(|w| cx.hoist_witness_pat(w))
-                .unwrap_or_else(|| witness_1.clone()),
-            witness_3: witnesses
-                .get(2)
-                .map(|w| cx.hoist_witness_pat(w))
-                .unwrap_or_else(|| witness_1.clone()),
+            witness_2: witnesses.get(1).map(|w| cx.print_witness_pat(w)).unwrap_or_default(),
+            witness_3: witnesses.get(2).map(|w| cx.print_witness_pat(w)).unwrap_or_default(),
             witness_1,
             remainder: witnesses.len().saturating_sub(3),
         }
@@ -49,19 +42,19 @@ impl<'tcx> Uncovered<'tcx> {
 #[derive(LintDiagnostic)]
 #[diag(pattern_analysis_overlapping_range_endpoints)]
 #[note]
-pub struct OverlappingRangeEndpoints<'tcx> {
+pub struct OverlappingRangeEndpoints {
     #[label]
     pub range: Span,
     #[subdiagnostic]
-    pub overlap: Vec<Overlap<'tcx>>,
+    pub overlap: Vec<Overlap>,
 }
 
-pub struct Overlap<'tcx> {
+pub struct Overlap {
     pub span: Span,
-    pub range: Pat<'tcx>,
+    pub range: String, // a printed pattern
 }
 
-impl<'tcx> Subdiagnostic for Overlap<'tcx> {
+impl Subdiagnostic for Overlap {
     fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
         self,
         diag: &mut Diag<'_, G>,
@@ -78,38 +71,38 @@ impl<'tcx> Subdiagnostic for Overlap<'tcx> {
 
 #[derive(LintDiagnostic)]
 #[diag(pattern_analysis_excluside_range_missing_max)]
-pub struct ExclusiveRangeMissingMax<'tcx> {
+pub struct ExclusiveRangeMissingMax {
     #[label]
     #[suggestion(code = "{suggestion}", applicability = "maybe-incorrect")]
     /// This is an exclusive range that looks like `lo..max` (i.e. doesn't match `max`).
     pub first_range: Span,
     /// Suggest `lo..=max` instead.
     pub suggestion: String,
-    pub max: Pat<'tcx>,
+    pub max: String, // a printed pattern
 }
 
 #[derive(LintDiagnostic)]
 #[diag(pattern_analysis_excluside_range_missing_gap)]
-pub struct ExclusiveRangeMissingGap<'tcx> {
+pub struct ExclusiveRangeMissingGap {
     #[label]
     #[suggestion(code = "{suggestion}", applicability = "maybe-incorrect")]
     /// This is an exclusive range that looks like `lo..gap` (i.e. doesn't match `gap`).
     pub first_range: Span,
-    pub gap: Pat<'tcx>,
+    pub gap: String, // a printed pattern
     /// Suggest `lo..=gap` instead.
     pub suggestion: String,
     #[subdiagnostic]
     /// All these ranges skipped over `gap` which we think is probably a mistake.
-    pub gap_with: Vec<GappedRange<'tcx>>,
+    pub gap_with: Vec<GappedRange>,
 }
 
-pub struct GappedRange<'tcx> {
+pub struct GappedRange {
     pub span: Span,
-    pub gap: Pat<'tcx>,
-    pub first_range: Pat<'tcx>,
+    pub gap: String,         // a printed pattern
+    pub first_range: String, // a printed pattern
 }
 
-impl<'tcx> Subdiagnostic for GappedRange<'tcx> {
+impl Subdiagnostic for GappedRange {
     fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
         self,
         diag: &mut Diag<'_, G>,
@@ -134,7 +127,7 @@ impl<'tcx> Subdiagnostic for GappedRange<'tcx> {
 pub(crate) struct NonExhaustiveOmittedPattern<'tcx> {
     pub scrut_ty: Ty<'tcx>,
     #[subdiagnostic]
-    pub uncovered: Uncovered<'tcx>,
+    pub uncovered: Uncovered,
 }
 
 #[derive(LintDiagnostic)]
diff --git a/compiler/rustc_pattern_analysis/src/lib.rs b/compiler/rustc_pattern_analysis/src/lib.rs
index a5c0b13c90b..6c9c848bb10 100644
--- a/compiler/rustc_pattern_analysis/src/lib.rs
+++ b/compiler/rustc_pattern_analysis/src/lib.rs
@@ -5,6 +5,7 @@
 // tidy-alphabetical-start
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
+#![cfg_attr(feature = "rustc", feature(let_chains))]
 // tidy-alphabetical-end
 
 pub mod constructor;
@@ -54,7 +55,6 @@ pub trait PatCx: Sized + fmt::Debug {
     type PatData: Clone;
 
     fn is_exhaustive_patterns_feature_on(&self) -> bool;
-    fn is_min_exhaustive_patterns_feature_on(&self) -> bool;
 
     /// The number of fields for this constructor.
     fn ctor_arity(&self, ctor: &Constructor<Self>, ty: &Self::Ty) -> usize;
diff --git a/compiler/rustc_pattern_analysis/src/lints.rs b/compiler/rustc_pattern_analysis/src/lints.rs
index 892aacd7ac5..6bcef0ec879 100644
--- a/compiler/rustc_pattern_analysis/src/lints.rs
+++ b/compiler/rustc_pattern_analysis/src/lints.rs
@@ -1,11 +1,12 @@
+use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
+use rustc_span::ErrorGuaranteed;
+use tracing::instrument;
+
 use crate::constructor::Constructor;
 use crate::errors::{NonExhaustiveOmittedPattern, NonExhaustiveOmittedPatternLintOnArm, Uncovered};
 use crate::pat_column::PatternColumn;
 use crate::rustc::{RevealedTy, RustcPatCtxt, WitnessPat};
 use crate::MatchArm;
-use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
-use rustc_span::ErrorGuaranteed;
-use tracing::instrument;
 
 /// Traverse the patterns to collect any variants of a non_exhaustive enum that fail to be mentioned
 /// in a given column.
diff --git a/compiler/rustc_pattern_analysis/src/pat.rs b/compiler/rustc_pattern_analysis/src/pat.rs
index a591c3c554b..d91deab160c 100644
--- a/compiler/rustc_pattern_analysis/src/pat.rs
+++ b/compiler/rustc_pattern_analysis/src/pat.rs
@@ -5,11 +5,10 @@ use std::fmt;
 
 use smallvec::{smallvec, SmallVec};
 
+use self::Constructor::*;
 use crate::constructor::{Constructor, Slice, SliceKind};
 use crate::{PatCx, PrivateUninhabitedField};
 
-use self::Constructor::*;
-
 /// A globally unique id to distinguish patterns.
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub(crate) struct PatId(u32);
diff --git a/compiler/rustc_pattern_analysis/src/rustc.rs b/compiler/rustc_pattern_analysis/src/rustc.rs
index 910dd4c6c1a..10b7968a1a7 100644
--- a/compiler/rustc_pattern_analysis/src/rustc.rs
+++ b/compiler/rustc_pattern_analysis/src/rustc.rs
@@ -7,7 +7,7 @@ use rustc_hir::HirId;
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::middle::stability::EvalResult;
 use rustc_middle::mir::{self, Const};
-use rustc_middle::thir::{self, FieldPat, Pat, PatKind, PatRange, PatRangeBoundary};
+use rustc_middle::thir::{self, Pat, PatKind, PatRange, PatRangeBoundary};
 use rustc_middle::ty::layout::IntegerExt;
 use rustc_middle::ty::{
     self, FieldDef, OpaqueTypeKey, ScalarInt, Ty, TyCtxt, TypeVisitableExt, VariantDef,
@@ -17,15 +17,17 @@ use rustc_session::lint;
 use rustc_span::{ErrorGuaranteed, Span, DUMMY_SP};
 use rustc_target::abi::{FieldIdx, Integer, VariantIdx, FIRST_VARIANT};
 
+use crate::constructor::Constructor::*;
 use crate::constructor::{
     IntRange, MaybeInfiniteInt, OpaqueId, RangeEnd, Slice, SliceKind, VariantVisibility,
 };
 use crate::lints::lint_nonexhaustive_missing_variants;
 use crate::pat_column::PatternColumn;
+use crate::rustc::print::EnumInfo;
 use crate::usefulness::{compute_match_usefulness, PlaceValidity};
 use crate::{errors, Captures, PatCx, PrivateUninhabitedField};
 
-use crate::constructor::Constructor::*;
+mod print;
 
 // Re-export rustc-specific versions of all these types.
 pub type Constructor<'p, 'tcx> = crate::constructor::Constructor<RustcPatCtxt<'p, 'tcx>>;
@@ -236,9 +238,7 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
                         let tys = cx.variant_sub_tys(ty, variant).map(|(field, ty)| {
                             let is_visible =
                                 adt.is_enum() || field.vis.is_accessible_from(cx.module, cx.tcx);
-                            let is_uninhabited = (cx.tcx.features().exhaustive_patterns
-                                || cx.tcx.features().min_exhaustive_patterns)
-                                && cx.is_uninhabited(*ty);
+                            let is_uninhabited = cx.is_uninhabited(*ty);
                             let skip = is_uninhabited && (!is_visible || is_non_exhaustive);
                             (ty, PrivateUninhabitedField(skip))
                         });
@@ -744,7 +744,7 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
     /// Note: it is possible to get `isize/usize::MAX+1` here, as explained in the doc for
     /// [`IntRange::split`]. This cannot be represented as a `Const`, so we represent it with
     /// `PosInfinity`.
-    pub(crate) fn hoist_pat_range_bdy(
+    fn hoist_pat_range_bdy(
         &self,
         miint: MaybeInfiniteInt,
         ty: RevealedTy<'tcx>,
@@ -774,8 +774,9 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
         }
     }
 
-    /// Convert back to a `thir::Pat` for diagnostic purposes.
-    pub(crate) fn hoist_pat_range(&self, range: &IntRange, ty: RevealedTy<'tcx>) -> Pat<'tcx> {
+    /// Convert to a [`print::Pat`] for diagnostic purposes.
+    fn hoist_pat_range(&self, range: &IntRange, ty: RevealedTy<'tcx>) -> print::Pat<'tcx> {
+        use print::{Pat, PatKind};
         use MaybeInfiniteInt::*;
         let cx = self;
         let kind = if matches!((range.lo, range.hi), (NegInfinity, PosInfinity)) {
@@ -809,83 +810,79 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
             PatKind::Range(Box::new(PatRange { lo, hi, end, ty: ty.inner() }))
         };
 
-        Pat { ty: ty.inner(), span: DUMMY_SP, kind }
+        Pat { ty: ty.inner(), kind }
     }
-    /// Convert back to a `thir::Pat` for diagnostic purposes. This panics for patterns that don't
+
+    /// Prints a [`WitnessPat`] to an owned string, for diagnostic purposes.
+    pub fn print_witness_pat(&self, pat: &WitnessPat<'p, 'tcx>) -> String {
+        // This works by converting the witness pattern to a `print::Pat`
+        // and then printing that, but callers don't need to know that.
+        self.hoist_witness_pat(pat).to_string()
+    }
+
+    /// Convert to a [`print::Pat`] for diagnostic purposes. This panics for patterns that don't
     /// appear in diagnostics, like float ranges.
-    pub fn hoist_witness_pat(&self, pat: &WitnessPat<'p, 'tcx>) -> Pat<'tcx> {
+    fn hoist_witness_pat(&self, pat: &WitnessPat<'p, 'tcx>) -> print::Pat<'tcx> {
+        use print::{FieldPat, Pat, PatKind};
         let cx = self;
-        let is_wildcard = |pat: &Pat<'_>| matches!(pat.kind, PatKind::Wild);
-        let mut subpatterns = pat.iter_fields().map(|p| Box::new(cx.hoist_witness_pat(p)));
+        let hoist = |p| Box::new(cx.hoist_witness_pat(p));
         let kind = match pat.ctor() {
             Bool(b) => PatKind::Constant { value: mir::Const::from_bool(cx.tcx, *b) },
             IntRange(range) => return self.hoist_pat_range(range, *pat.ty()),
-            Struct | Variant(_) | UnionField => match pat.ty().kind() {
-                ty::Tuple(..) => PatKind::Leaf {
-                    subpatterns: subpatterns
-                        .enumerate()
-                        .map(|(i, pattern)| FieldPat { field: FieldIdx::new(i), pattern })
-                        .collect(),
-                },
-                ty::Adt(adt_def, _) if adt_def.is_box() => {
-                    // Without `box_patterns`, the only legal pattern of type `Box` is `_` (outside
-                    // of `std`). So this branch is only reachable when the feature is enabled and
-                    // the pattern is a box pattern.
-                    PatKind::Deref { subpattern: subpatterns.next().unwrap() }
-                }
-                ty::Adt(adt_def, args) => {
-                    let variant_index = RustcPatCtxt::variant_index_for_adt(&pat.ctor(), *adt_def);
-                    let subpatterns = subpatterns
-                        .enumerate()
-                        .map(|(i, pattern)| FieldPat { field: FieldIdx::new(i), pattern })
-                        .collect();
+            Struct if pat.ty().is_box() => {
+                // Outside of the `alloc` crate, the only way to create a struct pattern
+                // of type `Box` is to use a `box` pattern via #[feature(box_patterns)].
+                PatKind::Box { subpattern: hoist(&pat.fields[0]) }
+            }
+            Struct | Variant(_) | UnionField => {
+                let enum_info = match *pat.ty().kind() {
+                    ty::Adt(adt_def, _) if adt_def.is_enum() => EnumInfo::Enum {
+                        adt_def,
+                        variant_index: RustcPatCtxt::variant_index_for_adt(pat.ctor(), adt_def),
+                    },
+                    ty::Adt(..) | ty::Tuple(..) => EnumInfo::NotEnum,
+                    _ => bug!("unexpected ctor for type {:?} {:?}", pat.ctor(), *pat.ty()),
+                };
 
-                    if adt_def.is_enum() {
-                        PatKind::Variant { adt_def: *adt_def, args, variant_index, subpatterns }
-                    } else {
-                        PatKind::Leaf { subpatterns }
-                    }
-                }
-                _ => bug!("unexpected ctor for type {:?} {:?}", pat.ctor(), *pat.ty()),
-            },
-            // Note: given the expansion of `&str` patterns done in `expand_pattern`, we should
-            // be careful to reconstruct the correct constant pattern here. However a string
-            // literal pattern will never be reported as a non-exhaustiveness witness, so we
-            // ignore this issue.
-            Ref => PatKind::Deref { subpattern: subpatterns.next().unwrap() },
+                let subpatterns = pat
+                    .iter_fields()
+                    .enumerate()
+                    .map(|(i, pat)| FieldPat { field: FieldIdx::new(i), pattern: hoist(pat) })
+                    .collect::<Vec<_>>();
+
+                PatKind::StructLike { enum_info, subpatterns }
+            }
+            Ref => PatKind::Deref { subpattern: hoist(&pat.fields[0]) },
             Slice(slice) => {
-                match slice.kind {
-                    SliceKind::FixedLen(_) => PatKind::Slice {
-                        prefix: subpatterns.collect(),
-                        slice: None,
-                        suffix: Box::new([]),
-                    },
-                    SliceKind::VarLen(prefix, _) => {
-                        let mut subpatterns = subpatterns.peekable();
-                        let mut prefix: Vec<_> = subpatterns.by_ref().take(prefix).collect();
-                        if slice.array_len.is_some() {
-                            // Improves diagnostics a bit: if the type is a known-size array, instead
-                            // of reporting `[x, _, .., _, y]`, we prefer to report `[x, .., y]`.
-                            // This is incorrect if the size is not known, since `[_, ..]` captures
-                            // arrays of lengths `>= 1` whereas `[..]` captures any length.
-                            while !prefix.is_empty() && is_wildcard(prefix.last().unwrap()) {
-                                prefix.pop();
-                            }
-                            while subpatterns.peek().is_some()
-                                && is_wildcard(subpatterns.peek().unwrap())
-                            {
-                                subpatterns.next();
-                            }
-                        }
-                        let suffix: Box<[_]> = subpatterns.collect();
-                        let wild = Pat::wildcard_from_ty(pat.ty().inner());
-                        PatKind::Slice {
-                            prefix: prefix.into_boxed_slice(),
-                            slice: Some(Box::new(wild)),
-                            suffix,
-                        }
+                let (prefix_len, has_dot_dot) = match slice.kind {
+                    SliceKind::FixedLen(len) => (len, false),
+                    SliceKind::VarLen(prefix_len, _) => (prefix_len, true),
+                };
+
+                let (mut prefix, mut suffix) = pat.fields.split_at(prefix_len);
+
+                // If the pattern contains a `..`, but is applied to values of statically-known
+                // length (arrays), then we can slightly simplify diagnostics by merging any
+                // adjacent wildcard patterns into the `..`: `[x, _, .., _, y]` => `[x, .., y]`.
+                // (This simplification isn't allowed for slice values, because in that case
+                // `[x, .., y]` would match some slices that `[x, _, .., _, y]` would not.)
+                if has_dot_dot && slice.array_len.is_some() {
+                    while let [rest @ .., last] = prefix
+                        && would_print_as_wildcard(cx.tcx, last)
+                    {
+                        prefix = rest;
+                    }
+                    while let [first, rest @ ..] = suffix
+                        && would_print_as_wildcard(cx.tcx, first)
+                    {
+                        suffix = rest;
                     }
                 }
+
+                let prefix = prefix.iter().map(hoist).collect();
+                let suffix = suffix.iter().map(hoist).collect();
+
+                PatKind::Slice { prefix, has_dot_dot, suffix }
             }
             &Str(value) => PatKind::Constant { value },
             Never if self.tcx.features().never_patterns => PatKind::Never,
@@ -899,7 +896,23 @@ impl<'p, 'tcx: 'p> RustcPatCtxt<'p, 'tcx> {
             }
         };
 
-        Pat { ty: pat.ty().inner(), span: DUMMY_SP, kind }
+        Pat { ty: pat.ty().inner(), kind }
+    }
+}
+
+/// Returns `true` if the given pattern would be printed as a wildcard (`_`).
+fn would_print_as_wildcard(tcx: TyCtxt<'_>, p: &WitnessPat<'_, '_>) -> bool {
+    match p.ctor() {
+        Constructor::IntRange(IntRange {
+            lo: MaybeInfiniteInt::NegInfinity,
+            hi: MaybeInfiniteInt::PosInfinity,
+        })
+        | Constructor::Wildcard
+        | Constructor::NonExhaustive
+        | Constructor::Hidden
+        | Constructor::PrivateUninhabited => true,
+        Constructor::Never if !tcx.features().never_patterns => true,
+        _ => false,
     }
 }
 
@@ -914,9 +927,6 @@ impl<'p, 'tcx: 'p> PatCx for RustcPatCtxt<'p, 'tcx> {
     fn is_exhaustive_patterns_feature_on(&self) -> bool {
         self.tcx.features().exhaustive_patterns
     }
-    fn is_min_exhaustive_patterns_feature_on(&self) -> bool {
-        self.tcx.features().min_exhaustive_patterns
-    }
 
     fn ctor_arity(&self, ctor: &crate::constructor::Constructor<Self>, ty: &Self::Ty) -> usize {
         self.ctor_arity(ctor, *ty)
@@ -966,7 +976,7 @@ impl<'p, 'tcx: 'p> PatCx for RustcPatCtxt<'p, 'tcx> {
         let overlaps: Vec<_> = overlaps_with
             .iter()
             .map(|pat| pat.data().span)
-            .map(|span| errors::Overlap { range: overlap_as_pat.clone(), span })
+            .map(|span| errors::Overlap { range: overlap_as_pat.to_string(), span })
             .collect();
         let pat_span = pat.data().span;
         self.tcx.emit_node_span_lint(
@@ -996,12 +1006,11 @@ impl<'p, 'tcx: 'p> PatCx for RustcPatCtxt<'p, 'tcx> {
         }
         // `pat` is an exclusive range like `lo..gap`. `gapped_with` contains ranges that start with
         // `gap+1`.
-        let suggested_range: thir::Pat<'_> = {
+        let suggested_range: String = {
             // Suggest `lo..=gap` instead.
-            let mut suggested_range = thir_pat.clone();
-            let thir::PatKind::Range(range) = &mut suggested_range.kind else { unreachable!() };
-            range.end = rustc_hir::RangeEnd::Included;
-            suggested_range
+            let mut suggested_range = PatRange::clone(range);
+            suggested_range.end = rustc_hir::RangeEnd::Included;
+            suggested_range.to_string()
         };
         let gap_as_pat = self.hoist_pat_range(&gap, *pat.ty());
         if gapped_with.is_empty() {
@@ -1014,9 +1023,9 @@ impl<'p, 'tcx: 'p> PatCx for RustcPatCtxt<'p, 'tcx> {
                     // Point at this range.
                     first_range: thir_pat.span,
                     // That's the gap that isn't covered.
-                    max: gap_as_pat.clone(),
+                    max: gap_as_pat.to_string(),
                     // Suggest `lo..=max` instead.
-                    suggestion: suggested_range.to_string(),
+                    suggestion: suggested_range,
                 },
             );
         } else {
@@ -1028,17 +1037,17 @@ impl<'p, 'tcx: 'p> PatCx for RustcPatCtxt<'p, 'tcx> {
                     // Point at this range.
                     first_range: thir_pat.span,
                     // That's the gap that isn't covered.
-                    gap: gap_as_pat.clone(),
+                    gap: gap_as_pat.to_string(),
                     // Suggest `lo..=gap` instead.
-                    suggestion: suggested_range.to_string(),
+                    suggestion: suggested_range,
                     // All these ranges skipped over `gap` which we think is probably a
                     // mistake.
                     gap_with: gapped_with
                         .iter()
                         .map(|pat| errors::GappedRange {
                             span: pat.data().span,
-                            gap: gap_as_pat.clone(),
-                            first_range: thir_pat.clone(),
+                            gap: gap_as_pat.to_string(),
+                            first_range: range.to_string(),
                         })
                         .collect(),
                 },
diff --git a/compiler/rustc_pattern_analysis/src/rustc/print.rs b/compiler/rustc_pattern_analysis/src/rustc/print.rs
new file mode 100644
index 00000000000..7d638714605
--- /dev/null
+++ b/compiler/rustc_pattern_analysis/src/rustc/print.rs
@@ -0,0 +1,219 @@
+//! Pattern analysis sometimes wants to print patterns as part of a user-visible
+//! diagnostic.
+//!
+//! Historically it did so by creating a synthetic [`thir::Pat`](rustc_middle::thir::Pat)
+//! and printing that, but doing so was making it hard to modify the THIR pattern
+//! representation for other purposes.
+//!
+//! So this module contains a forked copy of `thir::Pat` that is used _only_
+//! for diagnostics, and has been partly simplified to remove things that aren't
+//! needed for printing.
+
+use std::fmt;
+
+use rustc_middle::thir::PatRange;
+use rustc_middle::ty::{self, AdtDef, Ty, TyCtxt};
+use rustc_middle::{bug, mir};
+use rustc_span::sym;
+use rustc_target::abi::{FieldIdx, VariantIdx};
+
+#[derive(Clone, Debug)]
+pub(crate) struct FieldPat<'tcx> {
+    pub(crate) field: FieldIdx,
+    pub(crate) pattern: Box<Pat<'tcx>>,
+}
+
+#[derive(Clone, Debug)]
+pub(crate) struct Pat<'tcx> {
+    pub(crate) ty: Ty<'tcx>,
+    pub(crate) kind: PatKind<'tcx>,
+}
+
+#[derive(Clone, Debug)]
+pub(crate) enum PatKind<'tcx> {
+    Wild,
+
+    StructLike {
+        enum_info: EnumInfo<'tcx>,
+        subpatterns: Vec<FieldPat<'tcx>>,
+    },
+
+    Box {
+        subpattern: Box<Pat<'tcx>>,
+    },
+
+    Deref {
+        subpattern: Box<Pat<'tcx>>,
+    },
+
+    Constant {
+        value: mir::Const<'tcx>,
+    },
+
+    Range(Box<PatRange<'tcx>>),
+
+    Slice {
+        prefix: Box<[Box<Pat<'tcx>>]>,
+        /// True if this slice-like pattern should include a `..` between the
+        /// prefix and suffix.
+        has_dot_dot: bool,
+        suffix: Box<[Box<Pat<'tcx>>]>,
+    },
+
+    Never,
+}
+
+impl<'tcx> fmt::Display for Pat<'tcx> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.kind {
+            PatKind::Wild => write!(f, "_"),
+            PatKind::Never => write!(f, "!"),
+            PatKind::Box { ref subpattern } => write!(f, "box {subpattern}"),
+            PatKind::StructLike { ref enum_info, ref subpatterns } => {
+                ty::tls::with(|tcx| write_struct_like(f, tcx, self.ty, enum_info, subpatterns))
+            }
+            PatKind::Deref { ref subpattern } => write_ref_like(f, self.ty, subpattern),
+            PatKind::Constant { value } => write!(f, "{value}"),
+            PatKind::Range(ref range) => write!(f, "{range}"),
+            PatKind::Slice { ref prefix, has_dot_dot, ref suffix } => {
+                write_slice_like(f, prefix, has_dot_dot, suffix)
+            }
+        }
+    }
+}
+
+/// Returns a closure that will return `""` when called the first time,
+/// and then return `", "` when called any subsequent times.
+/// Useful for printing comma-separated lists.
+fn start_or_comma() -> impl FnMut() -> &'static str {
+    let mut first = true;
+    move || {
+        if first {
+            first = false;
+            ""
+        } else {
+            ", "
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub(crate) enum EnumInfo<'tcx> {
+    Enum { adt_def: AdtDef<'tcx>, variant_index: VariantIdx },
+    NotEnum,
+}
+
+fn write_struct_like<'tcx>(
+    f: &mut impl fmt::Write,
+    tcx: TyCtxt<'_>,
+    ty: Ty<'tcx>,
+    enum_info: &EnumInfo<'tcx>,
+    subpatterns: &[FieldPat<'tcx>],
+) -> fmt::Result {
+    let variant_and_name = match *enum_info {
+        EnumInfo::Enum { adt_def, variant_index } => {
+            let variant = adt_def.variant(variant_index);
+            let adt_did = adt_def.did();
+            let name = if tcx.is_diagnostic_item(sym::Option, adt_did)
+                || tcx.is_diagnostic_item(sym::Result, adt_did)
+            {
+                variant.name.to_string()
+            } else {
+                format!("{}::{}", tcx.def_path_str(adt_def.did()), variant.name)
+            };
+            Some((variant, name))
+        }
+        EnumInfo::NotEnum => ty.ty_adt_def().and_then(|adt_def| {
+            Some((adt_def.non_enum_variant(), tcx.def_path_str(adt_def.did())))
+        }),
+    };
+
+    let mut start_or_comma = start_or_comma();
+
+    if let Some((variant, name)) = &variant_and_name {
+        write!(f, "{name}")?;
+
+        // Only for Adt we can have `S {...}`,
+        // which we handle separately here.
+        if variant.ctor.is_none() {
+            write!(f, " {{ ")?;
+
+            let mut printed = 0;
+            for p in subpatterns {
+                if let PatKind::Wild = p.pattern.kind {
+                    continue;
+                }
+                let name = variant.fields[p.field].name;
+                write!(f, "{}{}: {}", start_or_comma(), name, p.pattern)?;
+                printed += 1;
+            }
+
+            let is_union = ty.ty_adt_def().is_some_and(|adt| adt.is_union());
+            if printed < variant.fields.len() && (!is_union || printed == 0) {
+                write!(f, "{}..", start_or_comma())?;
+            }
+
+            return write!(f, " }}");
+        }
+    }
+
+    let num_fields = variant_and_name.as_ref().map_or(subpatterns.len(), |(v, _)| v.fields.len());
+    if num_fields != 0 || variant_and_name.is_none() {
+        write!(f, "(")?;
+        for i in 0..num_fields {
+            write!(f, "{}", start_or_comma())?;
+
+            // Common case: the field is where we expect it.
+            if let Some(p) = subpatterns.get(i) {
+                if p.field.index() == i {
+                    write!(f, "{}", p.pattern)?;
+                    continue;
+                }
+            }
+
+            // Otherwise, we have to go looking for it.
+            if let Some(p) = subpatterns.iter().find(|p| p.field.index() == i) {
+                write!(f, "{}", p.pattern)?;
+            } else {
+                write!(f, "_")?;
+            }
+        }
+        write!(f, ")")?;
+    }
+
+    Ok(())
+}
+
+fn write_ref_like<'tcx>(
+    f: &mut impl fmt::Write,
+    ty: Ty<'tcx>,
+    subpattern: &Pat<'tcx>,
+) -> fmt::Result {
+    match ty.kind() {
+        ty::Ref(_, _, mutbl) => {
+            write!(f, "&{}", mutbl.prefix_str())?;
+        }
+        _ => bug!("{ty} is a bad ref pattern type"),
+    }
+    write!(f, "{subpattern}")
+}
+
+fn write_slice_like<'tcx>(
+    f: &mut impl fmt::Write,
+    prefix: &[Box<Pat<'tcx>>],
+    has_dot_dot: bool,
+    suffix: &[Box<Pat<'tcx>>],
+) -> fmt::Result {
+    let mut start_or_comma = start_or_comma();
+    write!(f, "[")?;
+    for p in prefix.iter() {
+        write!(f, "{}{}", start_or_comma(), p)?;
+    }
+    if has_dot_dot {
+        write!(f, "{}..", start_or_comma())?;
+    }
+    for p in suffix.iter() {
+        write!(f, "{}{}", start_or_comma(), p)?;
+    }
+    write!(f, "]")
+}
diff --git a/compiler/rustc_pattern_analysis/src/usefulness.rs b/compiler/rustc_pattern_analysis/src/usefulness.rs
index 76dc338e71c..6535afcc398 100644
--- a/compiler/rustc_pattern_analysis/src/usefulness.rs
+++ b/compiler/rustc_pattern_analysis/src/usefulness.rs
@@ -543,13 +543,11 @@
 //! recurse into subpatterns. That second part is done through [`PlaceValidity`], most notably
 //! [`PlaceValidity::specialize`].
 //!
-//! Having said all that, in practice we don't fully follow what's been presented in this section.
-//! Let's call "toplevel exception" the case where the match scrutinee itself has type `!` or
-//! `EmptyEnum`. First, on stable rust, we require `_` patterns for empty types in all cases apart
-//! from the toplevel exception. The `exhaustive_patterns` and `min_exaustive_patterns` allow
-//! omitting patterns in the cases described above. There's a final detail: in the toplevel
-//! exception or with the `exhaustive_patterns` feature, we ignore place validity when checking
-//! whether a pattern is required for exhaustiveness. I (Nadrieril) hope to deprecate this behavior.
+//! Having said all that, we don't fully follow what's been presented in this section. For
+//! backwards-compatibility, we ignore place validity when checking whether a pattern is required
+//! for exhaustiveness in two cases: when the `exhaustive_patterns` feature gate is on, or when the
+//! match scrutinee itself has type `!` or `EmptyEnum`. I (Nadrieril) hope to deprecate this
+//! exception.
 //!
 //!
 //!
@@ -709,18 +707,19 @@
 //! I (Nadrieril) prefer to put new tests in `ui/pattern/usefulness` unless there's a specific
 //! reason not to, for example if they crucially depend on a particular feature like `or_patterns`.
 
-use self::PlaceValidity::*;
-use crate::constructor::{Constructor, ConstructorSet, IntRange};
-use crate::pat::{DeconstructedPat, PatId, PatOrWild, WitnessPat};
-use crate::{Captures, MatchArm, PatCx, PrivateUninhabitedField};
+use std::fmt;
+
+#[cfg(feature = "rustc")]
+use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hash::{FxHashMap, FxHashSet};
 use rustc_index::bit_set::BitSet;
 use smallvec::{smallvec, SmallVec};
-use std::fmt;
 use tracing::{debug, instrument};
 
-#[cfg(feature = "rustc")]
-use rustc_data_structures::stack::ensure_sufficient_stack;
+use self::PlaceValidity::*;
+use crate::constructor::{Constructor, ConstructorSet, IntRange};
+use crate::pat::{DeconstructedPat, PatId, PatOrWild, WitnessPat};
+use crate::{Captures, MatchArm, PatCx, PrivateUninhabitedField};
 #[cfg(not(feature = "rustc"))]
 pub fn ensure_sufficient_stack<R>(f: impl FnOnce() -> R) -> R {
     f()
@@ -952,13 +951,10 @@ impl<Cx: PatCx> PlaceInfo<Cx> {
             self.is_scrutinee && matches!(ctors_for_ty, ConstructorSet::NoConstructors);
         // Whether empty patterns are counted as useful or not. We only warn an empty arm unreachable if
         // it is guaranteed unreachable by the opsem (i.e. if the place is `known_valid`).
-        let empty_arms_are_unreachable = self.validity.is_known_valid()
-            && (is_toplevel_exception
-                || cx.is_exhaustive_patterns_feature_on()
-                || cx.is_min_exhaustive_patterns_feature_on());
+        let empty_arms_are_unreachable = self.validity.is_known_valid();
         // Whether empty patterns can be omitted for exhaustiveness. We ignore place validity in the
         // toplevel exception and `exhaustive_patterns` cases for backwards compatibility.
-        let can_omit_empty_arms = empty_arms_are_unreachable
+        let can_omit_empty_arms = self.validity.is_known_valid()
             || is_toplevel_exception
             || cx.is_exhaustive_patterns_feature_on();