Add reachable_patterns lint to rfc-2008-non_exhaustive

Add linting on non_exhaustive structs and enum variants

Add ui tests for non_exhaustive reachable lint

Rename to non_exhaustive_omitted_patterns and avoid triggering on if let

Author: DevinR528

compiler/rustc_lint_defs/src/builtin.rs

@@ -3010,6 +3010,7 @@ declare_lint_pass! {
         UNSUPPORTED_CALLING_CONVENTIONS,
         BREAK_WITH_LABEL_AND_LOOP,
         UNUSED_ATTRIBUTES,
+        NON_EXHAUSTIVE_OMITTED_PATTERNS,
     ]
 }
 
@@ -3416,3 +3417,56 @@ declare_lint! {
     Warn,
     "`break` expression with label and unlabeled loop as value expression"
 }
+
+declare_lint! {
+    /// The `non_exhaustive_omitted_patterns` lint detects when a wildcard (`_` or `..`) in a
+    /// pattern for a `#[non_exhaustive]` struct or enum is reachable.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,ignore (needs separate crate)
+    /// // crate A
+    /// #[non_exhaustive]
+    /// pub enum Bar {
+    ///     A,
+    ///     B, // added variant in non breaking change
+    /// }
+    ///
+    /// // in crate B
+    /// match Bar::A {
+    ///     Bar::A => {},
+    ///     #[warn(non_exhaustive_omitted_patterns)]
+    ///     _ => {},
+    /// }
+    /// ```
+    ///
+    /// This will produce:
+    ///
+    /// ```text
+    /// warning: reachable patterns not covered of non exhaustive enum
+    ///    --> $DIR/reachable-patterns.rs:70:9
+    ///    |
+    /// LL |         _ => {}
+    ///    |         ^ pattern `B` not covered
+    ///    |
+    ///  note: the lint level is defined here
+    ///   --> $DIR/reachable-patterns.rs:69:16
+    ///    |
+    /// LL |         #[warn(non_exhaustive_omitted_patterns)]
+    ///    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+    ///    = help: ensure that all possible cases are being handled by adding the suggested match arms
+    ///    = note: the matched value is of type `Bar` and the `non_exhaustive_omitted_patterns` attribute was found
+    /// ```
+    ///
+    /// ### Explanation
+    ///
+    /// Structs and enums tagged with `#[non_exhaustive]` force the user to add a
+    /// (potentially redundant) wildcard when pattern-matching, to allow for future
+    /// addition of fields or variants. The `non_exhaustive_omitted_patterns` lint
+    /// detects when such a wildcard happens to actually catch some fields/variants.
+    /// In other words, when the match without the wildcard would not be exhaustive.
+    /// This lets the user be informed if new fields/variants were added.
+    pub NON_EXHAUSTIVE_OMITTED_PATTERNS,
+    Allow,
+    "detect when patterns of types marked `non_exhaustive` are missed",
+}

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -14,8 +14,9 @@ use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc_hir::{HirId, Pat};
 use rustc_middle::thir::PatKind;
 use rustc_middle::ty::{self, Ty, TyCtxt};
-use rustc_session::lint::builtin::BINDINGS_WITH_VARIANT_NAME;
-use rustc_session::lint::builtin::{IRREFUTABLE_LET_PATTERNS, UNREACHABLE_PATTERNS};
+use rustc_session::lint::builtin::{
+    BINDINGS_WITH_VARIANT_NAME, IRREFUTABLE_LET_PATTERNS, UNREACHABLE_PATTERNS,
+};
 use rustc_session::Session;
 use rustc_span::{DesugaringKind, ExpnKind, Span};
 use std::slice;
@@ -559,7 +560,7 @@ fn non_exhaustive_match<'p, 'tcx>(
     err.emit();
 }
 
-fn joined_uncovered_patterns(witnesses: &[super::Pat<'_>]) -> String {
+crate fn joined_uncovered_patterns(witnesses: &[super::Pat<'_>]) -> String {
     const LIMIT: usize = 3;
     match witnesses {
         [] => bug!(),
@@ -576,7 +577,7 @@ fn joined_uncovered_patterns(witnesses: &[super::Pat<'_>]) -> String {
     }
 }
 
-fn pattern_not_covered_label(witnesses: &[super::Pat<'_>], joined_patterns: &str) -> String {
+crate fn pattern_not_covered_label(witnesses: &[super::Pat<'_>], joined_patterns: &str) -> String {
     format!("pattern{} {} not covered", rustc_errors::pluralize!(witnesses.len()), joined_patterns)
 }
 

compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs

@@ -606,8 +606,9 @@ pub(super) enum Constructor<'tcx> {
     /// for those types for which we cannot list constructors explicitly, like `f64` and `str`.
     NonExhaustive,
     /// Stands for constructors that are not seen in the matrix, as explained in the documentation
-    /// for [`SplitWildcard`].
-    Missing,
+    /// for [`SplitWildcard`]. The carried `bool` is used for the `non_exhaustive_omitted_patterns`
+    /// lint.
+    Missing { nonexhaustive_enum_missing_real_variants: bool },
     /// Wildcard pattern.
     Wildcard,
 }
@@ -617,6 +618,10 @@ impl<'tcx> Constructor<'tcx> {
         matches!(self, Wildcard)
     }
 
+    pub(super) fn is_non_exhaustive(&self) -> bool {
+        matches!(self, NonExhaustive)
+    }
+
     fn as_int_range(&self) -> Option<&IntRange> {
         match self {
             IntRange(range) => Some(range),
@@ -756,7 +761,7 @@ impl<'tcx> Constructor<'tcx> {
             // Wildcards cover anything
             (_, Wildcard) => true,
             // The missing ctors are not covered by anything in the matrix except wildcards.
-            (Missing | Wildcard, _) => false,
+            (Missing { .. } | Wildcard, _) => false,
 
             (Single, Single) => true,
             (Variant(self_id), Variant(other_id)) => self_id == other_id,
@@ -829,7 +834,7 @@ impl<'tcx> Constructor<'tcx> {
                 .any(|other| slice.is_covered_by(other)),
             // This constructor is never covered by anything else
             NonExhaustive => false,
-            Str(..) | FloatRange(..) | Opaque | Missing | Wildcard => {
+            Str(..) | FloatRange(..) | Opaque | Missing { .. } | Wildcard => {
                 span_bug!(pcx.span, "found unexpected ctor in all_ctors: {:?}", self)
             }
         }
@@ -919,8 +924,14 @@ impl<'tcx> SplitWildcard<'tcx> {
                     && !cx.tcx.features().exhaustive_patterns
                     && !pcx.is_top_level;
 
-                if is_secretly_empty || is_declared_nonexhaustive {
+                if is_secretly_empty {
                     smallvec![NonExhaustive]
+                } else if is_declared_nonexhaustive {
+                    def.variants
+                        .indices()
+                        .map(|idx| Variant(idx))
+                        .chain(Some(NonExhaustive))
+                        .collect()
                 } else if cx.tcx.features().exhaustive_patterns {
                     // If `exhaustive_patterns` is enabled, we exclude variants known to be
                     // uninhabited.
@@ -975,6 +986,7 @@ impl<'tcx> SplitWildcard<'tcx> {
             // This type is one for which we cannot list constructors, like `str` or `f64`.
             _ => smallvec![NonExhaustive],
         };
+
         SplitWildcard { matrix_ctors: Vec::new(), all_ctors }
     }
 
@@ -1039,7 +1051,17 @@ impl<'tcx> SplitWildcard<'tcx> {
             // sometimes prefer reporting the list of constructors instead of just `_`.
             let report_when_all_missing = pcx.is_top_level && !IntRange::is_integral(pcx.ty);
             let ctor = if !self.matrix_ctors.is_empty() || report_when_all_missing {
-                Missing
+                if pcx.is_non_exhaustive {
+                    Missing {
+                        nonexhaustive_enum_missing_real_variants: self
+                            .iter_missing(pcx)
+                            .filter(|c| !c.is_non_exhaustive())
+                            .next()
+                            .is_some(),
+                    }
+                } else {
+                    Missing { nonexhaustive_enum_missing_real_variants: false }
+                }
             } else {
                 Wildcard
             };
@@ -1176,7 +1198,12 @@ impl<'p, 'tcx> Fields<'p, 'tcx> {
                 }
                 _ => bug!("bad slice pattern {:?} {:?}", constructor, ty),
             },
-            Str(..) | FloatRange(..) | IntRange(..) | NonExhaustive | Opaque | Missing
+            Str(..)
+            | FloatRange(..)
+            | IntRange(..)
+            | NonExhaustive
+            | Opaque
+            | Missing { .. }
             | Wildcard => Fields::Slice(&[]),
         };
         debug!("Fields::wildcards({:?}, {:?}) = {:#?}", constructor, ty, ret);
@@ -1189,15 +1216,18 @@ impl<'p, 'tcx> Fields<'p, 'tcx> {
     /// This is roughly the inverse of `specialize_constructor`.
     ///
     /// Examples:
-    /// `ctor`: `Constructor::Single`
-    /// `ty`: `Foo(u32, u32, u32)`
-    /// `self`: `[10, 20, _]`
+    ///
+    /// ```text
+    /// ctor: `Constructor::Single`
+    /// ty: `Foo(u32, u32, u32)`
+    /// self: `[10, 20, _]`
     /// returns `Foo(10, 20, _)`
     ///
-    /// `ctor`: `Constructor::Variant(Option::Some)`
-    /// `ty`: `Option<bool>`
-    /// `self`: `[false]`
+    /// ctor: `Constructor::Variant(Option::Some)`
+    /// ty: `Option<bool>`
+    /// self: `[false]`
     /// returns `Some(false)`
+    /// ```
     pub(super) fn apply(self, pcx: PatCtxt<'_, 'p, 'tcx>, ctor: &Constructor<'tcx>) -> Pat<'tcx> {
         let subpatterns_and_indices = self.patterns_and_indices();
         let mut subpatterns = subpatterns_and_indices.iter().map(|&(_, p)| p).cloned();
@@ -1265,7 +1295,7 @@ impl<'p, 'tcx> Fields<'p, 'tcx> {
             NonExhaustive => PatKind::Wild,
             Wildcard => return Pat::wildcard_from_ty(pcx.ty),
             Opaque => bug!("we should not try to apply an opaque constructor"),
-            Missing => bug!(
+            Missing { .. } => bug!(
                 "trying to apply the `Missing` constructor; this should have been done in `apply_constructors`"
             ),
         };

compiler/rustc_mir_build/src/thir/pattern/usefulness.rs

@@ -280,20 +280,23 @@
 //! The details are not necessary to understand this file, so we explain them in
 //! [`super::deconstruct_pat`]. Splitting is done by the [`Constructor::split`] function.
 
+use self::ArmType::*;
 use self::Usefulness::*;
-use self::WitnessPreference::*;
 
+use super::check_match::{joined_uncovered_patterns, pattern_not_covered_label};
 use super::deconstruct_pat::{Constructor, Fields, SplitWildcard};
 use super::{PatternFoldable, PatternFolder};
 
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashMap;
 
+use hir::def_id::DefId;
+use hir::HirId;
 use rustc_arena::TypedArena;
-use rustc_hir::def_id::DefId;
-use rustc_hir::HirId;
+use rustc_hir as hir;
 use rustc_middle::thir::{Pat, PatKind};
 use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
 use rustc_span::Span;
 
 use smallvec::{smallvec, SmallVec};
@@ -343,6 +346,8 @@ pub(super) struct PatCtxt<'a, 'p, 'tcx> {
     /// Whether the current pattern is the whole pattern as found in a match arm, or if it's a
     /// subpattern.
     pub(super) is_top_level: bool,
+    /// Wether the current pattern is from a `non_exhaustive` enum.
+    pub(super) is_non_exhaustive: bool,
 }
 
 impl<'a, 'p, 'tcx> fmt::Debug for PatCtxt<'a, 'p, 'tcx> {
@@ -862,7 +867,7 @@ impl<'p, 'tcx> SubPatSet<'p, 'tcx> {
 /// of potential unreachable sub-patterns (in the presence of or-patterns). When checking
 /// exhaustiveness of a whole match, we use the `WithWitnesses` variant, which carries a list of
 /// witnesses of non-exhaustiveness when there are any.
-/// Which variant to use is dictated by `WitnessPreference`.
+/// Which variant to use is dictated by `ArmType`.
 #[derive(Clone, Debug)]
 enum Usefulness<'p, 'tcx> {
     /// Carries a set of subpatterns that have been found to be reachable. If empty, this indicates
@@ -877,16 +882,24 @@ enum Usefulness<'p, 'tcx> {
 }
 
 impl<'p, 'tcx> Usefulness<'p, 'tcx> {
-    fn new_useful(preference: WitnessPreference) -> Self {
+    fn new_useful(preference: ArmType) -> Self {
         match preference {
-            ConstructWitness => WithWitnesses(vec![Witness(vec![])]),
-            LeaveOutWitness => NoWitnesses(SubPatSet::full()),
+            FakeExtraWildcard => WithWitnesses(vec![Witness(vec![])]),
+            RealArm => NoWitnesses(SubPatSet::full()),
         }
     }
-    fn new_not_useful(preference: WitnessPreference) -> Self {
+
+    fn new_not_useful(preference: ArmType) -> Self {
         match preference {
-            ConstructWitness => WithWitnesses(vec![]),
-            LeaveOutWitness => NoWitnesses(SubPatSet::empty()),
+            FakeExtraWildcard => WithWitnesses(vec![]),
+            RealArm => NoWitnesses(SubPatSet::empty()),
+        }
+    }
+
+    fn is_useful(&self) -> bool {
+        match self {
+            Usefulness::NoWitnesses(set) => !set.is_empty(),
+            Usefulness::WithWitnesses(witnesses) => !witnesses.is_empty(),
         }
     }
 
@@ -903,7 +916,7 @@ impl<'p, 'tcx> Usefulness<'p, 'tcx> {
 
     /// When trying several branches and each returns a `Usefulness`, we need to combine the
     /// results together.
-    fn merge(pref: WitnessPreference, usefulnesses: impl Iterator<Item = Self>) -> Self {
+    fn merge(pref: ArmType, usefulnesses: impl Iterator<Item = Self>) -> Self {
         let mut ret = Self::new_not_useful(pref);
         for u in usefulnesses {
             ret.extend(u);
@@ -926,7 +939,7 @@ impl<'p, 'tcx> Usefulness<'p, 'tcx> {
         }
     }
 
-    /// After calculating usefulness after a specialization, call this to recontruct a usefulness
+    /// After calculating usefulness after a specialization, call this to reconstruct a usefulness
     /// that makes sense for the matrix pre-specialization. This new usefulness can then be merged
     /// with the results of specializing with the other constructors.
     fn apply_constructor(
@@ -939,19 +952,31 @@ impl<'p, 'tcx> Usefulness<'p, 'tcx> {
         match self {
             WithWitnesses(witnesses) if witnesses.is_empty() => WithWitnesses(witnesses),
             WithWitnesses(witnesses) => {
-                let new_witnesses = if matches!(ctor, Constructor::Missing) {
-                    let mut split_wildcard = SplitWildcard::new(pcx);
-                    split_wildcard.split(pcx, matrix.head_ctors(pcx.cx));
-                    // Construct for each missing constructor a "wild" version of this
-                    // constructor, that matches everything that can be built with
-                    // it. For example, if `ctor` is a `Constructor::Variant` for
-                    // `Option::Some`, we get the pattern `Some(_)`.
-                    let new_patterns: Vec<_> = split_wildcard
-                        .iter_missing(pcx)
-                        .map(|missing_ctor| {
-                            Fields::wildcards(pcx, missing_ctor).apply(pcx, missing_ctor)
-                        })
-                        .collect();
+                let new_witnesses = if let Constructor::Missing { .. } = ctor {
+                    // We got the special `Missing` constructor, so each of the missing constructors
+                    // gives a new pattern that is not caught by the match. We list those patterns.
+                    let new_patterns = if pcx.is_non_exhaustive {
+                        // Here we don't want the user to try to list all variants, we want them to add
+                        // a wildcard, so we only suggest that.
+                        vec![
+                            Fields::wildcards(pcx, &Constructor::NonExhaustive)
+                                .apply(pcx, &Constructor::NonExhaustive),
+                        ]
+                    } else {
+                        let mut split_wildcard = SplitWildcard::new(pcx);
+                        split_wildcard.split(pcx, matrix.head_ctors(pcx.cx));
+                        // Construct for each missing constructor a "wild" version of this
+                        // constructor, that matches everything that can be built with
+                        // it. For example, if `ctor` is a `Constructor::Variant` for
+                        // `Option::Some`, we get the pattern `Some(_)`.
+                        split_wildcard
+                            .iter_missing(pcx)
+                            .map(|missing_ctor| {
+                                Fields::wildcards(pcx, missing_ctor).apply(pcx, missing_ctor)
+                            })
+                            .collect()
+                    };
+
                     witnesses
                         .into_iter()
                         .flat_map(|witness| {
@@ -976,9 +1001,9 @@ impl<'p, 'tcx> Usefulness<'p, 'tcx> {
 }
 
 #[derive(Copy, Clone, Debug)]
-enum WitnessPreference {
-    ConstructWitness,
-    LeaveOutWitness,
+enum ArmType {
+    FakeExtraWildcard,
+    RealArm,
 }
 
 /// A witness of non-exhaustiveness for error reporting, represented
@@ -1056,6 +1081,32 @@ impl<'tcx> Witness<'tcx> {
     }
 }
 
+/// Report that a match of a `non_exhaustive` enum marked with `non_exhaustive_omitted_patterns`
+/// is not exhaustive enough.
+///
+/// NB: The partner lint for structs lives in `compiler/rustc_typeck/src/check/pat.rs`.
+fn lint_non_exhaustive_omitted_patterns<'p, 'tcx>(
+    cx: &MatchCheckCtxt<'p, 'tcx>,
+    scrut_ty: Ty<'tcx>,
+    sp: Span,
+    hir_id: HirId,
+    witnesses: Vec<Pat<'tcx>>,
+) {
+    let joined_patterns = joined_uncovered_patterns(&witnesses);
+    cx.tcx.struct_span_lint_hir(NON_EXHAUSTIVE_OMITTED_PATTERNS, hir_id, sp, |build| {
+        let mut lint = build.build("some variants are not matched explicitly");
+        lint.span_label(sp, pattern_not_covered_label(&witnesses, &joined_patterns));
+        lint.help(
+            "ensure that all variants are matched explicitly by adding the suggested match arms",
+        );
+        lint.note(&format!(
+            "the matched value is of type `{}` and the `non_exhaustive_omitted_patterns` attribute was found",
+            scrut_ty,
+        ));
+        lint.emit();
+    });
+}
+
 /// Algorithm from <http://moscova.inria.fr/~maranget/papers/warn/index.html>.
 /// The algorithm from the paper has been modified to correctly handle empty
 /// types. The changes are:
@@ -1086,7 +1137,7 @@ fn is_useful<'p, 'tcx>(
     cx: &MatchCheckCtxt<'p, 'tcx>,
     matrix: &Matrix<'p, 'tcx>,
     v: &PatStack<'p, 'tcx>,
-    witness_preference: WitnessPreference,
+    witness_preference: ArmType,
     hir_id: HirId,
     is_under_guard: bool,
     is_top_level: bool,
@@ -1113,7 +1164,8 @@ fn is_useful<'p, 'tcx>(
 
     // FIXME(Nadrieril): Hack to work around type normalization issues (see #72476).
     let ty = matrix.heads().next().map_or(v.head().ty, |r| r.ty);
-    let pcx = PatCtxt { cx, ty, span: v.head().span, is_top_level };
+    let is_non_exhaustive = cx.is_foreign_non_exhaustive_enum(ty);
+    let pcx = PatCtxt { cx, ty, span: v.head().span, is_top_level, is_non_exhaustive };
 
     // If the first pattern is an or-pattern, expand it.
     let ret = if is_or_pat(v.head()) {
@@ -1148,6 +1200,7 @@ fn is_useful<'p, 'tcx>(
         }
         // We split the head constructor of `v`.
         let split_ctors = v_ctor.split(pcx, matrix.head_ctors(cx));
+        let is_non_exhaustive_and_wild = is_non_exhaustive && v_ctor.is_wildcard();
         // For each constructor, we compute whether there's a value that starts with it that would
         // witness the usefulness of `v`.
         let start_matrix = &matrix;
@@ -1160,10 +1213,46 @@ fn is_useful<'p, 'tcx>(
             let v = v.pop_head_constructor(&ctor_wild_subpatterns);
             let usefulness =
                 is_useful(cx, &spec_matrix, &v, witness_preference, hir_id, is_under_guard, false);
+
+            // When all the conditions are met we have a match with a `non_exhaustive` enum
+            // that has the potential to trigger the `non_exhaustive_omitted_patterns` lint.
+            // To understand the workings checkout `Constructor::split` and `SplitWildcard::new/into_ctors`
+            if is_non_exhaustive_and_wild
+                // We check that the match has a wildcard pattern and that that wildcard is useful,
+                // meaning there are variants that are covered by the wildcard. Without the check
+                // for `witness_preference` the lint would trigger on `if let NonExhaustiveEnum::A = foo {}`
+                && usefulness.is_useful() && matches!(witness_preference, RealArm)
+                && matches!(
+                    &ctor,
+                    Constructor::Missing { nonexhaustive_enum_missing_real_variants: true }
+                )
+            {
+                let patterns = {
+                    let mut split_wildcard = SplitWildcard::new(pcx);
+                    split_wildcard.split(pcx, matrix.head_ctors(pcx.cx));
+                    // Construct for each missing constructor a "wild" version of this
+                    // constructor, that matches everything that can be built with
+                    // it. For example, if `ctor` is a `Constructor::Variant` for
+                    // `Option::Some`, we get the pattern `Some(_)`.
+                    split_wildcard
+                        .iter_missing(pcx)
+                        // Filter out the `Constructor::NonExhaustive` variant it's meaningless
+                        // to our lint
+                        .filter(|c| !c.is_non_exhaustive())
+                        .map(|missing_ctor| {
+                            Fields::wildcards(pcx, missing_ctor).apply(pcx, missing_ctor)
+                        })
+                        .collect::<Vec<_>>()
+                };
+
+                lint_non_exhaustive_omitted_patterns(pcx.cx, pcx.ty, pcx.span, hir_id, patterns);
+            }
+
             usefulness.apply_constructor(pcx, start_matrix, &ctor, &ctor_wild_subpatterns)
         });
         Usefulness::merge(witness_preference, usefulnesses)
     };
+
     debug!(?ret);
     ret
 }
@@ -1214,8 +1303,7 @@ crate fn compute_match_usefulness<'p, 'tcx>(
         .copied()
         .map(|arm| {
             let v = PatStack::from_pattern(arm.pat);
-            let usefulness =
-                is_useful(cx, &matrix, &v, LeaveOutWitness, arm.hir_id, arm.has_guard, true);
+            let usefulness = is_useful(cx, &matrix, &v, RealArm, arm.hir_id, arm.has_guard, true);
             if !arm.has_guard {
                 matrix.push(v);
             }
@@ -1232,7 +1320,7 @@ crate fn compute_match_usefulness<'p, 'tcx>(
 
     let wild_pattern = cx.pattern_arena.alloc(Pat::wildcard_from_ty(scrut_ty));
     let v = PatStack::from_pattern(wild_pattern);
-    let usefulness = is_useful(cx, &matrix, &v, ConstructWitness, scrut_hir_id, false, true);
+    let usefulness = is_useful(cx, &matrix, &v, FakeExtraWildcard, scrut_hir_id, false, true);
     let non_exhaustiveness_witnesses = match usefulness {
         WithWitnesses(pats) => pats.into_iter().map(|w| w.single_pattern()).collect(),
         NoWitnesses(_) => bug!(),

compiler/rustc_typeck/src/check/pat.rs

@@ -11,6 +11,7 @@ use rustc_infer::infer;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_middle::ty::subst::GenericArg;
 use rustc_middle::ty::{self, Adt, BindingMode, Ty, TypeFoldable};
+use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
 use rustc_span::hygiene::DesugaringKind;
 use rustc_span::lev_distance::find_best_match_for_name;
 use rustc_span::source_map::{Span, Spanned};
@@ -1261,7 +1262,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         };
 
         // Require `..` if struct has non_exhaustive attribute.
-        if variant.is_field_list_non_exhaustive() && !adt.did.is_local() && !etc {
+        let non_exhaustive = variant.is_field_list_non_exhaustive() && !adt.did.is_local();
+        if non_exhaustive && !etc {
             self.error_foreign_non_exhaustive_spat(pat, adt.variant_descr(), fields.is_empty());
         }
 
@@ -1276,24 +1278,27 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             if etc {
                 tcx.sess.struct_span_err(pat.span, "`..` cannot be used in union patterns").emit();
             }
-        } else if !etc && !unmentioned_fields.is_empty() {
+        } else if !unmentioned_fields.is_empty() {
             let accessible_unmentioned_fields: Vec<_> = unmentioned_fields
                 .iter()
                 .copied()
                 .filter(|(field, _)| {
                     field.vis.is_accessible_from(tcx.parent_module(pat.hir_id).to_def_id(), tcx)
                 })
                 .collect();
-
-            if accessible_unmentioned_fields.is_empty() {
-                unmentioned_err = Some(self.error_no_accessible_fields(pat, &fields));
-            } else {
-                unmentioned_err = Some(self.error_unmentioned_fields(
-                    pat,
-                    &accessible_unmentioned_fields,
-                    accessible_unmentioned_fields.len() != unmentioned_fields.len(),
-                    &fields,
-                ));
+            if non_exhaustive {
+                self.non_exhaustive_reachable_pattern(pat, &accessible_unmentioned_fields, adt_ty)
+            } else if !etc {
+                if accessible_unmentioned_fields.is_empty() {
+                    unmentioned_err = Some(self.error_no_accessible_fields(pat, &fields));
+                } else {
+                    unmentioned_err = Some(self.error_unmentioned_fields(
+                        pat,
+                        &accessible_unmentioned_fields,
+                        accessible_unmentioned_fields.len() != unmentioned_fields.len(),
+                        &fields,
+                    ));
+                }
             }
         }
         match (inexistent_fields_err, unmentioned_err) {
@@ -1604,6 +1609,51 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         err
     }
 
+    /// Report that a pattern for a `#[non_exhaustive]` struct marked with `non_exhaustive_omitted_patterns`
+    /// is not exhaustive enough.
+    ///
+    /// Nb: the partner lint for enums lives in `compiler/rustc_mir_build/src/thir/pattern/usefulness.rs`.
+    fn non_exhaustive_reachable_pattern(
+        &self,
+        pat: &Pat<'_>,
+        unmentioned_fields: &[(&ty::FieldDef, Ident)],
+        ty: Ty<'tcx>,
+    ) {
+        fn joined_uncovered_patterns(witnesses: &[&Ident]) -> String {
+            const LIMIT: usize = 3;
+            match witnesses {
+                [] => bug!(),
+                [witness] => format!("`{}`", witness),
+                [head @ .., tail] if head.len() < LIMIT => {
+                    let head: Vec<_> = head.iter().map(<_>::to_string).collect();
+                    format!("`{}` and `{}`", head.join("`, `"), tail)
+                }
+                _ => {
+                    let (head, tail) = witnesses.split_at(LIMIT);
+                    let head: Vec<_> = head.iter().map(<_>::to_string).collect();
+                    format!("`{}` and {} more", head.join("`, `"), tail.len())
+                }
+            }
+        }
+        let joined_patterns = joined_uncovered_patterns(
+            &unmentioned_fields.iter().map(|(_, i)| i).collect::<Vec<_>>(),
+        );
+
+        self.tcx.struct_span_lint_hir(NON_EXHAUSTIVE_OMITTED_PATTERNS, pat.hir_id, pat.span, |build| {
+        let mut lint = build.build("some fields are not explicitly listed");
+        lint.span_label(pat.span, format!("field{} {} not listed", rustc_errors::pluralize!(unmentioned_fields.len()), joined_patterns));
+
+        lint.help(
+            "ensure that all fields are mentioned explicitly by adding the suggested fields",
+        );
+        lint.note(&format!(
+            "the pattern is of type `{}` and the `non_exhaustive_omitted_patterns` attribute was found",
+            ty,
+        ));
+        lint.emit();
+    });
+    }
+
     /// Returns a diagnostic reporting a struct pattern which does not mention some fields.
     ///
     /// ```text

src/test/ui/rfc-2008-non-exhaustive/auxiliary/enums.rs

@@ -4,8 +4,29 @@
 pub enum NonExhaustiveEnum {
     Unit,
     Tuple(u32),
-    Struct { field: u32 }
+    Struct { field: u32 },
+}
+
+#[non_exhaustive]
+pub enum NestedNonExhaustive {
+    A(NonExhaustiveEnum),
+    B,
+    C,
 }
 
 #[non_exhaustive]
 pub enum EmptyNonExhaustiveEnum {}
+
+pub enum VariantNonExhaustive {
+    #[non_exhaustive]
+    Bar {
+        x: u32,
+        y: u64,
+    },
+    Baz(u32, u16),
+}
+
+#[non_exhaustive]
+pub enum NonExhaustiveSingleVariant {
+    A(bool),
+}

src/test/ui/rfc-2008-non-exhaustive/auxiliary/structs.rs

@@ -1,3 +1,4 @@
+#[derive(Default)]
 #[non_exhaustive]
 pub struct NormalStruct {
     pub first_field: u16,
@@ -15,11 +16,18 @@ pub struct TupleStruct(pub u16, pub u16);
 pub struct FunctionalRecord {
     pub first_field: u16,
     pub second_field: u16,
-    pub third_field: bool
+    pub third_field: bool,
 }
 
 impl Default for FunctionalRecord {
     fn default() -> FunctionalRecord {
         FunctionalRecord { first_field: 640, second_field: 480, third_field: false }
     }
 }
+
+#[derive(Default)]
+#[non_exhaustive]
+pub struct NestedStruct {
+    pub foo: u16,
+    pub bar: NormalStruct,
+}

src/test/ui/rfc-2008-non-exhaustive/reachable-patterns.rs

@@ -0,0 +1,160 @@
+// Test that the `non_exhaustive_omitted_patterns` lint is triggered correctly.
+
+// aux-build:enums.rs
+extern crate enums;
+
+// aux-build:structs.rs
+extern crate structs;
+
+use enums::{
+    EmptyNonExhaustiveEnum, NestedNonExhaustive, NonExhaustiveEnum, NonExhaustiveSingleVariant,
+    VariantNonExhaustive,
+};
+use structs::{FunctionalRecord, NestedStruct, NormalStruct};
+
+#[non_exhaustive]
+#[derive(Default)]
+pub struct Foo {
+    a: u8,
+    b: usize,
+    c: String,
+}
+
+#[non_exhaustive]
+pub enum Bar {
+    A,
+    B,
+    C,
+}
+
+fn main() {
+    let enumeration = Bar::A;
+
+    // Ok: this is a crate local non_exhaustive enum
+    match enumeration {
+        Bar::A => {}
+        Bar::B => {}
+        #[deny(non_exhaustive_omitted_patterns)]
+        _ => {}
+    }
+
+    let non_enum = NonExhaustiveEnum::Unit;
+
+    // Ok: without the attribute
+    match non_enum {
+        NonExhaustiveEnum::Unit => {}
+        NonExhaustiveEnum::Tuple(_) => {}
+        _ => {}
+    }
+
+    match non_enum {
+        NonExhaustiveEnum::Unit => {}
+        NonExhaustiveEnum::Tuple(_) => {}
+        #[deny(non_exhaustive_omitted_patterns)]
+        _ => {}
+    }
+    //~^^ some variants are not matched explicitly
+
+    match non_enum {
+        NonExhaustiveEnum::Unit | NonExhaustiveEnum::Struct { .. } => {}
+        #[deny(non_exhaustive_omitted_patterns)]
+        _ => {}
+    }
+    //~^^ some variants are not matched explicitly
+
+    let x = 5;
+    match non_enum {
+        NonExhaustiveEnum::Unit if x > 10 => {}
+        NonExhaustiveEnum::Tuple(_) => {}
+        NonExhaustiveEnum::Struct { .. } => {}
+        #[deny(non_exhaustive_omitted_patterns)]
+        _ => {}
+    }
+    //~^^ some variants are not matched explicitly
+
+    // Ok: all covered and not `unreachable-patterns`
+    #[deny(unreachable_patterns)]
+    match non_enum {
+        NonExhaustiveEnum::Unit => {}
+        NonExhaustiveEnum::Tuple(_) => {}
+        NonExhaustiveEnum::Struct { .. } => {}
+        #[deny(non_exhaustive_omitted_patterns)]
+        _ => {}
+    }
+
+    #[deny(non_exhaustive_omitted_patterns)]
+    match NestedNonExhaustive::B {
+        NestedNonExhaustive::A(NonExhaustiveEnum::Unit) => {}
+        NestedNonExhaustive::A(_) => {}
+        NestedNonExhaustive::B => {}
+        _ => {}
+    }
+    //~^^ some variants are not matched explicitly
+    //~^^^^^ some variants are not matched explicitly
+
+    // The io::ErrorKind has many `unstable` fields how do they interact with this
+    // lint
+    #[deny(non_exhaustive_omitted_patterns)]
+    match std::io::ErrorKind::Other {
+        std::io::ErrorKind::NotFound => {}
+        std::io::ErrorKind::PermissionDenied => {}
+        std::io::ErrorKind::ConnectionRefused => {}
+        std::io::ErrorKind::ConnectionReset => {}
+        std::io::ErrorKind::ConnectionAborted => {}
+        std::io::ErrorKind::NotConnected => {}
+        std::io::ErrorKind::AddrInUse => {}
+        std::io::ErrorKind::AddrNotAvailable => {}
+        std::io::ErrorKind::BrokenPipe => {}
+        std::io::ErrorKind::AlreadyExists => {}
+        std::io::ErrorKind::WouldBlock => {}
+        std::io::ErrorKind::InvalidInput => {}
+        std::io::ErrorKind::InvalidData => {}
+        std::io::ErrorKind::TimedOut => {}
+        std::io::ErrorKind::WriteZero => {}
+        std::io::ErrorKind::Interrupted => {}
+        std::io::ErrorKind::Other => {}
+        std::io::ErrorKind::UnexpectedEof => {}
+        std::io::ErrorKind::Unsupported => {}
+        std::io::ErrorKind::OutOfMemory => {}
+        // All stable variants are above and unstable in `_`
+        _ => {}
+    }
+    //~^^ some variants are not matched explicitly
+
+    #[warn(non_exhaustive_omitted_patterns)]
+    match VariantNonExhaustive::Baz(1, 2) {
+        VariantNonExhaustive::Baz(_, _) => {}
+        VariantNonExhaustive::Bar { x, .. } => {}
+    }
+    //~^^ some fields are not explicitly listed
+
+    #[warn(non_exhaustive_omitted_patterns)]
+    let FunctionalRecord { first_field, second_field, .. } = FunctionalRecord::default();
+    //~^ some fields are not explicitly listed
+
+    // Ok: this is local
+    #[warn(non_exhaustive_omitted_patterns)]
+    let Foo { a, b, .. } = Foo::default();
+
+    #[warn(non_exhaustive_omitted_patterns)]
+    let NestedStruct { bar: NormalStruct { first_field, .. }, .. } = NestedStruct::default();
+    //~^ some fields are not explicitly listed
+    //~^^ some fields are not explicitly listed
+
+    // Ok: because this only has 1 variant
+    #[deny(non_exhaustive_omitted_patterns)]
+    match NonExhaustiveSingleVariant::A(true) {
+        NonExhaustiveSingleVariant::A(true) => {}
+        _ => {}
+    }
+
+    #[deny(non_exhaustive_omitted_patterns)]
+    match NonExhaustiveSingleVariant::A(true) {
+        _ => {}
+    }
+    //~^^ some variants are not matched explicitly
+
+    // Ok: we don't lint on `if let` expressions
+    #[deny(non_exhaustive_omitted_patterns)]
+    if let NonExhaustiveEnum::Tuple(_) = non_enum {}
+}

src/test/ui/rfc-2008-non-exhaustive/reachable-patterns.stderr

@@ -0,0 +1,146 @@
+warning: some fields are not explicitly listed
+  --> $DIR/reachable-patterns.rs:127:9
+   |
+LL |         VariantNonExhaustive::Bar { x, .. } => {}
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ field `y` not listed
+   |
+note: the lint level is defined here
+  --> $DIR/reachable-patterns.rs:124:12
+   |
+LL |     #[warn(non_exhaustive_omitted_patterns)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: ensure that all fields are mentioned explicitly by adding the suggested fields
+   = note: the pattern is of type `VariantNonExhaustive` and the `non_exhaustive_omitted_patterns` attribute was found
+
+warning: some fields are not explicitly listed
+  --> $DIR/reachable-patterns.rs:132:9
+   |
+LL |     let FunctionalRecord { first_field, second_field, .. } = FunctionalRecord::default();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ field `third_field` not listed
+   |
+note: the lint level is defined here
+  --> $DIR/reachable-patterns.rs:131:12
+   |
+LL |     #[warn(non_exhaustive_omitted_patterns)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: ensure that all fields are mentioned explicitly by adding the suggested fields
+   = note: the pattern is of type `FunctionalRecord` and the `non_exhaustive_omitted_patterns` attribute was found
+
+warning: some fields are not explicitly listed
+  --> $DIR/reachable-patterns.rs:140:29
+   |
+LL |     let NestedStruct { bar: NormalStruct { first_field, .. }, .. } = NestedStruct::default();
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ field `second_field` not listed
+   |
+note: the lint level is defined here
+  --> $DIR/reachable-patterns.rs:139:12
+   |
+LL |     #[warn(non_exhaustive_omitted_patterns)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: ensure that all fields are mentioned explicitly by adding the suggested fields
+   = note: the pattern is of type `NormalStruct` and the `non_exhaustive_omitted_patterns` attribute was found
+
+warning: some fields are not explicitly listed
+  --> $DIR/reachable-patterns.rs:140:9
+   |
+LL |     let NestedStruct { bar: NormalStruct { first_field, .. }, .. } = NestedStruct::default();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ field `foo` not listed
+   |
+   = help: ensure that all fields are mentioned explicitly by adding the suggested fields
+   = note: the pattern is of type `NestedStruct` and the `non_exhaustive_omitted_patterns` attribute was found
+
+error: some variants are not matched explicitly
+  --> $DIR/reachable-patterns.rs:54:9
+   |
+LL |         _ => {}
+   |         ^ pattern `Struct { .. }` not covered
+   |
+note: the lint level is defined here
+  --> $DIR/reachable-patterns.rs:53:16
+   |
+LL |         #[deny(non_exhaustive_omitted_patterns)]
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: ensure that all variants are matched explicitly by adding the suggested match arms
+   = note: the matched value is of type `NonExhaustiveEnum` and the `non_exhaustive_omitted_patterns` attribute was found
+
+error: some variants are not matched explicitly
+  --> $DIR/reachable-patterns.rs:61:9
+   |
+LL |         _ => {}
+   |         ^ pattern `Tuple(_)` not covered
+   |
+note: the lint level is defined here
+  --> $DIR/reachable-patterns.rs:60:16
+   |
+LL |         #[deny(non_exhaustive_omitted_patterns)]
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: ensure that all variants are matched explicitly by adding the suggested match arms
+   = note: the matched value is of type `NonExhaustiveEnum` and the `non_exhaustive_omitted_patterns` attribute was found
+
+error: some variants are not matched explicitly
+  --> $DIR/reachable-patterns.rs:71:9
+   |
+LL |         _ => {}
+   |         ^ pattern `Unit` not covered
+   |
+note: the lint level is defined here
+  --> $DIR/reachable-patterns.rs:70:16
+   |
+LL |         #[deny(non_exhaustive_omitted_patterns)]
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: ensure that all variants are matched explicitly by adding the suggested match arms
+   = note: the matched value is of type `NonExhaustiveEnum` and the `non_exhaustive_omitted_patterns` attribute was found
+
+error: some variants are not matched explicitly
+  --> $DIR/reachable-patterns.rs:88:32
+   |
+LL |         NestedNonExhaustive::A(_) => {}
+   |                                ^ patterns `Tuple(_)` and `Struct { .. }` not covered
+   |
+note: the lint level is defined here
+  --> $DIR/reachable-patterns.rs:85:12
+   |
+LL |     #[deny(non_exhaustive_omitted_patterns)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: ensure that all variants are matched explicitly by adding the suggested match arms
+   = note: the matched value is of type `NonExhaustiveEnum` and the `non_exhaustive_omitted_patterns` attribute was found
+
+error: some variants are not matched explicitly
+  --> $DIR/reachable-patterns.rs:90:9
+   |
+LL |         _ => {}
+   |         ^ pattern `C` not covered
+   |
+   = help: ensure that all variants are matched explicitly by adding the suggested match arms
+   = note: the matched value is of type `NestedNonExhaustive` and the `non_exhaustive_omitted_patterns` attribute was found
+
+error: some variants are not matched explicitly
+  --> $DIR/reachable-patterns.rs:120:9
+   |
+LL |         _ => {}
+   |         ^ patterns `HostUnreachable`, `NetworkUnreachable`, `NetworkDown` and 18 more not covered
+   |
+note: the lint level is defined here
+  --> $DIR/reachable-patterns.rs:97:12
+   |
+LL |     #[deny(non_exhaustive_omitted_patterns)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: ensure that all variants are matched explicitly by adding the suggested match arms
+   = note: the matched value is of type `ErrorKind` and the `non_exhaustive_omitted_patterns` attribute was found
+
+error: some variants are not matched explicitly
+  --> $DIR/reachable-patterns.rs:153:9
+   |
+LL |         _ => {}
+   |         ^ pattern `A(_)` not covered
+   |
+note: the lint level is defined here
+  --> $DIR/reachable-patterns.rs:151:12
+   |
+LL |     #[deny(non_exhaustive_omitted_patterns)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: ensure that all variants are matched explicitly by adding the suggested match arms
+   = note: the matched value is of type `NonExhaustiveSingleVariant` and the `non_exhaustive_omitted_patterns` attribute was found
+
+error: aborting due to 7 previous errors; 4 warnings emitted
+

src/test/ui/rfc-2008-non-exhaustive/struct.stderr

@@ -16,13 +16,13 @@ error[E0603]: tuple struct constructor `TupleStruct` is private
 LL |     let ts_explicit = structs::TupleStruct(640, 480);
    |                                ^^^^^^^^^^^ private tuple struct constructor
    |
-  ::: $DIR/auxiliary/structs.rs:11:24
+  ::: $DIR/auxiliary/structs.rs:12:24
    |
 LL | pub struct TupleStruct(pub u16, pub u16);
    |                        ---------------- a constructor is private if any of the fields is private
    |
 note: the tuple struct constructor `TupleStruct` is defined here
-  --> $DIR/auxiliary/structs.rs:11:1
+  --> $DIR/auxiliary/structs.rs:12:1
    |
 LL | pub struct TupleStruct(pub u16, pub u16);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -34,7 +34,7 @@ LL |     let us_explicit = structs::UnitStruct;
    |                                ^^^^^^^^^^ private unit struct
    |
 note: the unit struct `UnitStruct` is defined here
-  --> $DIR/auxiliary/structs.rs:8:1
+  --> $DIR/auxiliary/structs.rs:9:1
    |
 LL | pub struct UnitStruct;
    | ^^^^^^^^^^^^^^^^^^^^^^