Display witnesses of non-exhaustive match

Reporting format follows rustc and shows at most three witnesses.

crates/hir-ty/src/diagnostics/expr.rs

@@ -4,9 +4,10 @@
 
 use std::sync::Arc;
 
-use hir_def::{path::path, resolver::HasResolver, AssocItemId, DefWithBodyId, HasModule};
+use hir_def::{path::path, resolver::HasResolver, AdtId, AssocItemId, DefWithBodyId, HasModule};
 use hir_expand::name;
 use itertools::Either;
+use itertools::Itertools;
 use rustc_hash::FxHashSet;
 use typed_arena::Arena;
 
@@ -17,7 +18,8 @@ use crate::{
         deconstruct_pat::DeconstructedPat,
         usefulness::{compute_match_usefulness, MatchCheckCtx},
     },
-    InferenceResult, TyExt,
+    display::HirDisplay,
+    InferenceResult, Ty, TyExt,
 };
 
 pub(crate) use hir_def::{
@@ -37,6 +39,7 @@ pub enum BodyValidationDiagnostic {
     },
     MissingMatchArms {
         match_expr: ExprId,
+        uncovered_patterns: String,
     },
 }
 
@@ -211,10 +214,11 @@ impl ExprValidator {
         // https://github.com/rust-lang/rust/blob/f31622a50/compiler/rustc_mir_build/src/thir/pattern/check_match.rs#L200
 
         let witnesses = report.non_exhaustiveness_witnesses;
-        // FIXME Report witnesses
-        // eprintln!("compute_match_usefulness(..) -> {:?}", &witnesses);
         if !witnesses.is_empty() {
-            self.diagnostics.push(BodyValidationDiagnostic::MissingMatchArms { match_expr: id });
+            self.diagnostics.push(BodyValidationDiagnostic::MissingMatchArms {
+                match_expr: id,
+                uncovered_patterns: missing_match_arms(&cx, match_expr_ty, witnesses, arms),
+            });
         }
     }
 
@@ -367,3 +371,40 @@ fn types_of_subpatterns_do_match(pat: PatId, body: &Body, infer: &InferenceResul
     walk(pat, body, infer, &mut has_type_mismatches);
     !has_type_mismatches
 }
+
+fn missing_match_arms<'p>(
+    cx: &MatchCheckCtx<'_, 'p>,
+    scrut_ty: &Ty,
+    witnesses: Vec<DeconstructedPat<'p>>,
+    arms: &[MatchArm],
+) -> String {
+    let non_empty_enum = match scrut_ty.as_adt() {
+        Some((AdtId::EnumId(e), _)) => !cx.db.enum_data(e).variants.is_empty(),
+        _ => false,
+    };
+    if arms.is_empty() && !non_empty_enum {
+        format!("type `{}` is non-empty", scrut_ty.display(cx.db))
+    } else {
+        const LIMIT: usize = 3;
+        match &*witnesses {
+            [witness] => format!("`{}` not covered", witness.to_pat(&cx).display(cx.db)),
+            [head @ .., tail] if head.len() < LIMIT => {
+                let head: Vec<_> = head.iter().map(|w| w.to_pat(cx)).collect();
+                format!(
+                    "`{}` and `{}` not covered",
+                    head.iter().map(|p| p.display(cx.db)).join("`, `"),
+                    tail.to_pat(&cx).display(cx.db)
+                )
+            }
+            _ => {
+                let (head, tail) = witnesses.split_at(LIMIT);
+                let head: Vec<_> = head.iter().map(|w| w.to_pat(cx)).collect();
+                format!(
+                    "`{}` and {} more not covered",
+                    head.iter().map(|p| p.display(cx.db)).join("`, `"),
+                    tail.len()
+                )
+            }
+        }
+    }
+}

crates/hir-ty/src/diagnostics/match_check.rs

@@ -10,11 +10,19 @@ mod pat_util;
 pub(crate) mod deconstruct_pat;
 pub(crate) mod usefulness;
 
-use hir_def::{body::Body, expr::PatId, EnumVariantId, LocalFieldId, VariantId};
+use chalk_ir::Mutability;
+use hir_def::{
+    adt::VariantData, body::Body, expr::PatId, AdtId, EnumVariantId, HasModule, LocalFieldId,
+    VariantId,
+};
+use hir_expand::name::{name, Name};
 use stdx::{always, never};
 
 use crate::{
-    db::HirDatabase, infer::BindingMode, InferenceResult, Interner, Substitution, Ty, TyKind,
+    db::HirDatabase,
+    display::{HirDisplay, HirDisplayError, HirFormatter},
+    infer::BindingMode,
+    InferenceResult, Interner, Substitution, Ty, TyExt, TyKind,
 };
 
 use self::pat_util::EnumerateAndAdjustIterator;
@@ -49,6 +57,7 @@ pub(crate) enum PatKind {
 
     /// `x`, `ref x`, `x @ P`, etc.
     Binding {
+        name: Name,
         subpattern: Option<Pat>,
     },
 
@@ -148,7 +157,7 @@ impl<'a> PatCtxt<'a> {
                     }
                     _ => (),
                 }
-                PatKind::Binding { subpattern: self.lower_opt_pattern(subpat) }
+                PatKind::Binding { name: name.clone(), subpattern: self.lower_opt_pattern(subpat) }
             }
 
             hir_def::expr::Pat::TupleStruct { ref args, ellipsis, .. } if variant.is_some() => {
@@ -282,6 +291,127 @@ impl<'a> PatCtxt<'a> {
     }
 }
 
+impl HirDisplay for Pat {
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
+        match &*self.kind {
+            PatKind::Wild => write!(f, "_"),
+            PatKind::Binding { name, subpattern } => {
+                write!(f, "{name}")?;
+                if let Some(subpattern) = subpattern {
+                    write!(f, " @ ")?;
+                    subpattern.hir_fmt(f)?;
+                }
+                Ok(())
+            }
+            PatKind::Variant { subpatterns, .. } | PatKind::Leaf { subpatterns } => {
+                let variant = match *self.kind {
+                    PatKind::Variant { enum_variant, .. } => Some(VariantId::from(enum_variant)),
+                    _ => self.ty.as_adt().and_then(|(adt, _)| match adt {
+                        AdtId::StructId(s) => Some(s.into()),
+                        AdtId::UnionId(u) => Some(u.into()),
+                        AdtId::EnumId(_) => None,
+                    }),
+                };
+
+                if let Some(variant) = variant {
+                    match variant {
+                        VariantId::EnumVariantId(v) => {
+                            let data = f.db.enum_data(v.parent);
+                            write!(f, "{}", data.variants[v.local_id].name)?;
+                        }
+                        VariantId::StructId(s) => write!(f, "{}", f.db.struct_data(s).name)?,
+                        VariantId::UnionId(u) => write!(f, "{}", f.db.union_data(u).name)?,
+                    };
+
+                    let variant_data = variant.variant_data(f.db.upcast());
+                    if let VariantData::Record(rec_fields) = &*variant_data {
+                        write!(f, " {{ ")?;
+
+                        let mut printed = 0;
+                        let subpats = subpatterns
+                            .iter()
+                            .filter(|p| !matches!(*p.pattern.kind, PatKind::Wild))
+                            .map(|p| {
+                                printed += 1;
+                                WriteWith(move |f| {
+                                    write!(f, "{}: ", rec_fields[p.field].name)?;
+                                    p.pattern.hir_fmt(f)
+                                })
+                            });
+                        f.write_joined(subpats, ", ")?;
+
+                        if printed < rec_fields.len() {
+                            write!(f, "{}..", if printed > 0 { ", " } else { "" })?;
+                        }
+
+                        return write!(f, " }}");
+                    }
+                }
+
+                let num_fields = variant
+                    .map_or(subpatterns.len(), |v| v.variant_data(f.db.upcast()).fields().len());
+                if num_fields != 0 || variant.is_none() {
+                    write!(f, "(")?;
+                    let subpats = (0..num_fields).map(|i| {
+                        WriteWith(move |f| {
+                            let fid = LocalFieldId::from_raw((i as u32).into());
+                            if let Some(p) = subpatterns.get(i) {
+                                if p.field == fid {
+                                    return p.pattern.hir_fmt(f);
+                                }
+                            }
+                            if let Some(p) = subpatterns.iter().find(|p| p.field == fid) {
+                                p.pattern.hir_fmt(f)
+                            } else {
+                                write!(f, "_")
+                            }
+                        })
+                    });
+                    f.write_joined(subpats, ", ")?;
+                    if let (TyKind::Tuple(..), 1) = (self.ty.kind(Interner), num_fields) {
+                        write!(f, ",")?;
+                    }
+                    write!(f, ")")?;
+                }
+
+                Ok(())
+            }
+            PatKind::Deref { subpattern } => {
+                match self.ty.kind(Interner) {
+                    TyKind::Adt(adt, _) if is_box(adt.0, f.db) => write!(f, "box ")?,
+                    &TyKind::Ref(mutbl, ..) => {
+                        write!(f, "&{}", if mutbl == Mutability::Mut { "mut " } else { "" })?
+                    }
+                    _ => never!("{:?} is a bad Deref pattern type", self.ty),
+                }
+                subpattern.hir_fmt(f)
+            }
+            PatKind::LiteralBool { value } => write!(f, "{}", value),
+            PatKind::Or { pats } => f.write_joined(pats.iter(), " | "),
+        }
+    }
+}
+
+struct WriteWith<F>(F)
+where
+    F: Fn(&mut HirFormatter) -> Result<(), HirDisplayError>;
+
+impl<F> HirDisplay for WriteWith<F>
+where
+    F: Fn(&mut HirFormatter) -> Result<(), HirDisplayError>,
+{
+    fn hir_fmt(&self, f: &mut HirFormatter) -> Result<(), HirDisplayError> {
+        (self.0)(f)
+    }
+}
+
+fn is_box(adt: AdtId, db: &dyn HirDatabase) -> bool {
+    let owned_box = name![owned_box].to_smol_str();
+    let krate = adt.module(db.upcast()).krate();
+    let box_adt = db.lang_item(krate, owned_box).and_then(|it| it.as_struct()).map(AdtId::from);
+    Some(adt) == box_adt
+}
+
 pub(crate) trait PatternFoldable: Sized {
     fn fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
         self.super_fold_with(folder)
@@ -357,8 +487,8 @@ impl PatternFoldable for PatKind {
     fn super_fold_with<F: PatternFolder>(&self, folder: &mut F) -> Self {
         match self {
             PatKind::Wild => PatKind::Wild,
-            PatKind::Binding { subpattern } => {
-                PatKind::Binding { subpattern: subpattern.fold_with(folder) }
+            PatKind::Binding { name, subpattern } => {
+                PatKind::Binding { name: name.clone(), subpattern: subpattern.fold_with(folder) }
             }
             PatKind::Variant { substs, enum_variant, subpatterns } => PatKind::Variant {
                 substs: substs.fold_with(folder),

crates/hir-ty/src/diagnostics/match_check/deconstruct_pat.rs

@@ -51,13 +51,13 @@ use std::{
 use hir_def::{EnumVariantId, HasModule, LocalFieldId, VariantId};
 use smallvec::{smallvec, SmallVec};
 use stdx::never;
-use syntax::SmolStr;
 
 use crate::{infer::normalize, AdtId, Interner, Scalar, Ty, TyExt, TyKind};
 
 use super::{
+    is_box,
     usefulness::{helper::Captures, MatchCheckCtx, PatCtxt},
-    Pat, PatKind,
+    FieldPat, Pat, PatKind,
 };
 
 use self::Constructor::*;
@@ -144,6 +144,24 @@ impl IntRange {
         }
     }
 
+    fn to_pat(&self, _cx: &MatchCheckCtx, ty: Ty) -> Pat {
+        match ty.kind(Interner) {
+            TyKind::Scalar(Scalar::Bool) => {
+                let kind = match self.boundaries() {
+                    (0, 0) => PatKind::LiteralBool { value: false },
+                    (1, 1) => PatKind::LiteralBool { value: true },
+                    (0, 1) => PatKind::Wild,
+                    (lo, hi) => {
+                        never!("bad range for bool pattern: {}..={}", lo, hi);
+                        PatKind::Wild
+                    }
+                };
+                Pat { ty, kind: kind.into() }
+            }
+            _ => unimplemented!(),
+        }
+    }
+
     /// See `Constructor::is_covered_by`
     fn is_covered_by(&self, other: &Self) -> bool {
         if self.intersection(other).is_some() {
@@ -363,7 +381,7 @@ impl Constructor {
                 TyKind::Tuple(arity, ..) => arity,
                 TyKind::Ref(..) => 1,
                 TyKind::Adt(adt, ..) => {
-                    if adt_is_box(adt.0, pcx.cx) {
+                    if is_box(adt.0, pcx.cx.db) {
                         // The only legal patterns of type `Box` (outside `std`) are `_` and box
                         // patterns. If we're here we can assume this is a box pattern.
                         1
@@ -782,7 +800,7 @@ impl<'p> Fields<'p> {
                 }
                 TyKind::Ref(.., rty) => Fields::wildcards_from_tys(cx, once(rty.clone())),
                 &TyKind::Adt(AdtId(adt), ref substs) => {
-                    if adt_is_box(adt, cx) {
+                    if is_box(adt, cx.db) {
                         // The only legal patterns of type `Box` (outside `std`) are `_` and box
                         // patterns. If we're here we can assume this is a box pattern.
                         let subst_ty = substs.at(Interner, 0).assert_ty_ref(Interner).clone();
@@ -865,8 +883,8 @@ impl<'p> DeconstructedPat<'p> {
         let ctor;
         let fields;
         match pat.kind.as_ref() {
-            PatKind::Binding { subpattern: Some(subpat) } => return mkpat(subpat),
-            PatKind::Binding { subpattern: None } | PatKind::Wild => {
+            PatKind::Binding { subpattern: Some(subpat), .. } => return mkpat(subpat),
+            PatKind::Binding { subpattern: None, .. } | PatKind::Wild => {
                 ctor = Wildcard;
                 fields = Fields::empty();
             }
@@ -889,7 +907,7 @@ impl<'p> DeconstructedPat<'p> {
                         }
                         fields = Fields::from_iter(cx, wilds)
                     }
-                    TyKind::Adt(adt, substs) if adt_is_box(adt.0, cx) => {
+                    TyKind::Adt(adt, substs) if is_box(adt.0, cx.db) => {
                         // The only legal patterns of type `Box` (outside `std`) are `_` and box
                         // patterns. If we're here we can assume this is a box pattern.
                         // FIXME(Nadrieril): A `Box` can in theory be matched either with `Box(_,
@@ -963,10 +981,67 @@ impl<'p> DeconstructedPat<'p> {
         DeconstructedPat::new(ctor, fields, pat.ty.clone())
     }
 
-    // // FIXME(iDawer): implement reporting of noncovered patterns
-    // pub(crate) fn to_pat(&self, _cx: &MatchCheckCtx<'_, 'p>) -> Pat {
-    //     Pat { ty: self.ty.clone(), kind: PatKind::Wild.into() }
-    // }
+    pub(crate) fn to_pat(&self, cx: &MatchCheckCtx<'_, 'p>) -> Pat {
+        let mut subpatterns = self.iter_fields().map(|p| p.to_pat(cx));
+        let pat = match &self.ctor {
+            Single | Variant(_) => match self.ty.kind(Interner) {
+                TyKind::Tuple(..) => PatKind::Leaf {
+                    subpatterns: subpatterns
+                        .zip(0u32..)
+                        .map(|(p, i)| FieldPat {
+                            field: LocalFieldId::from_raw(i.into()),
+                            pattern: p,
+                        })
+                        .collect(),
+                },
+                TyKind::Adt(adt, _) if is_box(adt.0, cx.db) => {
+                    // 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() }
+                }
+                TyKind::Adt(adt, substs) => {
+                    let variant = self.ctor.variant_id_for_adt(adt.0);
+                    let subpatterns = Fields::list_variant_nonhidden_fields(cx, self.ty(), variant)
+                        .zip(subpatterns)
+                        .map(|((field, _ty), pattern)| FieldPat { field, pattern })
+                        .collect();
+
+                    if let VariantId::EnumVariantId(enum_variant) = variant {
+                        PatKind::Variant { substs: substs.clone(), enum_variant, subpatterns }
+                    } else {
+                        PatKind::Leaf { subpatterns }
+                    }
+                }
+                // 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.
+                TyKind::Ref(..) => PatKind::Deref { subpattern: subpatterns.next().unwrap() },
+                _ => {
+                    never!("unexpected ctor for type {:?} {:?}", self.ctor, self.ty);
+                    PatKind::Wild
+                }
+            },
+            &Slice(Slice { _unimplemented: _void }) => unimplemented!(),
+            &Str(_void) => unimplemented!(),
+            &FloatRange(_void) => unimplemented!(),
+            IntRange(range) => return range.to_pat(cx, self.ty.clone()),
+            Wildcard | NonExhaustive => PatKind::Wild,
+            Missing { .. } => {
+                never!(
+                    "trying to convert a `Missing` constructor into a `Pat`; this is probably a bug,
+                    `Missing` should have been processed in `apply_constructors`"
+                );
+                PatKind::Wild
+            }
+            Opaque | Or => {
+                never!("can't convert to pattern: {:?}", self.ctor);
+                PatKind::Wild
+            }
+        };
+        Pat { ty: self.ty.clone(), kind: Box::new(pat) }
+    }
 
     pub(super) fn is_or_pat(&self) -> bool {
         matches!(self.ctor, Or)
@@ -980,7 +1055,7 @@ impl<'p> DeconstructedPat<'p> {
         &self.ty
     }
 
-    pub(super) fn iter_fields<'a>(&'a self) -> impl Iterator<Item = &'a DeconstructedPat<'a>> + 'a {
+    pub(super) fn iter_fields<'a>(&'a self) -> impl Iterator<Item = &'p DeconstructedPat<'p>> + 'a {
         self.fields.iter_patterns()
     }
 
@@ -1023,11 +1098,3 @@ fn is_field_list_non_exhaustive(variant_id: VariantId, cx: &MatchCheckCtx<'_, '_
     };
     cx.db.attrs(attr_def_id).by_key("non_exhaustive").exists()
 }
-
-fn adt_is_box(adt: hir_def::AdtId, cx: &MatchCheckCtx<'_, '_>) -> bool {
-    use hir_def::lang_item::LangItemTarget;
-    match cx.db.lang_item(cx.module.krate(), SmolStr::new_inline("owned_box")) {
-        Some(LangItemTarget::StructId(box_id)) => adt == box_id.into(),
-        _ => false,
-    }
-}