Add diagnostic for _ expressions (typed holes)

crates/base-db/src/fixture.rs

@@ -21,6 +21,7 @@ use crate::{
 pub const WORKSPACE: SourceRootId = SourceRootId(0);
 
 pub trait WithFixture: Default + SourceDatabaseExt + 'static {
+    #[track_caller]
     fn with_single_file(ra_fixture: &str) -> (Self, FileId) {
         let fixture = ChangeFixture::parse(ra_fixture);
         let mut db = Self::default();
@@ -29,6 +30,7 @@ pub trait WithFixture: Default + SourceDatabaseExt + 'static {
         (db, fixture.files[0])
     }
 
+    #[track_caller]
     fn with_many_files(ra_fixture: &str) -> (Self, Vec<FileId>) {
         let fixture = ChangeFixture::parse(ra_fixture);
         let mut db = Self::default();
@@ -37,6 +39,7 @@ pub trait WithFixture: Default + SourceDatabaseExt + 'static {
         (db, fixture.files)
     }
 
+    #[track_caller]
     fn with_files(ra_fixture: &str) -> Self {
         let fixture = ChangeFixture::parse(ra_fixture);
         let mut db = Self::default();
@@ -45,6 +48,7 @@ pub trait WithFixture: Default + SourceDatabaseExt + 'static {
         db
     }
 
+    #[track_caller]
     fn with_files_extra_proc_macros(
         ra_fixture: &str,
         proc_macros: Vec<(String, ProcMacro)>,
@@ -56,18 +60,21 @@ pub trait WithFixture: Default + SourceDatabaseExt + 'static {
         db
     }
 
+    #[track_caller]
     fn with_position(ra_fixture: &str) -> (Self, FilePosition) {
         let (db, file_id, range_or_offset) = Self::with_range_or_offset(ra_fixture);
         let offset = range_or_offset.expect_offset();
         (db, FilePosition { file_id, offset })
     }
 
+    #[track_caller]
     fn with_range(ra_fixture: &str) -> (Self, FileRange) {
         let (db, file_id, range_or_offset) = Self::with_range_or_offset(ra_fixture);
         let range = range_or_offset.expect_range();
         (db, FileRange { file_id, range })
     }
 
+    #[track_caller]
     fn with_range_or_offset(ra_fixture: &str) -> (Self, FileId, RangeOrOffset) {
         let fixture = ChangeFixture::parse(ra_fixture);
         let mut db = Self::default();

crates/hir-def/src/resolver.rs

@@ -1046,6 +1046,12 @@ impl HasResolver for GenericDefId {
     }
 }
 
+impl HasResolver for EnumVariantId {
+    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
+        self.parent.resolver(db)
+    }
+}
+
 impl HasResolver for VariantId {
     fn resolver(self, db: &dyn DefDatabase) -> Resolver {
         match self {

crates/hir-ty/src/builder.rs

@@ -194,6 +194,19 @@ impl TyBuilder<()> {
         params.placeholder_subst(db)
     }
 
+    pub fn unknown_subst(db: &dyn HirDatabase, def: impl Into<GenericDefId>) -> Substitution {
+        let params = generics(db.upcast(), def.into());
+        Substitution::from_iter(
+            Interner,
+            params.iter_id().map(|id| match id {
+                either::Either::Left(_) => TyKind::Error.intern(Interner).cast(Interner),
+                either::Either::Right(id) => {
+                    unknown_const_as_generic(db.const_param_ty(id)).cast(Interner)
+                }
+            }),
+        )
+    }
+
     pub fn subst_for_def(
         db: &dyn HirDatabase,
         def: impl Into<GenericDefId>,

crates/hir-ty/src/infer.rs

@@ -215,6 +215,10 @@ pub enum InferenceDiagnostic {
         call_expr: ExprId,
         found: Ty,
     },
+    TypedHole {
+        expr: ExprId,
+        expected: Ty,
+    },
 }
 
 /// A mismatch between an expected and an inferred type.
@@ -600,29 +604,30 @@ impl<'a> InferenceContext<'a> {
             mismatch.actual = table.resolve_completely(mismatch.actual.clone());
         }
         result.diagnostics.retain_mut(|diagnostic| {
-            if let InferenceDiagnostic::ExpectedFunction { found: ty, .. }
-            | InferenceDiagnostic::UnresolvedField { receiver: ty, .. }
-            | InferenceDiagnostic::UnresolvedMethodCall { receiver: ty, .. } = diagnostic
-            {
-                *ty = table.resolve_completely(ty.clone());
-                // FIXME: Remove this when we are on par with rustc in terms of inference
-                if ty.contains_unknown() {
-                    return false;
-                }
+            use InferenceDiagnostic::*;
+            match diagnostic {
+                ExpectedFunction { found: ty, .. }
+                | UnresolvedField { receiver: ty, .. }
+                | UnresolvedMethodCall { receiver: ty, .. } => {
+                    *ty = table.resolve_completely(ty.clone());
+                    // FIXME: Remove this when we are on par with rustc in terms of inference
+                    if ty.contains_unknown() {
+                        return false;
+                    }
 
-                if let InferenceDiagnostic::UnresolvedMethodCall { field_with_same_name, .. } =
-                    diagnostic
-                {
-                    let clear = if let Some(ty) = field_with_same_name {
-                        *ty = table.resolve_completely(ty.clone());
-                        ty.contains_unknown()
-                    } else {
-                        false
-                    };
-                    if clear {
-                        *field_with_same_name = None;
+                    if let UnresolvedMethodCall { field_with_same_name, .. } = diagnostic {
+                        if let Some(ty) = field_with_same_name {
+                            *ty = table.resolve_completely(ty.clone());
+                            if ty.contains_unknown() {
+                                *field_with_same_name = None;
+                            }
+                        }
                     }
                 }
+                TypedHole { expected: ty, .. } => {
+                    *ty = table.resolve_completely(ty.clone());
+                }
+                _ => (),
             }
             true
         });

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

@@ -874,9 +874,15 @@ impl<'a> InferenceContext<'a> {
             },
             Expr::Underscore => {
                 // Underscore expressions may only appear in assignee expressions,
-                // which are handled by `infer_assignee_expr()`, so any underscore
-                // expression reaching this branch is an error.
-                self.err_ty()
+                // which are handled by `infer_assignee_expr()`.
+                // Any other underscore expression is an error, we render a specialized diagnostic
+                // to let the user know what type is expected though.
+                let expected = expected.to_option(&mut self.table).unwrap_or_else(|| self.err_ty());
+                self.push_diagnostic(InferenceDiagnostic::TypedHole {
+                    expr: tgt_expr,
+                    expected: expected.clone(),
+                });
+                expected
             }
         };
         // use a new type variable if we got unknown here
@@ -1001,12 +1007,13 @@ impl<'a> InferenceContext<'a> {
             }
             &Array::Repeat { initializer, repeat } => {
                 self.infer_expr_coerce(initializer, &Expectation::has_type(elem_ty.clone()));
-                self.infer_expr(
-                    repeat,
-                    &Expectation::HasType(
-                        TyKind::Scalar(Scalar::Uint(UintTy::Usize)).intern(Interner),
-                    ),
-                );
+                let usize = TyKind::Scalar(Scalar::Uint(UintTy::Usize)).intern(Interner);
+                match self.body[repeat] {
+                    Expr::Underscore => {
+                        self.write_expr_ty(repeat, usize);
+                    }
+                    _ => _ = self.infer_expr(repeat, &Expectation::HasType(usize)),
+                }
 
                 (
                     elem_ty,

crates/hir/src/diagnostics.rs

@@ -52,6 +52,7 @@ diagnostics![
     PrivateAssocItem,
     PrivateField,
     ReplaceFilterMapNextWithFindMap,
+    TypedHole,
     TypeMismatch,
     UndeclaredLabel,
     UnimplementedBuiltinMacro,
@@ -73,6 +74,12 @@ pub struct BreakOutsideOfLoop {
     pub bad_value_break: bool,
 }
 
+#[derive(Debug)]
+pub struct TypedHole {
+    pub expr: InFile<AstPtr<ast::Expr>>,
+    pub expected: Type,
+}
+
 #[derive(Debug)]
 pub struct UnresolvedModule {
     pub decl: InFile<AstPtr<ast::Module>>,

crates/hir/src/lib.rs

@@ -69,7 +69,8 @@ use hir_ty::{
     traits::FnTrait,
     AliasTy, CallableDefId, CallableSig, Canonical, CanonicalVarKinds, Cast, ClosureId,
     GenericArgData, Interner, ParamKind, QuantifiedWhereClause, Scalar, Substitution,
-    TraitEnvironment, TraitRefExt, Ty, TyBuilder, TyDefId, TyExt, TyKind, WhereClause,
+    TraitEnvironment, TraitRefExt, Ty, TyBuilder, TyDefId, TyExt, TyKind, ValueTyDefId,
+    WhereClause,
 };
 use itertools::Itertools;
 use nameres::diagnostics::DefDiagnosticKind;
@@ -91,10 +92,10 @@ pub use crate::{
         IncorrectCase, InvalidDeriveTarget, MacroDefError, MacroError, MacroExpansionParseError,
         MalformedDerive, MismatchedArgCount, MissingFields, MissingMatchArms, MissingUnsafe,
         MovedOutOfRef, NeedMut, NoSuchField, PrivateAssocItem, PrivateField,
-        ReplaceFilterMapNextWithFindMap, TypeMismatch, UndeclaredLabel, UnimplementedBuiltinMacro,
-        UnreachableLabel, UnresolvedExternCrate, UnresolvedField, UnresolvedImport,
-        UnresolvedMacroCall, UnresolvedMethodCall, UnresolvedModule, UnresolvedProcMacro,
-        UnusedMut,
+        ReplaceFilterMapNextWithFindMap, TypeMismatch, TypedHole, UndeclaredLabel,
+        UnimplementedBuiltinMacro, UnreachableLabel, UnresolvedExternCrate, UnresolvedField,
+        UnresolvedImport, UnresolvedMacroCall, UnresolvedMethodCall, UnresolvedModule,
+        UnresolvedProcMacro, UnusedMut,
     },
     has_source::HasSource,
     semantics::{PathResolution, Semantics, SemanticsScope, TypeInfo, VisibleTraits},
@@ -1005,6 +1006,10 @@ impl Struct {
         Type::from_def(db, self.id)
     }
 
+    pub fn constructor_ty(self, db: &dyn HirDatabase) -> Type {
+        Type::from_value_def(db, self.id)
+    }
+
     pub fn repr(self, db: &dyn HirDatabase) -> Option<ReprOptions> {
         db.struct_data(self.id).repr
     }
@@ -1042,6 +1047,10 @@ impl Union {
         Type::from_def(db, self.id)
     }
 
+    pub fn constructor_ty(self, db: &dyn HirDatabase) -> Type {
+        Type::from_value_def(db, self.id)
+    }
+
     pub fn fields(self, db: &dyn HirDatabase) -> Vec<Field> {
         db.union_data(self.id)
             .variant_data
@@ -1173,6 +1182,10 @@ impl Variant {
         self.parent
     }
 
+    pub fn constructor_ty(self, db: &dyn HirDatabase) -> Type {
+        Type::from_value_def(db, EnumVariantId { parent: self.parent.id, local_id: self.id })
+    }
+
     pub fn name(self, db: &dyn HirDatabase) -> Name {
         db.enum_data(self.parent.id).variants[self.id].name.clone()
     }
@@ -1574,6 +1587,16 @@ impl DefWithBody {
                     let expr = expr_syntax(expr);
                     acc.push(BreakOutsideOfLoop { expr, is_break, bad_value_break }.into())
                 }
+                hir_ty::InferenceDiagnostic::TypedHole { expr, expected } => {
+                    let expr = expr_syntax(*expr);
+                    acc.push(
+                        TypedHole {
+                            expr,
+                            expected: Type::new(db, DefWithBodyId::from(self), expected.clone()),
+                        }
+                        .into(),
+                    )
+                }
             }
         }
         for (pat_or_expr, mismatch) in infer.type_mismatches() {
@@ -1806,6 +1829,10 @@ impl Function {
         db.function_data(self.id).name.clone()
     }
 
+    pub fn ty(self, db: &dyn HirDatabase) -> Type {
+        Type::from_value_def(db, self.id)
+    }
+
     /// Get this function's return type
     pub fn ret_type(self, db: &dyn HirDatabase) -> Type {
         let resolver = self.id.resolver(db.upcast());
@@ -2085,11 +2112,7 @@ impl Const {
     }
 
     pub fn ty(self, db: &dyn HirDatabase) -> Type {
-        let data = db.const_data(self.id);
-        let resolver = self.id.resolver(db.upcast());
-        let ctx = hir_ty::TyLoweringContext::new(db, &resolver);
-        let ty = ctx.lower_ty(&data.type_ref);
-        Type::new_with_resolver_inner(db, &resolver, ty)
+        Type::from_value_def(db, self.id)
     }
 
     pub fn render_eval(self, db: &dyn HirDatabase) -> Result<String, ConstEvalError> {
@@ -2136,11 +2159,7 @@ impl Static {
     }
 
     pub fn ty(self, db: &dyn HirDatabase) -> Type {
-        let data = db.static_data(self.id);
-        let resolver = self.id.resolver(db.upcast());
-        let ctx = hir_ty::TyLoweringContext::new(db, &resolver);
-        let ty = ctx.lower_ty(&data.type_ref);
-        Type::new_with_resolver_inner(db, &resolver, ty)
+        Type::from_value_def(db, self.id)
     }
 }
 
@@ -3409,24 +3428,33 @@ impl Type {
         Type { env: environment, ty }
     }
 
-    fn from_def(db: &dyn HirDatabase, def: impl HasResolver + Into<TyDefId>) -> Type {
-        let ty_def = def.into();
-        let parent_subst = match ty_def {
-            TyDefId::TypeAliasId(id) => match id.lookup(db.upcast()).container {
-                ItemContainerId::TraitId(id) => {
-                    let subst = TyBuilder::subst_for_def(db, id, None).fill_with_unknown().build();
-                    Some(subst)
-                }
-                ItemContainerId::ImplId(id) => {
-                    let subst = TyBuilder::subst_for_def(db, id, None).fill_with_unknown().build();
-                    Some(subst)
-                }
-                _ => None,
+    fn from_def(db: &dyn HirDatabase, def: impl Into<TyDefId> + HasResolver) -> Type {
+        let ty = db.ty(def.into());
+        let substs = TyBuilder::unknown_subst(
+            db,
+            match def.into() {
+                TyDefId::AdtId(it) => GenericDefId::AdtId(it),
+                TyDefId::TypeAliasId(it) => GenericDefId::TypeAliasId(it),
+                TyDefId::BuiltinType(_) => return Type::new(db, def, ty.skip_binders().clone()),
             },
-            _ => None,
-        };
-        let ty = TyBuilder::def_ty(db, ty_def, parent_subst).fill_with_unknown().build();
-        Type::new(db, def, ty)
+        );
+        Type::new(db, def, ty.substitute(Interner, &substs))
+    }
+
+    fn from_value_def(db: &dyn HirDatabase, def: impl Into<ValueTyDefId> + HasResolver) -> Type {
+        let ty = db.value_ty(def.into());
+        let substs = TyBuilder::unknown_subst(
+            db,
+            match def.into() {
+                ValueTyDefId::ConstId(it) => GenericDefId::ConstId(it),
+                ValueTyDefId::FunctionId(it) => GenericDefId::FunctionId(it),
+                ValueTyDefId::StructId(it) => GenericDefId::AdtId(AdtId::StructId(it)),
+                ValueTyDefId::UnionId(it) => GenericDefId::AdtId(AdtId::UnionId(it)),
+                ValueTyDefId::EnumVariantId(it) => GenericDefId::EnumVariantId(it),
+                ValueTyDefId::StaticId(_) => return Type::new(db, def, ty.skip_binders().clone()),
+            },
+        );
+        Type::new(db, def, ty.substitute(Interner, &substs))
     }
 
     pub fn new_slice(ty: Type) -> Type {