Suggest using :: instead of . for enums in some cases.

Suggest replacing `.` with `::` when encountering "expected value, found enum":
- in a method-call expression and the method has the same name as a tuple variant
- in a field-access expression and the field has the same name as a unit or tuple variant

Author: zachs18

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -8,7 +8,7 @@ use rustc_ast::ptr::P;
 use rustc_ast::visit::{FnCtxt, FnKind, LifetimeCtxt, Visitor, walk_ty};
 use rustc_ast::{
     self as ast, AssocItemKind, DUMMY_NODE_ID, Expr, ExprKind, GenericParam, GenericParamKind,
-    Item, ItemKind, MethodCall, NodeId, Path, Ty, TyKind,
+    Item, ItemKind, MethodCall, NodeId, Path, PathSegment, Ty, TyKind,
 };
 use rustc_ast_pretty::pprust::where_bound_predicate_to_string;
 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
@@ -2469,31 +2469,73 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         def_id: DefId,
         span: Span,
     ) {
-        let Some(variants) = self.collect_enum_ctors(def_id) else {
+        let Some(variant_ctors) = self.collect_enum_ctors(def_id) else {
             err.note("you might have meant to use one of the enum's variants");
             return;
         };
 
-        let suggest_only_tuple_variants =
-            matches!(source, PathSource::TupleStruct(..)) || source.is_call();
-        if suggest_only_tuple_variants {
+        // If the expression is a field-access or method-call, try to find a variant with the field/method name
+        // that could have been intended, and suggest replacing the `.` with `::`.
+        // Otherwise, suggest adding `::VariantName` after the enum;
+        // and if the expression is call-like, only suggest tuple variants.
+        let (suggest_path_sep_dot_span, suggest_only_tuple_variants) = match source {
+            // `Type(a, b)` in a pattern, only suggest adding a tuple variant after `Type`.
+            PathSource::TupleStruct(..) => (None, true),
+            PathSource::Expr(Some(expr)) => match &expr.kind {
+                // `Type(a, b)`, only suggest adding a tuple variant after `Type`.
+                ExprKind::Call(..) => (None, true),
+                // `Type.Foo(a, b)`, suggest replacing `.` -> `::` if variant `Foo` exists and is a tuple variant,
+                // otherwise suggest adding a variant after `Type`.
+                ExprKind::MethodCall(box MethodCall {
+                    receiver,
+                    span,
+                    seg: PathSegment { ident, .. },
+                    ..
+                }) => {
+                    let dot_span = receiver.span.between(*span);
+                    let found_tuple_variant = variant_ctors.iter().any(|(path, _, ctor_kind)| {
+                        *ctor_kind == CtorKind::Fn
+                            && path.segments.last().is_some_and(|seg| seg.ident == *ident)
+                    });
+                    (found_tuple_variant.then_some(dot_span), false)
+                }
+                // `Type.Foo`, suggest replacing `.` -> `::` if variant `Foo` exists and is a unit or tuple variant,
+                // otherwise suggest adding a variant after `Type`.
+                ExprKind::Field(base, ident) => {
+                    let dot_span = base.span.between(ident.span);
+                    let found_tuple_or_unit_variant = variant_ctors.iter().any(|(path, ..)| {
+                        path.segments.last().is_some_and(|seg| seg.ident == *ident)
+                    });
+                    (found_tuple_or_unit_variant.then_some(dot_span), false)
+                }
+                _ => (None, false),
+            },
+            _ => (None, false),
+        };
+
+        if let Some(dot_span) = suggest_path_sep_dot_span {
+            err.span_suggestion_verbose(
+                dot_span,
+                "use the path separator to refer to a variant",
+                "::",
+                Applicability::MaybeIncorrect,
+            );
+        } else if suggest_only_tuple_variants {
             // Suggest only tuple variants regardless of whether they have fields and do not
             // suggest path with added parentheses.
-            let mut suggestable_variants = variants
+            let mut suggestable_variants = variant_ctors
                 .iter()
                 .filter(|(.., kind)| *kind == CtorKind::Fn)
                 .map(|(variant, ..)| path_names_to_string(variant))
                 .collect::<Vec<_>>();
             suggestable_variants.sort();
 
-            let non_suggestable_variant_count = variants.len() - suggestable_variants.len();
+            let non_suggestable_variant_count = variant_ctors.len() - suggestable_variants.len();
 
-            let source_msg = if source.is_call() {
-                "to construct"
-            } else if matches!(source, PathSource::TupleStruct(..)) {
+            let source_msg = if matches!(source, PathSource::TupleStruct(..)) {
                 "to match against"
             } else {
-                unreachable!()
+                "to construct"
             };
 
             if !suggestable_variants.is_empty() {
@@ -2512,7 +2554,7 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
             }
 
             // If the enum has no tuple variants..
-            if non_suggestable_variant_count == variants.len() {
+            if non_suggestable_variant_count == variant_ctors.len() {
                 err.help(format!("the enum has no tuple variants {source_msg}"));
             }
 
@@ -2535,7 +2577,7 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                 }
             };
 
-            let mut suggestable_variants = variants
+            let mut suggestable_variants = variant_ctors
                 .iter()
                 .filter(|(_, def_id, kind)| !needs_placeholder(*def_id, *kind))
                 .map(|(variant, _, kind)| (path_names_to_string(variant), kind))
@@ -2562,7 +2604,7 @@ impl<'ast, 'ra: 'ast, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                 );
             }
 
-            let mut suggestable_variants_with_placeholders = variants
+            let mut suggestable_variants_with_placeholders = variant_ctors
                 .iter()
                 .filter(|(_, def_id, kind)| needs_placeholder(*def_id, *kind))
                 .map(|(variant, _, kind)| (path_names_to_string(variant), kind))

tests/ui/resolve/enum-expected-value-suggest-variants.rs

@@ -0,0 +1,58 @@
+enum Foo {
+    //~^ HELP consider importing this tuple variant
+    A(u32),
+    B(u32),
+}
+
+enum Bar {
+    C(u32),
+    D(u32),
+    E,
+    F,
+}
+
+fn main() {
+    let _: Foo = Foo(0);
+    //~^ ERROR expected function
+    //~| HELP try to construct one of the enum's variants
+
+    let _: Foo = Foo.A(0);
+    //~^ ERROR expected value, found enum `Foo`
+    //~| HELP use the path separator to refer to a variant
+
+    let _: Foo = Foo.Bad(0);
+    //~^ ERROR expected value, found enum `Foo`
+    //~| HELP the following enum variants are available
+
+    let _: Bar = Bar(0);
+    //~^ ERROR expected function
+    //~| HELP try to construct one of the enum's variants
+    //~| HELP you might have meant to construct one of the enum's non-tuple variants
+
+    let _: Bar = Bar.C(0);
+    //~^ ERROR expected value, found enum `Bar`
+    //~| HELP use the path separator to refer to a variant
+
+    let _: Bar = Bar.E;
+    //~^ ERROR expected value, found enum `Bar`
+    //~| HELP use the path separator to refer to a variant
+
+    let _: Bar = Bar.Bad(0);
+    //~^ ERROR expected value, found enum `Bar`
+    //~| HELP you might have meant to use one of the following enum variants
+    //~| HELP alternatively, the following enum variants are also available
+
+    let _: Bar = Bar.Bad;
+    //~^ ERROR expected value, found enum `Bar`
+    //~| HELP you might have meant to use one of the following enum variants
+    //~| HELP alternatively, the following enum variants are also available
+
+    match Foo::A(42) {
+        A(..) => {}
+        //~^ ERROR cannot find tuple struct or tuple variant `A` in this scope
+        Foo(..) => {}
+        //~^ ERROR expected tuple struct or tuple variant
+        //~| HELP try to match against one of the enum's variants
+        _ => {}
+    }
+}