Make specifying repr optional for fieldless enums

Author: fee1-dead

compiler/rustc_error_codes/src/error_codes/E0732.md

@@ -4,16 +4,19 @@ Erroneous code example:
 
 ```compile_fail,E0732
 enum Enum { // error!
-    Unit = 1,
-    Tuple() = 2,
-    Struct{} = 3,
+    Unit = 3,
+    Tuple(u16) = 2,
+    Struct {
+        a: u8,
+        b: u16,
+    } = 1,
 }
 # fn main() {}
 ```
 
-A `#[repr(inttype)]` must be provided on an `enum` if it has a non-unit
-variant with a discriminant, or where there are both unit variants with
-discriminants and non-unit variants. This restriction ensures that there
+A `#[repr(inttype)]` must be provided on an `enum` if it not fieldless and
+has a discriminant, or where there are both fieldless variants with
+discriminants and variants that have fields. This restriction ensures that there
 is a well-defined way to extract a variant's discriminant from a value;
 for instance:
 

compiler/rustc_typeck/src/check/check.rs

@@ -1384,12 +1384,17 @@ fn check_enum<'tcx>(
     }
 
     if tcx.adt_def(def_id).repr.int.is_none() {
-        let is_unit = |var: &hir::Variant<'_>| matches!(var.data, hir::VariantData::Unit(..));
+        let is_fieldless = |var: &hir::Variant<'_>| match var.data {
+            hir::VariantData::Unit(_) => true,
+            hir::VariantData::Tuple(fields, _) | hir::VariantData::Struct(fields, _) => {
+                fields.is_empty()
+            }
+        };
 
         let has_disr = |var: &hir::Variant<'_>| var.disr_expr.is_some();
-        let has_non_units = vs.iter().any(|var| !is_unit(var));
-        let disr_units = vs.iter().any(|var| is_unit(&var) && has_disr(&var));
-        let disr_non_unit = vs.iter().any(|var| !is_unit(&var) && has_disr(&var));
+        let has_non_units = vs.iter().any(|var| !is_fieldless(var));
+        let disr_units = vs.iter().any(|var| is_fieldless(&var) && has_disr(&var));
+        let disr_non_unit = vs.iter().any(|var| !is_fieldless(&var) && has_disr(&var));
 
         if disr_non_unit || (disr_units && has_non_units) {
             let mut err =

src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.rs

@@ -1,8 +1,9 @@
-#![crate_type="lib"]
-
+#![crate_type = "lib"]
 enum Enum {
-//~^ ERROR `#[repr(inttype)]` must be specified
-  Unit = 1,
-  Tuple() = 2,
-  Struct{} = 3,
+    //~^ ERROR `#[repr(inttype)]` must be specified
+    Unit = 5,
+    Tuple(u8) = 3,
+    Struct {
+        foo: u16
+    } = 1,
 }

src/test/ui/enum-discriminant/arbitrary_enum_discriminant-no-repr.stderr

@@ -1,11 +1,12 @@
 error[E0732]: `#[repr(inttype)]` must be specified
-  --> $DIR/arbitrary_enum_discriminant-no-repr.rs:3:1
+  --> $DIR/arbitrary_enum_discriminant-no-repr.rs:2:1
    |
 LL | / enum Enum {
 LL | |
-LL | |   Unit = 1,
-LL | |   Tuple() = 2,
-LL | |   Struct{} = 3,
+LL | |     Unit = 5,
+LL | |     Tuple(u8) = 3,
+...  |
+LL | |     } = 1,
 LL | | }
    | |_^
 

src/test/ui/enum-discriminant/fieldless-no-repr.rs

@@ -0,0 +1,8 @@
+// check-pass
+#![crate_type="lib"]
+
+enum Enum {
+  Unit = 1,
+  Tuple() = 2,
+  Struct{} = 3,
+}