Add a macro that derives TryFrom<u32> for fieldless enums

compiler/rustc_macros/src/lib.rs

@@ -20,6 +20,7 @@
 mod query;
 mod serialize;
 mod symbols;
+mod try_from;
 mod type_foldable;
 mod type_visitable;
 
@@ -165,3 +166,12 @@ pub fn extension(attr: TokenStream, input: TokenStream) -> TokenStream {
         suggestion_part,
         applicability)] => diagnostics::subdiagnostic_derive
 );
+
+decl_derive! {
+    [TryFromU32] =>
+    /// Derives `TryFrom<u32>` for the annotated `enum`, which must have no fields.
+    /// Each variant maps to the value it would produce under an `as u32` cast.
+    ///
+    /// The error type is `u32`.
+    try_from::try_from_u32
+}

compiler/rustc_macros/src/try_from.rs

@@ -0,0 +1,53 @@
+use proc_macro2::TokenStream;
+use quote::{quote, quote_spanned};
+use syn::Data;
+use syn::spanned::Spanned;
+use synstructure::Structure;
+
+pub(crate) fn try_from_u32(s: Structure<'_>) -> TokenStream {
+    let span_error = |span, message: &str| {
+        quote_spanned! { span => const _: () = ::core::compile_error!(#message); }
+    };
+
+    // Must be applied to an enum type.
+    if let Some(span) = match &s.ast().data {
+        Data::Enum(_) => None,
+        Data::Struct(s) => Some(s.struct_token.span()),
+        Data::Union(u) => Some(u.union_token.span()),
+    } {
+        return span_error(span, "type is not an enum (TryFromU32)");
+    }
+
+    // The enum's variants must not have fields.
+    let variant_field_errors = s
+        .variants()
+        .iter()
+        .filter_map(|v| v.ast().fields.iter().map(|f| f.span()).next())
+        .map(|span| span_error(span, "enum variant cannot have fields (TryFromU32)"))
+        .collect::<TokenStream>();
+    if !variant_field_errors.is_empty() {
+        return variant_field_errors;
+    }
+
+    let ctor = s
+        .variants()
+        .iter()
+        .map(|v| v.construct(|_, _| -> TokenStream { unreachable!() }))
+        .collect::<Vec<_>>();
+    s.gen_impl(quote! {
+        // The surrounding code might have shadowed these identifiers.
+        use ::core::convert::TryFrom;
+        use ::core::primitive::u32;
+        use ::core::result::Result::{self, Ok, Err};
+
+        gen impl TryFrom<u32> for @Self {
+            type Error = u32;
+
+            #[allow(deprecated)] // Don't warn about deprecated variants.
+            fn try_from(value: u32) -> Result<Self, Self::Error> {
+                #( if value == const { #ctor as u32 } { return Ok(#ctor) } )*
+                Err(value)
+            }
+        }
+    })
+}

tests/ui-fulldeps/try-from-u32/errors.rs

@@ -0,0 +1,24 @@
+#![feature(rustc_private)]
+//@ edition: 2021
+
+// Checks the error messages produced by `#[derive(TryFromU32)]`.
+
+extern crate rustc_macros;
+
+use rustc_macros::TryFromU32;
+
+#[derive(TryFromU32)]
+struct MyStruct {} //~ type is not an enum
+
+#[derive(TryFromU32)]
+enum NonTrivial {
+    A,
+    B(),
+    C {},
+    D(bool),                //~ enum variant cannot have fields
+    E(bool, bool),          //~ enum variant cannot have fields
+    F { x: bool },          //~ enum variant cannot have fields
+    G { x: bool, y: bool }, //~ enum variant cannot have fields
+}
+
+fn main() {}

tests/ui-fulldeps/try-from-u32/errors.stderr

@@ -0,0 +1,32 @@
+error: type is not an enum (TryFromU32)
+  --> $DIR/errors.rs:11:1
+   |
+LL | struct MyStruct {}
+   | ^^^^^^
+
+error: enum variant cannot have fields (TryFromU32)
+  --> $DIR/errors.rs:18:7
+   |
+LL |     D(bool),
+   |       ^^^^
+
+error: enum variant cannot have fields (TryFromU32)
+  --> $DIR/errors.rs:19:7
+   |
+LL |     E(bool, bool),
+   |       ^^^^
+
+error: enum variant cannot have fields (TryFromU32)
+  --> $DIR/errors.rs:20:9
+   |
+LL |     F { x: bool },
+   |         ^
+
+error: enum variant cannot have fields (TryFromU32)
+  --> $DIR/errors.rs:21:9
+   |
+LL |     G { x: bool, y: bool },
+   |         ^
+
+error: aborting due to 5 previous errors
+

tests/ui-fulldeps/try-from-u32/hygiene.rs

@@ -0,0 +1,32 @@
+#![feature(rustc_private)]
+//@ edition: 2021
+//@ check-pass
+
+// Checks that the derive macro still works even if the surrounding code has
+// shadowed the relevant library types.
+
+extern crate rustc_macros;
+
+mod submod {
+    use rustc_macros::TryFromU32;
+
+    struct Result;
+    trait TryFrom {}
+    #[allow(non_camel_case_types)]
+    struct u32;
+    struct Ok;
+    struct Err;
+    mod core {}
+    mod std {}
+
+    #[derive(TryFromU32)]
+    pub(crate) enum MyEnum {
+        Zero,
+        One,
+    }
+}
+
+fn main() {
+    use submod::MyEnum;
+    let _: Result<MyEnum, u32> = MyEnum::try_from(1u32);
+}

tests/ui-fulldeps/try-from-u32/values.rs

@@ -0,0 +1,36 @@
+#![feature(assert_matches)]
+#![feature(rustc_private)]
+//@ edition: 2021
+//@ run-pass
+
+// Checks the values accepted by the `TryFrom<u32>` impl produced by `#[derive(TryFromU32)]`.
+
+extern crate rustc_macros;
+
+use core::assert_matches::assert_matches;
+use rustc_macros::TryFromU32;
+
+#[derive(TryFromU32, Debug, PartialEq)]
+#[repr(u32)]
+enum Repr {
+    Zero,
+    One(),
+    Seven = 7,
+}
+
+#[derive(TryFromU32, Debug)]
+enum NoRepr {
+    Zero,
+    One,
+}
+
+fn main() {
+    assert_eq!(Repr::try_from(0u32), Ok(Repr::Zero));
+    assert_eq!(Repr::try_from(1u32), Ok(Repr::One()));
+    assert_eq!(Repr::try_from(2u32), Err(2));
+    assert_eq!(Repr::try_from(7u32), Ok(Repr::Seven));
+
+    assert_matches!(NoRepr::try_from(0u32), Ok(NoRepr::Zero));
+    assert_matches!(NoRepr::try_from(1u32), Ok(NoRepr::One));
+    assert_matches!(NoRepr::try_from(2u32), Err(2));
+}