Implement #[derive(From)]

compiler/rustc_builtin_macros/messages.ftl

@@ -222,6 +222,8 @@ builtin_macros_format_unused_args = multiple unused formatting arguments
 
 builtin_macros_format_use_positional = consider using a positional formatting argument instead
 
+builtin_macros_from_wrong_target = `#[derive(From)]` can only be used on structs with a single field
+
 builtin_macros_multiple_default_attrs = multiple `#[default]` attributes
     .note = only one `#[default]` attribute is needed
     .label = `#[default]` used here

compiler/rustc_builtin_macros/src/deriving/from.rs

@@ -0,0 +1,121 @@
+use rustc_ast as ast;
+use rustc_ast::ptr::P;
+use rustc_ast::{FieldDef, Item, ItemKind, VariantData};
+use rustc_expand::base::{Annotatable, ExtCtxt};
+use rustc_span::{Ident, Span, kw, sym};
+use thin_vec::thin_vec;
+
+use crate::deriving::generic::ty::{Bounds, Path, PathKind, Ty};
+use crate::deriving::generic::{
+    BlockOrExpr, FieldlessVariantsStrategy, MethodDef, SubstructureFields, TraitDef,
+    combine_substructure,
+};
+use crate::deriving::pathvec_std;
+use crate::errors;
+
+/// Generate an implementation of the `From` trait, provided that `item`
+/// is a struct or a tuple struct with exactly one field.
+pub(crate) fn expand_deriving_from(
+    cx: &ExtCtxt<'_>,
+    span: Span,
+    mitem: &ast::MetaItem,
+    item: &Annotatable,
+    push: &mut dyn FnMut(Annotatable),
+    is_const: bool,
+) {
+    let mut visitor = ExtractNonSingleFieldStruct { cx, field: None };
+    item.visit_with(&mut visitor);
+
+    // Make sure that the derive is only invoked on single-field [tuple] structs.
+    // From this point below, we know that there is exactly one field.
+    let Some(field) = visitor.field else { return };
+
+    let path = Path::new_(
+        pathvec_std!(convert::From),
+        vec![Box::new(Ty::AstTy(field.ty.clone()))],
+        PathKind::Std,
+    );
+
+    // Generate code like this:
+    //
+    // struct S(u32);
+    // #[automatically_derived]
+    // impl ::core::convert::From<u32> for S {
+    //     #[inline]
+    //     fn from(value: u32) -> S {
+    //         Self(value)
+    //     }
+    // }
+    let from_trait_def = TraitDef {
+        span,
+        path,
+        skip_path_as_bound: true,
+        needs_copy_as_bound_if_packed: false,
+        additional_bounds: Vec::new(),
+        supports_unions: false,
+        methods: vec![MethodDef {
+            name: sym::from,
+            generics: Bounds { bounds: vec![] },
+            explicit_self: false,
+            nonself_args: vec![(Ty::AstTy(field.ty), sym::value)],
+            ret_ty: Ty::Self_,
+            attributes: thin_vec![cx.attr_word(sym::inline, span)],
+            fieldless_variants_strategy: FieldlessVariantsStrategy::Default,
+            combine_substructure: combine_substructure(Box::new(|cx, span, substructure| {
+                let self_kw = Ident::new(kw::SelfUpper, span);
+                let expr: P<ast::Expr> = match substructure.fields {
+                    SubstructureFields::StaticStruct(variant, _) => match variant {
+                        // Self {
+                        //     field: value
+                        // }
+                        VariantData::Struct { .. } => cx.expr_struct_ident(
+                            span,
+                            self_kw,
+                            thin_vec![cx.field_imm(
+                                span,
+                                field.ident.unwrap(),
+                                cx.expr_ident(span, Ident::new(sym::value, span))
+                            )],
+                        ),
+                        // Self(value)
+                        VariantData::Tuple(_, _) => cx.expr_call_ident(
+                            span,
+                            self_kw,
+                            thin_vec![cx.expr_ident(span, Ident::new(sym::value, span))],
+                        ),
+                        variant => {
+                            cx.dcx().bug(format!("Invalid derive(From) ADT variant: {variant:?}"));
+                        }
+                    },
+                    _ => cx.dcx().bug("Invalid derive(From) ADT input"),
+                };
+                BlockOrExpr::new_expr(expr)
+            })),
+        }],
+        associated_types: Vec::new(),
+        is_const,
+        is_staged_api_crate: cx.ecfg.features.staged_api(),
+    };
+
+    from_trait_def.expand(cx, mitem, item, push);
+}
+
+struct ExtractNonSingleFieldStruct<'a, 'b> {
+    cx: &'a ExtCtxt<'b>,
+    field: Option<FieldDef>,
+}
+
+impl<'a, 'b> rustc_ast::visit::Visitor<'a> for ExtractNonSingleFieldStruct<'a, 'b> {
+    fn visit_item(&mut self, item: &'a Item) -> Self::Result {
+        match &item.kind {
+            ItemKind::Struct(_, _, data) => match data.fields() {
+                [field] => self.field = Some(field.clone()),
+                _ => {}
+            },
+            _ => {}
+        };
+        if self.field.is_none() {
+            self.cx.dcx().emit_err(errors::DeriveFromWrongTarget { span: item.span });
+        }
+    }
+}

compiler/rustc_builtin_macros/src/deriving/generic/ty.rs

@@ -3,7 +3,7 @@
 
 pub(crate) use Ty::*;
 use rustc_ast::ptr::P;
-use rustc_ast::{self as ast, Expr, GenericArg, GenericParamKind, Generics, SelfKind};
+use rustc_ast::{self as ast, Expr, GenericArg, GenericParamKind, Generics, SelfKind, TyKind};
 use rustc_expand::base::ExtCtxt;
 use rustc_span::source_map::respan;
 use rustc_span::{DUMMY_SP, Ident, Span, Symbol, kw};
@@ -66,7 +66,7 @@ impl Path {
     }
 }
 
-/// A type. Supports pointers, Self, and literals.
+/// A type. Supports pointers, Self, literals, unit or an arbitrary AST path.
 #[derive(Clone)]
 pub(crate) enum Ty {
     Self_,
@@ -77,6 +77,8 @@ pub(crate) enum Ty {
     Path(Path),
     /// For () return types.
     Unit,
+    /// An arbitrary type.
+    AstTy(P<ast::Ty>),
 }
 
 pub(crate) fn self_ref() -> Ty {
@@ -102,6 +104,7 @@ impl Ty {
                 let ty = ast::TyKind::Tup(ThinVec::new());
                 cx.ty(span, ty)
             }
+            AstTy(ty) => ty.clone(),
         }
     }
 
@@ -133,6 +136,10 @@ impl Ty {
                 cx.path_all(span, false, vec![self_ty], params)
             }
             Path(p) => p.to_path(cx, span, self_ty, generics),
+            AstTy(ty) => match &ty.kind {
+                TyKind::Path(_, path) => path.clone(),
+                _ => cx.dcx().span_bug(span, "non-path in a path in generic `derive`"),
+            },
             Ref(..) => cx.dcx().span_bug(span, "ref in a path in generic `derive`"),
             Unit => cx.dcx().span_bug(span, "unit in a path in generic `derive`"),
         }

compiler/rustc_builtin_macros/src/deriving/mod.rs

@@ -24,6 +24,7 @@ pub(crate) mod clone;
 pub(crate) mod coerce_pointee;
 pub(crate) mod debug;
 pub(crate) mod default;
+pub(crate) mod from;
 pub(crate) mod hash;
 
 #[path = "cmp/eq.rs"]

compiler/rustc_builtin_macros/src/errors.rs

@@ -446,6 +446,13 @@ pub(crate) struct DefaultHasArg {
     pub(crate) span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(builtin_macros_from_wrong_target)]
+pub(crate) struct DeriveFromWrongTarget {
+    #[primary_span]
+    pub(crate) span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(builtin_macros_derive_macro_call)]
 pub(crate) struct DeriveMacroCall {

compiler/rustc_builtin_macros/src/lib.rs

@@ -139,6 +139,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
         PartialEq: partial_eq::expand_deriving_partial_eq,
         PartialOrd: partial_ord::expand_deriving_partial_ord,
         CoercePointee: coerce_pointee::expand_deriving_coerce_pointee,
+        From: from::expand_deriving_from,
     }
 
     let client = rustc_proc_macro::bridge::client::Client::expand1(rustc_proc_macro::quote);

compiler/rustc_feature/src/unstable.rs

@@ -471,6 +471,8 @@ declare_features! (
     (unstable, deprecated_suggestion, "1.61.0", Some(94785)),
     /// Allows deref patterns.
     (incomplete, deref_patterns, "1.79.0", Some(87121)),
+    /// Allows deriving the From trait on single-field structs.
+    (unstable, derive_from, "CURRENT_RUSTC_VERSION", Some(144889)),
     /// Tells rustdoc to automatically generate `#[doc(cfg(...))]`.
     (unstable, doc_auto_cfg, "1.58.0", Some(43781)),
     /// Allows `#[doc(cfg(...))]`.

compiler/rustc_span/src/symbol.rs

@@ -390,6 +390,7 @@ symbols! {
         __D,
         __H,
         __S,
+        __T,
         __awaitee,
         __try_var,
         _t,
@@ -741,6 +742,7 @@ symbols! {
         contracts_ensures,
         contracts_internals,
         contracts_requires,
+        convert,
         convert_identity,
         copy,
         copy_closures,
@@ -842,6 +844,7 @@ symbols! {
         derive_const,
         derive_const_issue: "118304",
         derive_default_enum,
+        derive_from,
         derive_smart_pointer,
         destruct,
         destructuring_assignment,
@@ -2320,6 +2323,7 @@ symbols! {
         va_start,
         val,
         validity,
+        value,
         values,
         var,
         variant_count,

library/core/src/macros/mod.rs

@@ -1767,4 +1767,15 @@ pub(crate) mod builtin {
     pub macro deref($pat:pat) {
         builtin # deref($pat)
     }
+
+    /// Derive macro generating an impl of the trait `From`.
+    /// Currently, it can only be used on single-field structs.
+    // Note that the macro is in a different module than the `From` trait,
+    // to avoid triggering an unstable feature being used if someone imports
+    // `std::convert::From`.
+    #[rustc_builtin_macro]
+    #[unstable(feature = "derive_from", issue = "144889")]
+    pub macro From($item: item) {
+        /* compiler built-in */
+    }
 }

library/core/src/prelude/v1.rs

@@ -117,3 +117,10 @@ pub use crate::macros::builtin::deref;
     reason = "`type_alias_impl_trait` has open design concerns"
 )]
 pub use crate::macros::builtin::define_opaque;
+
+#[unstable(
+    feature = "derive_from",
+    issue = "144889",
+    reason = "`derive(From)` is unstable"
+)]
+pub use crate::macros::builtin::From;

tests/ui/deriving/deriving-from-wrong-target.rs

@@ -0,0 +1,52 @@
+//@ edition: 2021
+//@ check-fail
+
+#![feature(derive_from)]
+#![allow(dead_code)]
+
+#[derive(From)]
+struct S1;
+//~^ ERROR `#[derive(From)]` can only be used on structs with a single field
+
+#[derive(From)]
+struct S2 {}
+//~^ ERROR `#[derive(From)]` can only be used on structs with a single field
+
+#[derive(From)]
+struct S3(u32, bool);
+//~^ ERROR `#[derive(From)]` can only be used on structs with a single field
+
+#[derive(From)]
+//~v ERROR `#[derive(From)]` can only be used on structs with a single field
+struct S4 {
+    a: u32,
+    b: bool,
+}
+
+#[derive(From)]
+enum E1 {}
+//~^ ERROR `#[derive(From)]` can only be used on structs with a single field
+
+#[derive(From)]
+//~v ERROR `#[derive(From)]` can only be used on structs with a single field
+enum E2 {
+    V1,
+    V2,
+}
+
+#[derive(From)]
+//~v ERROR `#[derive(From)]` can only be used on structs with a single field
+enum E3 {
+    V1(u32),
+    V2(bool),
+}
+
+#[derive(From)]
+//~^ ERROR the size for values of type `T` cannot be known at compilation time [E0277]
+//~| ERROR the size for values of type `T` cannot be known at compilation time [E0277]
+struct SUnsizedField<T: ?Sized> {
+    last: T,
+    //~^ ERROR the size for values of type `T` cannot be known at compilation time [E0277]
+}
+
+fn main() {}