Implement `#[derive(From)]`

4 files changed, 158 insertions, 5 deletions

diff --git a/compiler/rustc_builtin_macros/messages.ftl b/compiler/rustc_builtin_macros/messages.ftl
index eb3c40cc593..358c0d3db46 100644
--- a/compiler/rustc_builtin_macros/messages.ftl
+++ b/compiler/rustc_builtin_macros/messages.ftl
@@ -222,6 +222,15 @@ builtin_macros_format_unused_args = multiple unused formatting arguments
 
 builtin_macros_format_use_positional = consider using a positional formatting argument instead
 
+builtin_macros_derive_from_wrong_target = `#[derive(From)]` used on {$kind}
+
+builtin_macros_derive_from_wrong_field_count = `#[derive(From)]` used on a struct with {$multiple_fields ->
+    [true] multiple fields
+    *[false] no fields
+}
+
+builtin_macros_derive_from_usage_note = `#[derive(From)]` can only be used on structs with exactly one field
+
 builtin_macros_multiple_default_attrs = multiple `#[default]` attributes
     .note = only one `#[default]` attribute is needed
     .label = `#[default]` used here
diff --git a/compiler/rustc_builtin_macros/src/deriving/from.rs b/compiler/rustc_builtin_macros/src/deriving/from.rs
index 79fd4caa440..ef0e6ca324a 100644
--- a/compiler/rustc_builtin_macros/src/deriving/from.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/from.rs
@@ -1,13 +1,132 @@
 use rustc_ast as ast;
-use rustc_expand::base::{Annotatable, ExtCtxt};
-use rustc_span::{Span, sym};
+use rustc_ast::{ItemKind, VariantData};
+use rustc_errors::MultiSpan;
+use rustc_expand::base::{Annotatable, DummyResult, 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,
+    annotatable: &Annotatable,
     push: &mut dyn FnMut(Annotatable),
     is_const: bool,
 ) {
+    let Annotatable::Item(item) = &annotatable else {
+        cx.dcx().bug("derive(From) used on something else than an item");
+    };
+
+    // #[derive(From)] is currently usable only on structs with exactly one field.
+    let field = if let ItemKind::Struct(_, _, data) = &item.kind
+        && let [field] = data.fields()
+    {
+        Some(field.clone())
+    } else {
+        None
+    };
+
+    let from_type = match &field {
+        Some(field) => Ty::AstTy(field.ty.clone()),
+        // We don't have a type to put into From<...> if we don't have a single field, so just put
+        // unit there.
+        None => Ty::Unit,
+    };
+    let path =
+        Path::new_(pathvec_std!(convert::From), vec![Box::new(from_type.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![(from_type, 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 Some(field) = &field else {
+                    let item_span = item.kind.ident().map(|ident| ident.span).unwrap_or(item.span);
+                    let err_span = MultiSpan::from_spans(vec![span, item_span]);
+                    let error = match &item.kind {
+                        ItemKind::Struct(_, _, data) => {
+                            cx.dcx().emit_err(errors::DeriveFromWrongFieldCount {
+                                span: err_span,
+                                multiple_fields: data.fields().len() > 1,
+                            })
+                        }
+                        ItemKind::Enum(_, _, _) | ItemKind::Union(_, _, _) => {
+                            cx.dcx().emit_err(errors::DeriveFromWrongTarget {
+                                span: err_span,
+                                kind: &format!("{} {}", item.kind.article(), item.kind.descr()),
+                            })
+                        }
+                        _ => cx.dcx().bug("Invalid derive(From) ADT input"),
+                    };
+
+                    return BlockOrExpr::new_expr(DummyResult::raw_expr(span, Some(error)));
+                };
+
+                let self_kw = Ident::new(kw::SelfUpper, span);
+                let expr: Box<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, annotatable, push);
 }
diff --git a/compiler/rustc_builtin_macros/src/deriving/generic/ty.rs b/compiler/rustc_builtin_macros/src/deriving/generic/ty.rs
index 00e70b21cf4..1458553d492 100644
--- a/compiler/rustc_builtin_macros/src/deriving/generic/ty.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/generic/ty.rs
@@ -2,7 +2,7 @@
 //! when specifying impls to be derived.
 
 pub(crate) use Ty::*;
-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};
@@ -65,7 +65,7 @@ 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_,
@@ -76,6 +76,8 @@ pub(crate) enum Ty {
     Path(Path),
     /// For () return types.
     Unit,
+    /// An arbitrary type.
+    AstTy(Box<ast::Ty>),
 }
 
 pub(crate) fn self_ref() -> Ty {
@@ -101,6 +103,7 @@ impl Ty {
                 let ty = ast::TyKind::Tup(ThinVec::new());
                 cx.ty(span, ty)
             }
+            AstTy(ty) => ty.clone(),
         }
     }
 
@@ -132,6 +135,10 @@ 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`"),
         }
diff --git a/compiler/rustc_builtin_macros/src/errors.rs b/compiler/rustc_builtin_macros/src/errors.rs
index bb520db75b9..54e8f750337 100644
--- a/compiler/rustc_builtin_macros/src/errors.rs
+++ b/compiler/rustc_builtin_macros/src/errors.rs
@@ -447,6 +447,24 @@ pub(crate) struct DefaultHasArg {
 }
 
 #[derive(Diagnostic)]
+#[diag(builtin_macros_derive_from_wrong_target)]
+#[note(builtin_macros_derive_from_usage_note)]
+pub(crate) struct DeriveFromWrongTarget<'a> {
+    #[primary_span]
+    pub(crate) span: MultiSpan,
+    pub(crate) kind: &'a str,
+}
+
+#[derive(Diagnostic)]
+#[diag(builtin_macros_derive_from_wrong_field_count)]
+#[note(builtin_macros_derive_from_usage_note)]
+pub(crate) struct DeriveFromWrongFieldCount {
+    #[primary_span]
+    pub(crate) span: MultiSpan,
+    pub(crate) multiple_fields: bool,
+}
+
+#[derive(Diagnostic)]
 #[diag(builtin_macros_derive_macro_call)]
 pub(crate) struct DeriveMacroCall {
     #[primary_span]