Add typed Decodal derive support

crates/decodal-derive/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "decodal-derive"
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+repository.workspace = true
+readme.workspace = true
+description = "Derive macro for generating Decodal schemas and typed decoders from Rust structs."
+keywords = ["decodal", "derive", "proc-macro", "schema"]
+categories = ["config", "development-tools::procedural-macro-helpers"]
+
+[lib]
+proc-macro = true
+
+[dependencies]
+proc-macro2 = "1"
+quote = "1"
+syn = { version = "2", features = ["full"] }
+
+[dev-dependencies]
+decodal = { version = "0.1", path = "../decodal-core" }

crates/decodal-derive/src/lib.rs

@@ -0,0 +1,212 @@
+use proc_macro::TokenStream;
+use proc_macro2::TokenStream as TokenStream2;
+use quote::{format_ident, quote};
+use syn::{
+    Data, DeriveInput, Expr, Fields, LitStr, Result, Type, parse_macro_input, spanned::Spanned,
+};
+
+#[proc_macro_derive(Decodal, attributes(decodal))]
+pub fn derive_decodal(input: TokenStream) -> TokenStream {
+    match expand_decodal(parse_macro_input!(input as DeriveInput)) {
+        Ok(tokens) => tokens.into(),
+        Err(error) => error.to_compile_error().into(),
+    }
+}
+
+fn expand_decodal(input: DeriveInput) -> Result<TokenStream2> {
+    let name = input.ident;
+    let fields = match input.data {
+        Data::Struct(data) => match data.fields {
+            Fields::Named(fields) => fields.named,
+            _ => {
+                return Err(syn::Error::new(
+                    data.struct_token.span(),
+                    "Decodal derive supports structs with named fields only",
+                ));
+            }
+        },
+        _ => {
+            return Err(syn::Error::new(
+                name.span(),
+                "Decodal derive supports structs only",
+            ));
+        }
+    };
+
+    let mut schema_fields = Vec::new();
+    let mut decode_fields = Vec::new();
+
+    for field in fields {
+        let ident = field.ident.expect("named field");
+        let ty = field.ty;
+        let attrs = FieldAttrs::from_attrs(&field.attrs, &ident.to_string())?;
+        let path = attrs.rename.clone();
+        let schema_value = schema_expr(&ty, &attrs)?;
+        let decode_value = decode_expr(&ty, &path, &attrs);
+
+        schema_fields.push(quote! {
+            (#path, { #schema_value })
+        });
+        decode_fields.push(quote! {
+            #ident: #decode_value
+        });
+    }
+
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+
+    Ok(quote! {
+        impl #impl_generics ::decodal::DecodalSchema for #name #ty_generics #where_clause {
+            fn decodal_schema() -> ::decodal::HostValue {
+                ::decodal::HostValue::object_from_paths([
+                    #(#schema_fields),*
+                ])
+            }
+        }
+
+        impl #impl_generics ::decodal::DecodalDecode for #name #ty_generics #where_clause {
+            fn decodal_decode(data: &::decodal::Data) -> ::decodal::DecodeResult<Self> {
+                Ok(Self {
+                    #(#decode_fields),*
+                })
+            }
+        }
+    })
+}
+
+#[derive(Default)]
+struct FieldAttrs {
+    rename: String,
+    default: Option<DefaultAttr>,
+    constraints: Vec<ConstraintAttr>,
+}
+
+impl FieldAttrs {
+    fn from_attrs(attrs: &[syn::Attribute], fallback_name: &str) -> Result<Self> {
+        let mut output = Self {
+            rename: fallback_name.to_string(),
+            ..Self::default()
+        };
+
+        for attr in attrs {
+            if !attr.path().is_ident("decodal") {
+                continue;
+            }
+            attr.parse_nested_meta(|meta| {
+                if meta.path.is_ident("rename") {
+                    let value: LitStr = meta.value()?.parse()?;
+                    output.rename = value.value();
+                    return Ok(());
+                }
+                if meta.path.is_ident("default") {
+                    if meta.input.peek(syn::Token![=]) {
+                        let expr: Expr = meta.value()?.parse()?;
+                        output.default = Some(DefaultAttr::Expr(expr));
+                    } else {
+                        output.default = Some(DefaultAttr::Default);
+                    }
+                    return Ok(());
+                }
+                if let Some(kind) = ConstraintKind::from_path(&meta.path) {
+                    let expr: Expr = meta.value()?.parse()?;
+                    output.constraints.push(ConstraintAttr { kind, expr });
+                    return Ok(());
+                }
+                Err(meta.error("unsupported decodal attribute"))
+            })?;
+        }
+
+        Ok(output)
+    }
+}
+
+enum DefaultAttr {
+    Default,
+    Expr(Expr),
+}
+
+struct ConstraintAttr {
+    kind: ConstraintKind,
+    expr: Expr,
+}
+
+#[derive(Clone, Copy)]
+enum ConstraintKind {
+    Gt,
+    Gte,
+    Lt,
+    Lte,
+}
+
+impl ConstraintKind {
+    fn from_path(path: &syn::Path) -> Option<Self> {
+        if path.is_ident("gt") {
+            Some(Self::Gt)
+        } else if path.is_ident("gte") {
+            Some(Self::Gte)
+        } else if path.is_ident("lt") {
+            Some(Self::Lt)
+        } else if path.is_ident("lte") {
+            Some(Self::Lte)
+        } else {
+            None
+        }
+    }
+
+    fn method(self) -> syn::Ident {
+        match self {
+            Self::Gt => format_ident!("gt"),
+            Self::Gte => format_ident!("gte"),
+            Self::Lt => format_ident!("lt"),
+            Self::Lte => format_ident!("lte"),
+        }
+    }
+}
+
+fn schema_expr(ty: &Type, attrs: &FieldAttrs) -> Result<TokenStream2> {
+    let mut tokens = quote! {
+        let mut schema = <#ty as ::decodal::DecodalSchema>::decodal_schema();
+    };
+
+    for constraint in &attrs.constraints {
+        let method = constraint.kind.method();
+        let expr = &constraint.expr;
+        tokens.extend(quote! {
+            schema = schema.#method(#expr);
+        });
+    }
+
+    if let Some(default) = &attrs.default {
+        let default_expr = match default {
+            DefaultAttr::Default => quote! { <#ty as ::core::default::Default>::default() },
+            DefaultAttr::Expr(expr) => quote! { (#expr) },
+        };
+        tokens.extend(quote! {
+            schema = schema
+                .default(::decodal::IntoHostValue::into_host_value(#default_expr))
+                .expect("Decodal derive generated a valid default");
+        });
+    }
+
+    tokens.extend(quote! { schema });
+    Ok(tokens)
+}
+
+fn decode_expr(ty: &Type, path: &str, attrs: &FieldAttrs) -> TokenStream2 {
+    let missing = if let Some(default) = &attrs.default {
+        match default {
+            DefaultAttr::Default => quote! { <#ty as ::core::default::Default>::default() },
+            DefaultAttr::Expr(expr) => quote! { ::core::convert::Into::into(#expr) },
+        }
+    } else {
+        quote! { return Err(::decodal::DecodeError::new(#path, "missing field")); }
+    };
+
+    quote! {
+        if let Some(value) = ::decodal::data_at_path(data, #path) {
+            <#ty as ::decodal::DecodalDecode>::decodal_decode(value)
+                .map_err(|error| ::decodal::prefix_decode_error(#path, error))?
+        } else {
+            #missing
+        }
+    }
+}

crates/decodal-derive/tests/derive.rs

@@ -0,0 +1,79 @@
+use decodal::{Data, DecodalDecode, DecodalSchema, EmptyLoader, Engine};
+use decodal_derive::Decodal;
+
+#[derive(Debug, PartialEq, Decodal)]
+struct Service {
+    name: String,
+    #[decodal(gt = 443, default = 8443)]
+    port: i64,
+    #[decodal(default = true, rename = "feature.enable")]
+    feature_enabled: bool,
+    #[decodal(default)]
+    tags: Vec<String>,
+}
+
+#[test]
+fn derives_schema_and_decode() {
+    let mut engine = Engine::new(EmptyLoader);
+    engine
+        .bind_global("Service", Service::decodal_schema())
+        .unwrap();
+    let module = engine
+        .add_root_source(
+            "test",
+            "test",
+            r#"
+            Service & {
+              name = "api";
+              port = 9443;
+              feature.enable = false;
+              tags = ["web", "prod"];
+            }
+            "#,
+        )
+        .unwrap();
+    let value = engine.eval_module(module).unwrap();
+    let data = engine.materialize(&value).unwrap();
+    let service = Service::decodal_decode(&data).unwrap();
+    assert_eq!(
+        service,
+        Service {
+            name: "api".into(),
+            port: 9443,
+            feature_enabled: false,
+            tags: vec!["web".into(), "prod".into()],
+        }
+    );
+}
+
+#[test]
+fn defaults_are_available_in_schema() {
+    let mut engine = Engine::new(EmptyLoader);
+    engine
+        .bind_global("Service", Service::decodal_schema())
+        .unwrap();
+    let module = engine
+        .add_root_source(
+            "test",
+            "test",
+            r#"
+            Service & {
+              name = "api";
+            }
+            "#,
+        )
+        .unwrap();
+    let value = engine.eval_module(module).unwrap();
+    let data = engine.materialize(&value).unwrap();
+    let service = Service::decodal_decode(&data).unwrap();
+    assert_eq!(service.port, 8443);
+    assert!(service.feature_enabled);
+    assert!(service.tags.is_empty());
+}
+
+#[test]
+fn decode_reports_field_path() {
+    let data = Data::Object(vec![]);
+    let error = Service::decodal_decode(&data).unwrap_err();
+    assert_eq!(error.path, "name");
+}