Implement valuable value pointer

tests/tests/pointer.rs

@@ -0,0 +1,79 @@
+use valuable::{pointer, Valuable, Value, Visit};
+
+#[derive(Valuable)]
+struct Struct1 {
+    x: String,
+    y: Struct2,
+}
+
+#[derive(Valuable)]
+struct Struct2 {
+    z: String,
+}
+
+#[derive(Default)]
+struct CollectValues(Vec<String>);
+
+impl Visit for CollectValues {
+    fn visit_value(&mut self, value: Value<'_>) {
+        self.0.push(format!("{:?}", value));
+    }
+}
+
+#[test]
+fn basic() {
+    let value = Struct1 {
+        x: "a".to_owned(),
+        y: Struct2 { z: "b".to_owned() },
+    };
+
+    let mut visitor = CollectValues::default();
+    value.visit_pointer(
+        pointer::Pointer::new(&[pointer::Segment::Field("x")]),
+        &mut visitor,
+    );
+    assert_eq!(visitor.0.len(), 1);
+    assert_eq!(visitor.0[0], r#""a""#);
+
+    let mut visitor = CollectValues::default();
+    value.visit_pointer(
+        pointer::Pointer::new(&[pointer::Segment::Field("y")]),
+        &mut visitor,
+    );
+    assert_eq!(visitor.0.len(), 1);
+    assert_eq!(visitor.0[0], r#"Struct2 { z: "b" }"#);
+
+    let mut visitor = CollectValues::default();
+    value.visit_pointer(
+        pointer::Pointer::new(&[pointer::Segment::Field("y"), pointer::Segment::Field("z")]),
+        &mut visitor,
+    );
+    assert_eq!(visitor.0.len(), 1);
+    assert_eq!(visitor.0[0], r#""b""#);
+}
+
+#[cfg(feature = "derive")]
+#[test]
+fn visit_pointer_macro() {
+    use valuable::visit_pointer;
+
+    let value = Struct1 {
+        x: "a".to_owned(),
+        y: Struct2 { z: "b".to_owned() },
+    };
+
+    let mut visitor = CollectValues::default();
+    visit_pointer!(value.x, visitor);
+    assert_eq!(visitor.0.len(), 1);
+    assert_eq!(visitor.0[0], r#""a""#);
+
+    let mut visitor = CollectValues::default();
+    visit_pointer!(value.y, visitor);
+    assert_eq!(visitor.0.len(), 1);
+    assert_eq!(visitor.0[0], r#"Struct2 { z: "b" }"#);
+
+    let mut visitor = CollectValues::default();
+    visit_pointer!(value.y.z, visitor);
+    assert_eq!(visitor.0.len(), 1);
+    assert_eq!(visitor.0[0], r#""b""#);
+}

valuable-derive/Cargo.toml

@@ -12,7 +12,7 @@ proc-macro = true
 [dependencies]
 proc-macro2 = "1.0"
 quote = "1.0"
-syn = { version = "1.0.60", features = ["extra-traits"] }
+syn = { version = "1.0.60", features = ["full", "extra-traits"] }
 
 [dev-dependencies]
 valuable = { path = "../valuable", features = ["derive"] }

valuable-derive/src/expand.rs → valuable-derive/src/derive.rs

@@ -312,7 +312,7 @@ fn allowed_lints() -> TokenStream {
     }
 }
 
-fn respan(tokens: TokenStream, span: &impl ToTokens) -> TokenStream {
+pub(crate) fn respan(tokens: TokenStream, span: &impl ToTokens) -> TokenStream {
     let mut iter = span.to_token_stream().into_iter();
     // `Span` on stable Rust has a limitation that only points to the first
     // token, not the whole tokens. We can work around this limitation by

valuable-derive/src/lib.rs

@@ -1,9 +1,10 @@
-extern crate proc_macro;
+#![warn(rust_2018_idioms, unreachable_pub)]
 
-mod expand;
+mod derive;
+mod pointer;
 
 use proc_macro::TokenStream;
-use syn::parse_macro_input;
+use syn::{parse_macro_input, Error};
 
 /// Derive a `Valuable` implementation for a struct or enum.
 ///
@@ -26,5 +27,12 @@ use syn::parse_macro_input;
 #[proc_macro_derive(Valuable, attributes(valuable))]
 pub fn derive_valuable(input: TokenStream) -> TokenStream {
     let mut input = parse_macro_input!(input as syn::DeriveInput);
-    expand::derive_valuable(&mut input).into()
+    derive::derive_valuable(&mut input).into()
+}
+
+#[proc_macro]
+pub fn visit_pointer(input: TokenStream) -> TokenStream {
+    pointer::visit_pointer(input.into())
+        .unwrap_or_else(Error::into_compile_error)
+        .into()
 }

valuable-derive/src/pointer.rs

@@ -0,0 +1,94 @@
+use std::collections::VecDeque;
+
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::parse::{Parse, ParseStream};
+use syn::{Expr, Result, Token};
+
+use crate::derive::respan;
+
+pub(crate) fn visit_pointer(input: TokenStream) -> Result<TokenStream> {
+    let Input {
+        expr,
+        segments,
+        visit,
+    } = syn::parse2(input)?;
+
+    let segments = segments.iter().map(|segment| match segment {
+        Segment::Member(syn::Member::Named(ident)) => {
+            let literal = ident.to_string();
+            quote! {
+                ::valuable::pointer::Segment::Field(#literal),
+            }
+        }
+        Segment::Member(syn::Member::Unnamed(index)) => {
+            quote! {
+                ::valuable::pointer::Segment::TupleIndex(#index),
+            }
+        }
+        Segment::Index(expr) => {
+            let expr = respan(quote! { &#expr }, expr);
+            quote! {
+                ::valuable::pointer::Segment::Index(
+                    ::valuable::Valuable::as_value(#expr)
+                ),
+            }
+        }
+    });
+
+    let visit_pointer = respan(quote! { ::valuable::Valuable::visit_pointer }, &expr);
+    Ok(quote! {
+        #visit_pointer(
+            &#expr,
+            ::valuable::pointer::Pointer::new(&[
+                #(#segments)*
+            ]),
+            &mut #visit,
+        )
+    })
+}
+
+struct Input {
+    expr: Expr,
+    segments: VecDeque<Segment>,
+    visit: Expr,
+}
+
+enum Segment {
+    Member(syn::Member),
+    Index(Box<Expr>),
+}
+
+impl Parse for Input {
+    fn parse(input: ParseStream<'_>) -> Result<Self> {
+        let mut chain = input.parse()?;
+        let _: Token![,] = input.parse()?;
+        let visit = input.parse()?;
+        let _: Option<Token![,]> = input.parse()?;
+
+        let mut segments = VecDeque::new();
+        let expr;
+        loop {
+            match chain {
+                Expr::Field(e) => {
+                    chain = *e.base;
+                    segments.push_front(Segment::Member(e.member))
+                }
+                Expr::Index(e) => {
+                    chain = *e.expr;
+                    segments.push_front(Segment::Index(e.index))
+                }
+                e => {
+                    expr = e;
+                    break;
+                }
+            }
+        }
+
+        Ok(Self {
+            expr,
+            segments,
+            visit,
+        })
+    }
+}

valuable/src/lib.rs

@@ -115,6 +115,8 @@ pub use mappable::Mappable;
 mod named_values;
 pub use named_values::NamedValues;
 
+pub mod pointer;
+
 mod slice;
 pub use slice::Slice;