Adds support for #[wasm_bindgen(typescript_custom_section)].

crates/backend/src/ast.rs

@@ -22,6 +22,8 @@ pub struct Program {
     /// objects" in the sense that they represent a JS object with a particular
     /// shape in JIT parlance.
     pub dictionaries: Vec<Dictionary>,
+    /// custom typescript sections to be included in the definition file
+    pub typescript_custom_sections: Vec<String>,
 }
 
 /// A rust to js interface. Allows interaction with rust objects/functions

crates/backend/src/encode.rs

@@ -46,6 +46,7 @@ fn shared_program<'a>(prog: &'a ast::Program, intern: &'a Interner)
         imports: prog.imports.iter()
             .map(|a| shared_import(a, intern))
             .collect::<Result<Vec<_>, _>>()?,
+        typescript_custom_sections: prog.typescript_custom_sections.iter().map(|x| -> &'a str { &x }).collect(),
         // version: shared::version(),
         // schema_version: shared::SCHEMA_VERSION.to_string(),
     })

crates/cli-support/src/js/mod.rs

@@ -2143,6 +2143,10 @@ impl<'a, 'b> SubContext<'a, 'b> {
         for s in self.program.structs.iter() {
             self.generate_struct(s)?;
         }
+        for s in self.program.typescript_custom_sections.iter() {
+            self.cx.typescript.push_str(s);
+            self.cx.typescript.push_str("\n\n");
+        }
 
         Ok(())
     }

crates/macro-support/src/parser.rs

@@ -207,6 +207,13 @@ impl BindgenAttrs {
             _ => false,
         })
     }
+
+    fn typescript_custom_section(&self) -> bool {
+        self.attrs.iter().any(|a| match *a {
+            BindgenAttr::TypescriptCustomSection => true,
+            _ => false,
+        })
+    }
 }
 
 impl Parse for BindgenAttrs {
@@ -244,6 +251,7 @@ pub enum BindgenAttr {
     Extends(syn::Path),
     VendorPrefix(Ident),
     Variadic,
+    TypescriptCustomSection,
 }
 
 impl Parse for BindgenAttr {
@@ -334,6 +342,9 @@ impl Parse for BindgenAttr {
             };
             return Ok(BindgenAttr::JsName(val, span));
         }
+        if attr == "typescript_custom_section" {
+            return Ok(BindgenAttr::TypescriptCustomSection);
+        }
 
         Err(original.error("unknown attribute"))
     }
@@ -810,11 +821,20 @@ impl<'a> MacroParse<(Option<BindgenAttrs>, &'a mut TokenStream)> for syn::Item {
                 e.to_tokens(tokens);
                 e.macro_parse(program, ())?;
             }
-            _ => bail_span!(
-                self,
-                "#[wasm_bindgen] can only be applied to a function, \
-                 struct, enum, impl, or extern block"
-            ),
+            syn::Item::Const(mut c) => {
+                let opts = match opts {
+                    Some(opts) => opts,
+                    None => BindgenAttrs::find(&mut c.attrs)?,
+                };
+                c.macro_parse(program, opts)?;
+            }
+            _ => {
+                bail_span!(
+                    self,
+                    "#[wasm_bindgen] can only be applied to a function, \
+                    struct, enum, impl, or extern block",
+                );
+            }
         }
 
         Ok(())
@@ -992,6 +1012,29 @@ impl MacroParse<()> for syn::ItemEnum {
     }
 }
 
+impl MacroParse<BindgenAttrs> for syn::ItemConst {
+    fn macro_parse(self, program: &mut ast::Program, opts: BindgenAttrs) -> Result<(), Diagnostic> {
+        // Shortcut
+        if !opts.typescript_custom_section() {
+            bail_span!(self, "#[wasm_bindgen] will not work on constants unless you are defining a #[wasm_bindgen(typescript_custom_section)].");
+        }
+
+        match *self.expr {
+            syn::Expr::Lit(syn::ExprLit {
+                lit: syn::Lit::Str(litstr),
+                ..
+            }) => {
+                program.typescript_custom_sections.push(litstr.value());
+            },
+            _ => {
+                bail_span!(self, "Expected a string literal to be used with #[wasm_bindgen(typescript_custom_section)].");
+            },
+        }
+
+        Ok(())
+    }
+}
+
 impl MacroParse<BindgenAttrs> for syn::ItemForeignMod {
     fn macro_parse(self, program: &mut ast::Program, opts: BindgenAttrs) -> Result<(), Diagnostic> {
         let mut errors = Vec::new();

crates/shared/src/lib.rs

@@ -12,6 +12,7 @@ struct Program<'a> {
     enums: Vec<Enum<'a>>,
     imports: Vec<Import<'a>>,
     structs: Vec<Struct<'a>>,
+    typescript_custom_sections: Vec<&'a str>,
     // version: &'a str,
     // schema_version: &'a str,
 }

guide/src/SUMMARY.md

@@ -76,6 +76,7 @@
       - [`constructor`](./reference/attributes/on-rust-exports/constructor.md)
       - [`js_name = Blah`](./reference/attributes/on-rust-exports/js_name.md)
       - [`readonly`](./reference/attributes/on-rust-exports/readonly.md)
+      - [`typescript_custom_section`](./reference/attributes/on-rust-exports/typescript_custom_section.md)
 
 --------------------------------------------------------------------------------
 

guide/src/reference/attributes/on-rust-exports/typescript_custom_section.md

@@ -0,0 +1,34 @@
+# `typescript_custom_section`
+
+When added to a `const` `&'static str`, it will append the contents of the
+string to the `.d.ts` file exported by `wasm-bindgen-cli` (when the
+`--typescript` flag is enabled).
+
+```rust
+#[wasm_bindgen(typescript_custom_section)]
+const TS_APPEND_CONTENT: &'static str = r#"
+
+export type Coords = { "latitude": number, "longitude": number, }; 
+
+"#;
+```
+
+The primary target for this feature is for code generation. For example, you
+can author a macro that allows you to export a TypeScript definition alongside
+the definition of a struct or Rust type.
+
+```rust
+#[derive(MyTypescriptExport)]
+struct Coords {
+    latitude: u32,
+    longitude: u32,
+}
+```
+
+The proc_derive_macro "MyTypescriptExport" can export its own
+`#[wasm_bindgen(typescript_custom_section)]` section, which would then be
+picked up by wasm-bindgen-cli. This would be equivalent to the contents of
+the TS_APPEND_CONTENT string in the first example.
+
+This feature allows plain data objects to be typechecked in Rust and in
+TypeScript by outputing a type definition generated at compile time.