Fix derive_dialect! proc macro for use from external crates

Author: Alexander Beedie

derive/src/dialect.rs

@@ -120,24 +120,20 @@ impl Parse for DeriveDialectInput {
 
 /// Entry point for the `derive_dialect!` macro
 pub(crate) fn derive_dialect(input: DeriveDialectInput) -> proc_macro::TokenStream {
-    let err = |msg: String| {
-        Error::new(proc_macro2::Span::call_site(), msg)
-            .to_compile_error()
-            .into()
-    };
+    match derive_dialect_inner(input) {
+        Ok(tokens) => tokens.into(),
+        Err(e) => e.to_compile_error().into(),
+    }
+}
 
-    let source = match read_dialect_mod_file() {
-        Ok(s) => s,
-        Err(e) => return err(format!("Failed to read dialect/mod.rs: {e}")),
-    };
-    let file: File = match syn::parse_str(&source) {
-        Ok(f) => f,
-        Err(e) => return err(format!("Failed to parse source: {e}")),
-    };
-    let methods = match extract_dialect_methods(&file) {
-        Ok(m) => m,
-        Err(e) => return e.to_compile_error().into(),
-    };
+fn derive_dialect_inner(input: DeriveDialectInput) -> syn::Result<TokenStream> {
+    let call_site = proc_macro2::Span::call_site();
+
+    let source = read_dialect_mod_file()
+        .map_err(|e| Error::new(call_site, format!("Failed to read dialect/mod.rs: {e}")))?;
+    let file: File = syn::parse_str::<File>(&source)
+        .map_err(|e| Error::new(call_site, format!("Failed to parse source: {e}")))?;
+    let methods = extract_dialect_methods(&file)?;
 
     // Validate overrides
     let bool_names: HashSet<_> = methods
@@ -147,20 +143,23 @@ pub(crate) fn derive_dialect(input: DeriveDialectInput) -> proc_macro::TokenStre
         .collect();
     for (key, value) in &input.overrides {
         let key_str = key.to_string();
-        let err = |msg| Error::new(key.span(), msg).to_compile_error().into();
         match value {
             Override::Bool(_) if !bool_names.contains(&key_str) => {
-                return err(format!("Unknown boolean method `{key_str}`"));
+                return Err(Error::new(
+                    key.span(),
+                    format!("Unknown boolean method `{key_str}`"),
+                ));
             }
             Override::Char(_) | Override::None if key_str != "identifier_quote_style" => {
-                return err(format!(
-                    "Char/None only valid for `identifier_quote_style`, not `{key_str}`"
+                return Err(Error::new(
+                    key.span(),
+                    format!("Char/None only valid for `identifier_quote_style`, not `{key_str}`"),
                 ));
             }
             _ => {}
         }
     }
-    generate_derived_dialect(&input, &methods).into()
+    Ok(generate_derived_dialect(&input, &methods))
 }
 
 /// Generate the complete derived `Dialect` implementation
@@ -258,11 +257,55 @@ fn extract_param_names(sig: &Signature) -> Vec<&Ident> {
 }
 
 /// Read the `dialect/mod.rs` file that contains the Dialect trait.
+///
+/// Searches for the file in the following order:
+/// 1. `$CARGO_MANIFEST_DIR/src/dialect/mod.rs` - works when the macro is
+///    invoked from within the `sqlparser` crate itself (e.g. in tests).
+/// 2. `<sqlparser_derive dir>/../src/dialect/mod.rs` - works when
+///    `sqlparser_derive` lives in a workspace alongside the main crate
+///    (the standard `derive/` layout).
+/// 3. Sibling directories of the compiled `sqlparser_derive` crate in the
+///    Cargo registry - works when an external crate uses `derive_dialect!`
+///    via a registry dependency.
 fn read_dialect_mod_file() -> Result<String, String> {
-    let manifest_dir =
-        std::env::var("CARGO_MANIFEST_DIR").map_err(|_| "CARGO_MANIFEST_DIR not set")?;
-    let path = std::path::Path::new(&manifest_dir).join("src/dialect/mod.rs");
-    std::fs::read_to_string(&path).map_err(|e| format!("Failed to read {}: {e}", path.display()))
+    use std::path::{Path, PathBuf};
+
+    const DERIVE_CRATE_DIR: &str = env!("CARGO_MANIFEST_DIR");
+    let derive_dir = Path::new(DERIVE_CRATE_DIR);
+    let mut candidates: Vec<PathBuf> = Vec::new();
+
+    // The crate being compiled (eg: within sqlparser).
+    if let Ok(manifest_dir) = std::env::var("CARGO_MANIFEST_DIR") {
+        candidates.push(Path::new(&manifest_dir).join("src/dialect/mod.rs"));
+    }
+    // Workspace layout: the main crate is the parent of `derive/`.
+    candidates.push(derive_dir.join("../src/dialect/mod.rs"));
+
+    // Cargo registry: look for sibling `sqlparser-*` directories (prefer newest).
+    if let Some(parent) = derive_dir.parent() {
+        if let Ok(entries) = std::fs::read_dir(parent) {
+            let mut siblings: Vec<_> = entries
+                .filter_map(|e| e.ok())
+                .filter(|e| {
+                    let name = e.file_name();
+                    let name = name.to_string_lossy();
+                    name.starts_with("sqlparser-") && !name.starts_with("sqlparser-derive")
+                })
+                .collect();
+            siblings.sort_by(|a, b| b.file_name().cmp(&a.file_name()));
+            candidates.extend(siblings.into_iter().map(|e| e.path().join("src/dialect/mod.rs")));
+        }
+    }
+    for path in &candidates {
+        if let Ok(content) = std::fs::read_to_string(path) {
+            return Ok(content);
+        }
+    }
+    Err(format!(
+        "Could not find `sqlparser` dialect/mod.rs file. \
+         Searched in $CARGO_MANIFEST_DIR/src/dialect/mod.rs and \
+         the `sqlparser_derive` crate at {DERIVE_CRATE_DIR}"
+    ))
 }
 
 /// Extract all methods from the `Dialect` trait (excluding `dialect` for TypeId)