feat: support implicit template placeholders

CHANGELOG.md

@@ -6,6 +6,24 @@ All notable changes to this project will be documented in this file.
 ### Added
 - _Nothing yet._
 
+## [0.6.0] - 2025-10-08
+
+### Added
+- Recognised empty placeholder bodies (`{}` / `{:?}`) as implicit positional
+  identifiers, numbering them by appearance and exposing the new
+  `TemplateIdentifier::Implicit` variant in the template API.
+- Propagated the implicit identifier metadata through
+  `template_support::TemplateIdentifierSpec`, ensuring derive-generated display
+  implementations resolve tuple fields in placeholder order.
+
+### Fixed
+- Preserved `TemplateError::EmptyPlaceholder` diagnostics for whitespace-only
+  placeholders, matching previous error reporting for invalid bodies.
+
+### Tests
+- Added parser regressions covering implicit placeholder sequencing and the
+  whitespace-only error path.
+
 ## [0.5.15] - 2025-10-07
 
 ### Added

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "masterror"
-version = "0.5.15"
+version = "0.6.0"
 rust-version = "1.90"
 edition = "2024"
 license = "MIT OR Apache-2.0"
@@ -49,8 +49,8 @@ turnkey = []
 openapi = ["dep:utoipa"]
 
 [workspace.dependencies]
-masterror-derive = { version = "0.1.7", path = "masterror-derive" }
-masterror-template = { version = "0.1.4", path = "masterror-template" }
+masterror-derive = { version = "0.2.0", path = "masterror-derive" }
+masterror-template = { version = "0.2.0", path = "masterror-template" }
 
 [dependencies]
 masterror-derive = { workspace = true }

README.md

@@ -29,9 +29,9 @@ Stable categories, conservative HTTP mapping, no `unsafe`.
 
 ~~~toml
 [dependencies]
-masterror = { version = "0.5.15", default-features = false }
+masterror = { version = "0.6.0", default-features = false }
 # or with features:
-# masterror = { version = "0.5.15", features = [
+# masterror = { version = "0.6.0", features = [
 #   "axum", "actix", "openapi", "serde_json",
 #   "sqlx", "sqlx-migrate", "reqwest", "redis",
 #   "validator", "config", "tokio", "multipart",
@@ -66,10 +66,10 @@ masterror = { version = "0.5.15", default-features = false }
 ~~~toml
 [dependencies]
 # lean core
-masterror = { version = "0.5.15", default-features = false }
+masterror = { version = "0.6.0", default-features = false }
 
 # with Axum/Actix + JSON + integrations
-# masterror = { version = "0.5.15", features = [
+# masterror = { version = "0.6.0", features = [
 #   "axum", "actix", "openapi", "serde_json",
 #   "sqlx", "sqlx-migrate", "reqwest", "redis",
 #   "validator", "config", "tokio", "multipart",
@@ -383,13 +383,13 @@ assert_eq!(resp.status, 401);
 Minimal core:
 
 ~~~toml
-masterror = { version = "0.5.15", default-features = false }
+masterror = { version = "0.6.0", default-features = false }
 ~~~
 
 API (Axum + JSON + deps):
 
 ~~~toml
-masterror = { version = "0.5.15", features = [
+masterror = { version = "0.6.0", features = [
   "axum", "serde_json", "openapi",
   "sqlx", "reqwest", "redis", "validator", "config", "tokio"
 ] }
@@ -398,7 +398,7 @@ masterror = { version = "0.5.15", features = [
 API (Actix + JSON + deps):
 
 ~~~toml
-masterror = { version = "0.5.15", features = [
+masterror = { version = "0.6.0", features = [
   "actix", "serde_json", "openapi",
   "sqlx", "reqwest", "redis", "validator", "config", "tokio"
 ] }

masterror-derive/Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "masterror-derive"
 rust-version = "1.90"
-version = "0.1.7"
+version = "0.2.0"
 edition = "2024"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/RAprogramm/masterror"

masterror-derive/src/display.rs

@@ -249,6 +249,10 @@ fn struct_placeholder_expr(
             }
         }
         TemplateIdentifierSpec::Positional(index) => fields
+            .get_positional(*index)
+            .map(|field| struct_field_expr(field, placeholder.formatter))
+            .ok_or_else(|| placeholder_error(placeholder.span, &placeholder.identifier)),
+        TemplateIdentifierSpec::Implicit(index) => fields
             .get_positional(*index)
             .map(|field| struct_field_expr(field, placeholder.formatter))
             .ok_or_else(|| placeholder_error(placeholder.span, &placeholder.identifier))
@@ -298,6 +302,15 @@ fn variant_tuple_placeholder(
             Err(placeholder_error(placeholder.span, &placeholder.identifier))
         }
         TemplateIdentifierSpec::Positional(index) => bindings
+            .get(*index)
+            .map(|binding| {
+                ResolvedPlaceholderExpr::with(
+                    quote!(#binding),
+                    needs_pointer_value(placeholder.formatter)
+                )
+            })
+            .ok_or_else(|| placeholder_error(placeholder.span, &placeholder.identifier)),
+        TemplateIdentifierSpec::Implicit(index) => bindings
             .get(*index)
             .map(|binding| {
                 ResolvedPlaceholderExpr::with(
@@ -338,6 +351,10 @@ fn variant_named_placeholder(
         TemplateIdentifierSpec::Positional(index) => Err(placeholder_error(
             placeholder.span,
             &TemplateIdentifierSpec::Positional(*index)
+        )),
+        TemplateIdentifierSpec::Implicit(index) => Err(placeholder_error(
+            placeholder.span,
+            &TemplateIdentifierSpec::Implicit(*index)
         ))
     }
 }

masterror-derive/src/input.rs

@@ -826,5 +826,8 @@ pub fn placeholder_error(span: Span, identifier: &TemplateIdentifierSpec) -> Err
         TemplateIdentifierSpec::Positional(index) => {
             Error::new(span, format!("field `{}` is not available", index))
         }
+        TemplateIdentifierSpec::Implicit(index) => {
+            Error::new(span, format!("field `{}` is not available", index))
+        }
     }
 }

masterror-derive/src/template_support.rs

@@ -27,7 +27,8 @@ pub struct TemplatePlaceholderSpec {
 #[derive(Debug, Clone)]
 pub enum TemplateIdentifierSpec {
     Named(String),
-    Positional(usize)
+    Positional(usize),
+    Implicit(usize)
 }
 
 pub fn parse_display_template(lit: LitStr) -> Result<DisplayTemplate, Error> {
@@ -49,6 +50,7 @@ pub fn parse_display_template(lit: LitStr) -> Result<DisplayTemplate, Error> {
                     TemplateIdentifier::Positional(index) => {
                         TemplateIdentifierSpec::Positional(*index)
                     }
+                    TemplateIdentifier::Implicit(index) => TemplateIdentifierSpec::Implicit(*index)
                 };
 
                 segments.push(TemplateSegmentSpec::Placeholder(TemplatePlaceholderSpec {

masterror-template/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "masterror-template"
-version = "0.1.4"
+version = "0.2.0"
 rust-version = "1.90"
 edition = "2024"
 repository = "https://github.com/RAprogramm/masterror"

masterror-template/src/template.rs

@@ -139,6 +139,9 @@ impl<'a> TemplatePlaceholder<'a> {
 /// Placeholder identifier parsed from the template.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum TemplateIdentifier<'a> {
+    /// Implicit positional index inferred from the placeholder order (`{}` /
+    /// `{:?}` / etc.).
+    Implicit(usize),
     /// Positional index (`{0}` / `{1:?}` / etc.).
     Positional(usize),
     /// Named field (`{name}` / `{kind:?}` / etc.).
@@ -150,7 +153,7 @@ impl<'a> TemplateIdentifier<'a> {
     pub const fn as_str(&self) -> Option<&'a str> {
         match self {
             Self::Named(value) => Some(value),
-            Self::Positional(_) => None
+            Self::Positional(_) | Self::Implicit(_) => None
         }
     }
 }
@@ -569,6 +572,32 @@ mod tests {
         assert_eq!(placeholders[1].identifier(), &named("message"));
     }
 
+    #[test]
+    fn parses_implicit_identifiers() {
+        let template = ErrorTemplate::parse("{}, {:?}, {name}, {}").expect("parse");
+        let mut placeholders = template.placeholders();
+
+        let first = placeholders.next().expect("first placeholder");
+        assert_eq!(first.identifier(), &TemplateIdentifier::Implicit(0));
+        assert_eq!(first.formatter(), TemplateFormatter::Display);
+
+        let second = placeholders.next().expect("second placeholder");
+        assert_eq!(second.identifier(), &TemplateIdentifier::Implicit(1));
+        assert_eq!(
+            second.formatter(),
+            TemplateFormatter::Debug {
+                alternate: false
+            }
+        );
+
+        let third = placeholders.next().expect("third placeholder");
+        assert_eq!(third.identifier(), &named("name"));
+
+        let fourth = placeholders.next().expect("fourth placeholder");
+        assert_eq!(fourth.identifier(), &TemplateIdentifier::Implicit(2));
+        assert!(placeholders.next().is_none());
+    }
+
     #[test]
     fn parses_debug_formatter() {
         let template = ErrorTemplate::parse("{0:#?}").expect("parse");