Support indices preceding the base symbol, revamping #699 (#825)

Breaking change: abbreviate attach() attachment params, top -> t, bottom -> b

2 files changed, 14 insertions, 4 deletions

diff --git a/src/eval/library.rs b/src/eval/library.rs
index 0c635864..a92d8bd1 100644
--- a/src/eval/library.rs
+++ b/src/eval/library.rs
@@ -87,8 +87,18 @@ pub struct LangItems {
     /// Matched delimiters in math: `[x + y]`.
     pub math_delimited: fn(open: Content, body: Content, close: Content) -> Content,
     /// A base with optional attachments in math: `a_1^2`.
-    pub math_attach:
-        fn(base: Content, bottom: Option<Content>, top: Option<Content>) -> Content,
+    #[allow(clippy::type_complexity)]
+    pub math_attach: fn(
+        base: Content,
+        // Positioned smartly.
+        top: Option<Content>,
+        bottom: Option<Content>,
+        // Fixed positions.
+        topleft: Option<Content>,
+        bottomleft: Option<Content>,
+        topright: Option<Content>,
+        bottomright: Option<Content>,
+    ) -> Content,
     /// A base with an accent: `arrow(x)`.
     pub math_accent: fn(base: Content, accent: char) -> Content,
     /// A fraction in math: `x/2`.
diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index d8f49d66..d2ca0e74 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -708,9 +708,9 @@ impl Eval for ast::MathAttach {
     #[tracing::instrument(name = "MathAttach::eval", skip_all)]
     fn eval(&self, vm: &mut Vm) -> SourceResult<Self::Output> {
         let base = self.base().eval_display(vm)?;
-        let bottom = self.bottom().map(|expr| expr.eval_display(vm)).transpose()?;
         let top = self.top().map(|expr| expr.eval_display(vm)).transpose()?;
-        Ok((vm.items.math_attach)(base, bottom, top))
+        let bottom = self.bottom().map(|expr| expr.eval_display(vm)).transpose()?;
+        Ok((vm.items.math_attach)(base, top, bottom, None, None, None, None))
     }
 }