craft-math: operator names take their LaTeX scripts directly again (restores #1019)

#1019 (22161d2d) wrote \sin^2\theta as \sin ^{2}{\theta } so it reads back as
the same tree. The squash merge of #1038, whose branch predated it, put
math_latex.rs and the corpus test back as they were before. This applies
#1019's change again, unchanged, with its test.

crates/math/src/math_latex.rs

@@ -982,7 +982,12 @@ fn script_node(kind: ScriptKind, base: &[MNode], sub: &[MNode], sup: &[MNode], o
         braced(base, output, depth);
         return;
     }
-    braced(base, output, depth);
+    // An operator name takes its scripts directly (`\sin^{2}`): in braces it would no longer apply
+    // to what follows it when read back.
+    match base {
+        [MNode::Run(r)] if is_function_name(r) => run_node(r, output),
+        _ => braced(base, output, depth),
+    }
     if matches!(kind, ScriptKind::Sub | ScriptKind::SubSup) {
         output.push('_');
         braced(sub, output, depth);
@@ -993,6 +998,11 @@ fn script_node(kind: ScriptKind, base: &[MNode], sub: &[MNode], sup: &[MNode], o
     }
 }
 
+/// Is `r` an operator name the writer writes as a command (`\sin`, `\log`)?
+fn is_function_name(r: &MRun) -> bool {
+    !r.nor && r.sty == Some(MSty::Plain) && r.scr == MScr::Roman && FUNCTION_NAMES.contains(&r.text.as_str())
+}
+
 fn radical_node(degree: &[MNode], degree_hidden: bool, expression: &[MNode], output: &mut String, depth: usize) {
     output.push_str("\\sqrt");
     if !degree_hidden && !degree.is_empty() {
@@ -1289,4 +1299,18 @@ mod tests {
         // The writer keeps its own names.
         assert_eq!(to_latex(&parse_latex(r"\infinity")), to_latex(&parse_latex(r"\infty")));
     }
+
+    /// An operator name with a script still applies to its argument when the LaTeX is read back.
+    #[test]
+    fn operator_names_take_their_scripts_directly() {
+        for src in [r"\sin^2\theta+\cos^2\theta=1", r"\log_2 x", r"\ln^{3}(x+1)"] {
+            let tree = parse_latex(src);
+            let tex = to_latex(&tree);
+            assert_eq!(crate::math::to_linear(&parse_latex(&tex)), crate::math::to_linear(&tree), "{src}: {tex}");
+            assert!(!tex.contains(r"{\sin }") && !tex.contains(r"{\log }"), "{tex}");
+        }
+        assert!(to_latex(&parse_latex(r"\sin^2\theta")).starts_with(r"\sin ^{2}"));
+        // Other bases keep their braces.
+        assert_eq!(to_latex(&parse_latex("x^2")), "{x}^{2}");
+    }
 }

crates/math/tests/corpus.rs

@@ -18,10 +18,6 @@ fn equations() -> Vec<(String, math::Arg, bool)> {
     v
 }
 
-/// Equations whose LaTeX this crate writes in a form it reads back differently (a function
-/// raised to a power: `\sin^2\theta` comes back as `(\sin)^2 θ`). Known; kept visible here.
-const LATEX_ROUND_TRIP_GAPS: &[&str] = &["func"];
-
 #[test]
 fn every_corpus_equation_reads_without_unknown_commands() {
     for (id, tree, _) in equations() {
@@ -54,10 +50,8 @@ fn every_corpus_equation_survives_mathml_and_latex() {
         let lin = math::to_linear(&tree);
         let ml = mathml::to_mathml(&tree, display, "");
         assert_eq!(mathml::parse(&ml).and_then(|m| mathml::read(&m)), Some((lin.clone(), display)), "{id}: {ml}");
-        if !LATEX_ROUND_TRIP_GAPS.contains(&id.as_str()) {
-            let tex = math_latex::to_latex(&tree);
-            assert_eq!(math::to_linear(&math_latex::parse_latex(&tex)), lin, "{id}: {tex}");
-        }
+        let tex = math_latex::to_latex(&tree);
+        assert_eq!(math::to_linear(&math_latex::parse_latex(&tex)), lin, "{id}: {tex}");
     }
 }