[ty] Truncate Literal type display in some situations (#20928)

crates/ty_python_semantic/resources/mdtest/diagnostics/union_call.md

@@ -138,3 +138,27 @@ def _(n: int):
     # error: [unknown-argument]
     y = f("foo", name="bar", unknown="quux")
 ```
+
+### Truncation for long unions and literals
+
+This test demonstrates a call where the expected type is a large mixed union. The diagnostic must
+therefore truncate the long expected union type to avoid overwhelming output.
+
+```py
+from typing import Literal, Union
+
+class A: ...
+class B: ...
+class C: ...
+class D: ...
+class E: ...
+class F: ...
+
+def f1(x: Union[Literal[1, 2, 3, 4, 5, 6, 7, 8], A, B, C, D, E, F]) -> int:
+    return 0
+
+def _(n: int):
+    x = n
+    # error: [invalid-argument-type]
+    f1(x)
+```

crates/ty_python_semantic/resources/mdtest/snapshots/union_call.md_-_Calling_a_union_of_f…_-_Try_to_cover_all_pos…_-_Truncation_for_long_…_(ec94b5e857284ef3).snap

@@ -0,0 +1,56 @@
+---
+source: crates/ty_test/src/lib.rs
+assertion_line: 427
+expression: snapshot
+---
+---
+mdtest name: union_call.md - Calling a union of function types - Try to cover all possible reasons - Truncation for long unions and literals
+mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/union_call.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing import Literal, Union
+ 2 | 
+ 3 | class A: ...
+ 4 | class B: ...
+ 5 | class C: ...
+ 6 | class D: ...
+ 7 | class E: ...
+ 8 | class F: ...
+ 9 | 
+10 | def f1(x: Union[Literal[1, 2, 3, 4, 5, 6, 7, 8], A, B, C, D, E, F]) -> int:
+11 |     return 0
+12 | 
+13 | def _(n: int):
+14 |     x = n
+15 |     # error: [invalid-argument-type]
+16 |     f1(x)
+```
+
+# Diagnostics
+
+```
+error[invalid-argument-type]: Argument to function `f1` is incorrect
+  --> src/mdtest_snippet.py:16:8
+   |
+14 |     x = n
+15 |     # error: [invalid-argument-type]
+16 |     f1(x)
+   |        ^ Expected `Literal[1, 2, 3, 4, 5, ... omitted 3 literals] | A | B | ... omitted 4 union elements`, found `int`
+   |
+info: Function defined here
+  --> src/mdtest_snippet.py:10:5
+   |
+ 8 | class F: ...
+ 9 |
+10 | def f1(x: Union[Literal[1, 2, 3, 4, 5, 6, 7, 8], A, B, C, D, E, F]) -> int:
+   |     ^^ ----------------------------------------------------------- Parameter declared here
+11 |     return 0
+   |
+info: rule `invalid-argument-type` is enabled by default
+
+```

crates/ty_python_semantic/src/types/display.rs

@@ -38,7 +38,7 @@ pub struct DisplaySettings<'db> {
     /// Class names that should be displayed fully qualified
     /// (e.g., `module.ClassName` instead of just `ClassName`)
     pub qualified: Rc<FxHashMap<&'db str, QualificationLevel>>,
-    /// Whether long unions are displayed in full
+    /// Whether long unions and literals are displayed in full
     pub preserve_full_unions: bool,
 }
 
@@ -1328,6 +1328,44 @@ impl Display for DisplayParameter<'_> {
     }
 }
 
+#[derive(Debug, Copy, Clone)]
+struct TruncationPolicy {
+    max: usize,
+    max_when_elided: usize,
+}
+
+impl TruncationPolicy {
+    fn display_limit(self, total: usize, preserve_full: bool) -> usize {
+        if preserve_full {
+            return total;
+        }
+        let limit = if total > self.max {
+            self.max_when_elided
+        } else {
+            self.max
+        };
+        limit.min(total)
+    }
+}
+
+#[derive(Debug)]
+struct DisplayOmitted {
+    count: usize,
+    singular: &'static str,
+    plural: &'static str,
+}
+
+impl Display for DisplayOmitted {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        let noun = if self.count == 1 {
+            self.singular
+        } else {
+            self.plural
+        };
+        write!(f, "... omitted {} {}", self.count, noun)
+    }
+}
+
 impl<'db> UnionType<'db> {
     fn display_with(
         &'db self,
@@ -1348,8 +1386,10 @@ struct DisplayUnionType<'db> {
     settings: DisplaySettings<'db>,
 }
 
-const MAX_DISPLAYED_UNION_ITEMS: usize = 5;
-const MAX_DISPLAYED_UNION_ITEMS_WHEN_ELIDED: usize = 3;
+const UNION_POLICY: TruncationPolicy = TruncationPolicy {
+    max: 5,
+    max_when_elided: 3,
+};
 
 impl Display for DisplayUnionType<'_> {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
@@ -1379,16 +1419,8 @@ impl Display for DisplayUnionType<'_> {
 
         let mut join = f.join(" | ");
 
-        let display_limit = if self.settings.preserve_full_unions {
-            total_entries
-        } else {
-            let limit = if total_entries > MAX_DISPLAYED_UNION_ITEMS {
-                MAX_DISPLAYED_UNION_ITEMS_WHEN_ELIDED
-            } else {
-                MAX_DISPLAYED_UNION_ITEMS
-            };
-            limit.min(total_entries)
-        };
+        let display_limit =
+            UNION_POLICY.display_limit(total_entries, self.settings.preserve_full_unions);
 
         let mut condensed_types = Some(condensed_types);
         let mut displayed_entries = 0usize;
@@ -1420,8 +1452,10 @@ impl Display for DisplayUnionType<'_> {
         if !self.settings.preserve_full_unions {
             let omitted_entries = total_entries.saturating_sub(displayed_entries);
             if omitted_entries > 0 {
-                join.entry(&DisplayUnionOmitted {
+                join.entry(&DisplayOmitted {
                     count: omitted_entries,
+                    singular: "union element",
+                    plural: "union elements",
                 });
             }
         }
@@ -1437,38 +1471,45 @@ impl fmt::Debug for DisplayUnionType<'_> {
         Display::fmt(self, f)
     }
 }
-
-struct DisplayUnionOmitted {
-    count: usize,
-}
-
-impl Display for DisplayUnionOmitted {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        let plural = if self.count == 1 {
-            "element"
-        } else {
-            "elements"
-        };
-        write!(f, "... omitted {} union {}", self.count, plural)
-    }
-}
-
 struct DisplayLiteralGroup<'db> {
     literals: Vec<Type<'db>>,
     db: &'db dyn Db,
     settings: DisplaySettings<'db>,
 }
 
+const LITERAL_POLICY: TruncationPolicy = TruncationPolicy {
+    max: 7,
+    max_when_elided: 5,
+};
+
 impl Display for DisplayLiteralGroup<'_> {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
         f.write_str("Literal[")?;
-        f.join(", ")
-            .entries(
-                self.literals
-                    .iter()
-                    .map(|ty| ty.representation(self.db, self.settings.singleline())),
-            )
-            .finish()?;
+
+        let total_entries = self.literals.len();
+
+        let display_limit =
+            LITERAL_POLICY.display_limit(total_entries, self.settings.preserve_full_unions);
+
+        let mut join = f.join(", ");
+
+        for lit in self.literals.iter().take(display_limit) {
+            let rep = lit.representation(self.db, self.settings.singleline());
+            join.entry(&rep);
+        }
+
+        if !self.settings.preserve_full_unions {
+            let omitted_entries = total_entries.saturating_sub(display_limit);
+            if omitted_entries > 0 {
+                join.entry(&DisplayOmitted {
+                    count: omitted_entries,
+                    singular: "literal",
+                    plural: "literals",
+                });
+            }
+        }
+
+        join.finish()?;
         f.write_str("]")
     }
 }