1 files changed, 142 insertions, 0 deletions
diff --git a/src/tools/rust-analyzer/crates/ide/src/inlay_hints/discriminant.rs b/src/tools/rust-analyzer/crates/ide/src/inlay_hints/discriminant.rs
new file mode 100644
index 000000000..f32c4bdf2
--- /dev/null
+++ b/src/tools/rust-analyzer/crates/ide/src/inlay_hints/discriminant.rs
@@ -0,0 +1,142 @@
+//! Implementation of "enum variant discriminant" inlay hints:
+//! ```no_run
+//! enum Foo {
+//!    Bar/* = 0*/,
+//! }
+//! ```
+use ide_db::{base_db::FileId, famous_defs::FamousDefs};
+use syntax::ast::{self, AstNode, HasName};
+
+use crate::{DiscriminantHints, InlayHint, InlayHintsConfig, InlayKind, InlayTooltip};
+
+pub(super) fn hints(
+    acc: &mut Vec<InlayHint>,
+    FamousDefs(sema, _): &FamousDefs<'_, '_>,
+    config: &InlayHintsConfig,
+    _: FileId,
+    variant: &ast::Variant,
+) -> Option<()> {
+    let field_list = match config.discriminant_hints {
+        DiscriminantHints::Always => variant.field_list(),
+        DiscriminantHints::Fieldless => match variant.field_list() {
+            Some(_) => return None,
+            None => None,
+        },
+        DiscriminantHints::Never => return None,
+    };
+
+    if variant.eq_token().is_some() {
+        return None;
+    }
+
+    let name = variant.name()?;
+
+    let descended = sema.descend_node_into_attributes(variant.clone()).pop();
+    let desc_pat = descended.as_ref().unwrap_or(variant);
+    let v = sema.to_def(desc_pat)?;
+    let d = v.eval(sema.db);
+
+    acc.push(InlayHint {
+        range: match field_list {
+            Some(field_list) => name.syntax().text_range().cover(field_list.syntax().text_range()),
+            None => name.syntax().text_range(),
+        },
+        kind: InlayKind::DiscriminantHint,
+        label: match &d {
+            Ok(v) => format!("{}", v).into(),
+            Err(_) => "?".into(),
+        },
+        tooltip: Some(InlayTooltip::String(match &d {
+            Ok(_) => "enum variant discriminant".into(),
+            Err(e) => format!("{e:?}").into(),
+        })),
+    });
+
+    Some(())
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::inlay_hints::{
+        tests::{check_with_config, DISABLED_CONFIG},
+        DiscriminantHints, InlayHintsConfig,
+    };
+
+    #[track_caller]
+    fn check_discriminants(ra_fixture: &str) {
+        check_with_config(
+            InlayHintsConfig { discriminant_hints: DiscriminantHints::Always, ..DISABLED_CONFIG },
+            ra_fixture,
+        );
+    }
+
+    #[track_caller]
+    fn check_discriminants_fieldless(ra_fixture: &str) {
+        check_with_config(
+            InlayHintsConfig {
+                discriminant_hints: DiscriminantHints::Fieldless,
+                ..DISABLED_CONFIG
+            },
+            ra_fixture,
+        );
+    }
+
+    #[test]
+    fn fieldless() {
+        check_discriminants(
+            r#"
+enum Enum {
+    Variant,
+  //^^^^^^^0
+    Variant1,
+  //^^^^^^^^1
+    Variant2,
+  //^^^^^^^^2
+    Variant5 = 5,
+    Variant6,
+  //^^^^^^^^6
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn datacarrying_mixed() {
+        check_discriminants(
+            r#"
+enum Enum {
+    Variant(),
+  //^^^^^^^^^0
+    Variant1,
+  //^^^^^^^^1
+    Variant2 {},
+  //^^^^^^^^^^^2
+    Variant3,
+  //^^^^^^^^3
+    Variant5 = 5,
+    Variant6,
+  //^^^^^^^^6
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn datacarrying_mixed_fieldless_set() {
+        check_discriminants_fieldless(
+            r#"
+enum Enum {
+    Variant(),
+    Variant1,
+  //^^^^^^^^1
+    Variant2 {},
+    Variant3,
+  //^^^^^^^^3
+    Variant5 = 5,
+    Variant6,
+  //^^^^^^^^6
+}
+"#,
+        );
+    }
+}