Add variance-related information to lifetime error messages

[rust.git] / compiler / rustc_middle / src / ty / sty.rs

diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index c1f71fbbfa41ad485012b9942faf2ebf766f6779..1d9ff512288db9b05e140f6f4d224fa35b60f36a 100644 (file)
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -2181,3 +2181,55 @@ pub fn is_trivially_sized(&self, tcx: TyCtxt<'tcx>) -> bool {
         }
     }
 }
+
+/// Extra information about why we ended up with a particular variance.
+/// This is only used to add more information to error messages, and
+/// has no effect on soundness. While choosing the 'wrong' `VarianceDiagInfo`
+/// may lead to confusing notes in error messages, it will never cause
+/// a miscompilation or unsoundness.
+///
+/// When in doubt, use `VarianceDiagInfo::default()`
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub enum VarianceDiagInfo<'tcx> {
+    /// No additional information - this is the default.
+    /// We will not add any additional information to error messages.
+    None,
+    /// We switched our variance because a type occurs inside
+    /// the generic argument of a mutable reference or pointer
+    /// (`*mut T` or `&mut T`). In either case, our variance
+    /// will always be `Invariant`.
+    Mut {
+        /// Tracks whether we had a mutable pointer or reference,
+        /// for better error messages
+        kind: VarianceDiagMutKind,
+        /// The type parameter of the mutable pointer/reference
+        /// (the `T` in `&mut T` or `*mut T`).
+        ty: Ty<'tcx>,
+    },
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub enum VarianceDiagMutKind {
+    /// A mutable raw pointer (`*mut T`)
+    RawPtr,
+    /// A mutable reference (`&mut T`)
+    Ref,
+}
+
+impl<'tcx> VarianceDiagInfo<'tcx> {
+    /// Mirrors `Variance::xform` - used to 'combine' the existing
+    /// and new `VarianceDiagInfo`s when our variance changes.
+    pub fn xform(self, other: VarianceDiagInfo<'tcx>) -> VarianceDiagInfo<'tcx> {
+        // For now, just use the first `VarianceDiagInfo::Mut` that we see
+        match self {
+            VarianceDiagInfo::None => other,
+            VarianceDiagInfo::Mut { .. } => self,
+        }
+    }
+}
+
+impl<'tcx> Default for VarianceDiagInfo<'tcx> {
+    fn default() -> Self {
+        Self::None
+    }
+}