be even more precise about "cast" vs "coercion"

compiler/rustc_middle/src/mir/query.rs

@@ -234,8 +234,8 @@ pub enum ConstraintCategory<'tcx> {
     UseAsStatic,
     TypeAnnotation,
     Cast {
-        /// Whether this cast is a coercion.
-        is_coercion: bool,
+        /// Whether this cast is a coercion that was automatically inserted by the compiler.
+        is_implicit_coercion: bool,
         /// Whether this is an unsizing coercion and if yes, this contains the target type.
         /// Region variables are erased to ReErased.
         #[derive_where(skip)]

compiler/rustc_middle/src/mir/statement.rs

@@ -432,7 +432,7 @@ impl<'tcx> Rvalue<'tcx> {
                 | CastKind::IntToFloat
                 | CastKind::FnPtrToPtr
                 | CastKind::PtrToPtr
-                | CastKind::PointerCoercion(_)
+                | CastKind::PointerCoercion(_, _)
                 | CastKind::PointerWithExposedProvenance
                 | CastKind::Transmute,
                 _,

compiler/rustc_middle/src/mir/syntax.rs

@@ -579,7 +579,7 @@ pub struct CopyNonOverlapping<'tcx> {
     pub count: Operand<'tcx>,
 }
 
-/// Represents how a `TerminatorKind::Call` was constructed, used for diagnostics
+/// Represents how a [`TerminatorKind::Call`] was constructed, used for diagnostics.
 #[derive(Clone, Copy, TyEncodable, TyDecodable, Debug, PartialEq, Hash, HashStable)]
 #[derive(TypeFoldable, TypeVisitable)]
 pub enum CallSource {
@@ -1403,7 +1403,7 @@ pub enum CastKind {
     /// * [`PointerCoercion::MutToConstPointer`]
     ///
     /// Both are runtime nops, so should be [`CastKind::PtrToPtr`] instead in runtime MIR.
-    PointerCoercion(PointerCoercion),
+    PointerCoercion(PointerCoercion, CoercionSource),
     IntToInt,
     FloatToInt,
     FloatToFloat,
@@ -1419,6 +1419,15 @@ pub enum CastKind {
     Transmute,
 }
 
+/// Represents how a [`CastKind::PointerCoercion`] was constructed, used for diagnostics.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
+pub enum CoercionSource {
+    /// The coercion was manually written by the user with an `as` cast.
+    AsCast,
+    /// The coercion was automatically inserted by the compiler.
+    Implicit,
+}
+
 #[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, Hash, HashStable)]
 #[derive(TypeFoldable, TypeVisitable)]
 pub enum AggregateKind<'tcx> {