Add a note for implicit temporaries on &mut (fn or const)

Author: sharnoff

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -1836,6 +1836,8 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             "value referencing"
         };
 
+        let mut implicit_temporary = None;
+
         let (place_desc, note) = if let Some(place_desc) = opt_place_desc {
             let local_kind = if let Some(local) = borrow.borrowed_place.as_local() {
                 match self.body.local_kind(local) {
@@ -1861,7 +1863,22 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             let root_place =
                 self.prefixes(borrow.borrowed_place.as_ref(), PrefixSet::All).last().unwrap();
             let local = root_place.local;
-            match self.body.local_kind(local) {
+            let local_kind = self.body.local_kind(local);
+            if let LocalKind::Temp = local_kind {
+                // Mutable references to constants and functions will implicitly create
+                // temporaries, even though immutable references don't. We use these below to add a
+                // note about this.
+                match &self.body.local_decls[local].local_info {
+                    Some(box LocalInfo::FnDefRef) => {
+                        implicit_temporary = Some(("function", "function pointer"));
+                    }
+                    Some(box LocalInfo::ConstRef { .. }) => {
+                        implicit_temporary = Some(("constant", "value"));
+                    }
+                    _ => {}
+                }
+            }
+            match local_kind {
                 LocalKind::ReturnPointer | LocalKind::Temp => {
                     ("temporary value".to_string(), "temporary value created here".to_string())
                 }
@@ -1906,6 +1923,13 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             }
         }
 
+        if let Some((item_name, value_desc)) = implicit_temporary {
+            err.note(format!(
+                "mutably borrowing a {} implicitly copies the {} to a temporary",
+                item_name, value_desc
+            ));
+        }
+
         Some(err)
     }
 

compiler/rustc_middle/src/mir/mod.rs

@@ -900,6 +900,8 @@ pub enum LocalInfo<'tcx> {
     StaticRef { def_id: DefId, is_thread_local: bool },
     /// A temporary created that references the const with the given `DefId`
     ConstRef { def_id: DefId },
+    /// A temporary created that references a function
+    FnDefRef,
     /// A temporary created during the creation of an aggregate
     /// (e.g. a temporary for `foo` in `MyStruct { my_field: foo }`)
     AggregateTemp,

compiler/rustc_middle/src/thir.rs

@@ -432,8 +432,8 @@ pub enum ExprKind<'tcx> {
         lit: ty::ScalarInt,
         user_ty: UserTy<'tcx>,
     },
-    /// A literal of a ZST type.
-    ZstLiteral {
+    /// Named functions, associated functions, and constructors (including `Self(...)`)
+    NamedFn {
         user_ty: UserTy<'tcx>,
     },
     /// Associated constants and named constants

compiler/rustc_middle/src/thir/visit.rs

@@ -136,7 +136,7 @@ pub fn walk_expr<'a, 'tcx: 'a, V: Visitor<'a, 'tcx>>(visitor: &mut V, expr: &Exp
         }) => {}
         Literal { lit: _, neg: _ } => {}
         NonHirLiteral { lit: _, user_ty: _ } => {}
-        ZstLiteral { user_ty: _ } => {}
+        NamedFn { user_ty: _ } => {}
         NamedConst { def_id: _, substs: _, user_ty: _ } => {}
         ConstParam { param: _, def_id: _ } => {}
         StaticRef { alloc_id: _, ty: _, def_id: _ } => {}

compiler/rustc_mir_build/src/build/custom/parse/instruction.rs

@@ -153,7 +153,7 @@ impl<'tcx, 'body> ParseCtxt<'tcx, 'body> {
             ExprKind::Literal { .. }
             | ExprKind::NamedConst { .. }
             | ExprKind::NonHirLiteral { .. }
-            | ExprKind::ZstLiteral { .. }
+            | ExprKind::NamedFn { .. }
             | ExprKind::ConstParam { .. }
             | ExprKind::ConstBlock { .. } => {
                 Ok(Operand::Constant(Box::new(

compiler/rustc_mir_build/src/build/expr/as_constant.rs

@@ -68,7 +68,7 @@ pub fn as_constant_inner<'tcx>(
 
             Constant { span, user_ty, literal }
         }
-        ExprKind::ZstLiteral { ref user_ty } => {
+        ExprKind::NamedFn { ref user_ty } => {
             let user_ty = user_ty.as_ref().map(push_cuta).flatten();
 
             let literal = ConstantKind::Val(ConstValue::ZeroSized, ty);

compiler/rustc_mir_build/src/build/expr/as_place.rs

@@ -554,7 +554,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             | ExprKind::Literal { .. }
             | ExprKind::NamedConst { .. }
             | ExprKind::NonHirLiteral { .. }
-            | ExprKind::ZstLiteral { .. }
+            | ExprKind::NamedFn { .. }
             | ExprKind::ConstParam { .. }
             | ExprKind::ConstBlock { .. }
             | ExprKind::StaticRef { .. }

compiler/rustc_mir_build/src/build/expr/as_rvalue.rs

@@ -459,7 +459,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             ExprKind::Literal { .. }
             | ExprKind::NamedConst { .. }
             | ExprKind::NonHirLiteral { .. }
-            | ExprKind::ZstLiteral { .. }
+            | ExprKind::NamedFn { .. }
             | ExprKind::ConstParam { .. }
             | ExprKind::ConstBlock { .. }
             | ExprKind::StaticRef { .. } => {

compiler/rustc_mir_build/src/build/expr/as_temp.rs

@@ -70,6 +70,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 ExprKind::NamedConst { def_id, .. } | ExprKind::ConstParam { def_id, .. } => {
                     local_decl.local_info = Some(Box::new(LocalInfo::ConstRef { def_id }));
                 }
+                ExprKind::NamedFn { .. } => {
+                    local_decl.local_info = Some(Box::new(LocalInfo::FnDefRef));
+                }
                 _ => {}
             }
             this.local_decls.push(local_decl)

compiler/rustc_mir_build/src/build/expr/category.rs

@@ -72,7 +72,7 @@ impl Category {
             ExprKind::ConstBlock { .. }
             | ExprKind::Literal { .. }
             | ExprKind::NonHirLiteral { .. }
-            | ExprKind::ZstLiteral { .. }
+            | ExprKind::NamedFn { .. }
             | ExprKind::ConstParam { .. }
             | ExprKind::StaticRef { .. }
             | ExprKind::NamedConst { .. } => Some(Category::Constant),