fix/update teach_note from 'escaping mutable ref/ptr' const-check

Author: RalfJung

compiler/rustc_const_eval/messages.ftl

@@ -134,14 +134,16 @@ const_eval_incompatible_return_types =
 const_eval_incompatible_types =
     calling a function with argument of type {$callee_ty} passing data of type {$caller_ty}
 
-const_eval_interior_mutable_data_refer =
+const_eval_interior_mutable_ref_escaping =
     {const_eval_const_context}s cannot refer to interior mutable data
     .label = this borrow of an interior mutable value may end up in the final value
     .help = to fix this, the value can be extracted to a separate `static` item and then referenced
     .teach_note =
-        A constant containing interior mutable data behind a reference can allow you to modify that data.
-        This would make multiple uses of a constant to be able to see different values and allow circumventing
-        the `Send` and `Sync` requirements for shared mutable data, which is unsound.
+        References that escape into the final value of a constant or static must be immutable.
+        This is to avoid accidentally creating shared mutable state.
+
+
+        If you really want global mutable state, try using an interior mutable `static` or a `static mut`.
 
 const_eval_intern_kind = {$kind ->
     [static] static
@@ -229,6 +231,24 @@ const_eval_modified_global =
 
 const_eval_mutable_ptr_in_final = encountered mutable pointer in final value of {const_eval_intern_kind}
 
+const_eval_mutable_raw_escaping =
+    raw mutable pointers are not allowed in the final value of {const_eval_const_context}s
+    .teach_note =
+        Pointers that escape into the final value of a constant or static must be immutable.
+        This is to avoid accidentally creating shared mutable state.
+
+
+        If you really want global mutable state, try using an interior mutable `static` or a `static mut`.
+
+const_eval_mutable_ref_escaping =
+    mutable references are not allowed in the final value of {const_eval_const_context}s
+    .teach_note =
+        References that escape into the final value of a constant or static must be immutable.
+        This is to avoid accidentally creating shared mutable state.
+
+
+        If you really want global mutable state, try using an interior mutable `static` or a `static mut`.
+
 const_eval_nested_static_in_thread_local = #[thread_local] does not support implicit nested statics, please create explicit static items and refer to them instead
 const_eval_non_const_fmt_macro_call =
     cannot call non-const formatting macro in {const_eval_const_context}s
@@ -364,30 +384,11 @@ const_eval_unallowed_fn_pointer_call = function pointer calls are not allowed in
 const_eval_unallowed_heap_allocations =
     allocations are not allowed in {const_eval_const_context}s
     .label = allocation not allowed in {const_eval_const_context}s
-    .teach_note = The value of statics and constants must be known at compile time, and they live for the entire lifetime of a program. Creating a boxed value allocates memory on the heap at runtime, and therefore cannot be done at compile time.
+    .teach_note =
+        The runtime heap is not yet available at compile-time, so no runtime heap allocations can be created.
 
 const_eval_unallowed_inline_asm =
     inline assembly is not allowed in {const_eval_const_context}s
-const_eval_unallowed_mutable_raw =
-    raw mutable pointers are not allowed in the final value of {const_eval_const_context}s
-    .teach_note =
-        References in statics and constants may only refer to immutable values.
-
-
-        Statics are shared everywhere, and if they refer to mutable data one might violate memory
-        safety since holding multiple mutable references to shared data is not allowed.
-
-
-        If you really want global mutable state, try using static mut or a global UnsafeCell.
-
-const_eval_unallowed_mutable_refs =
-    mutable references are not allowed in the final value of {const_eval_const_context}s
-    .teach_note =
-        Statics are shared everywhere, and if they refer to mutable data one might violate memory
-        safety since holding multiple mutable references to shared data is not allowed.
-
-
-        If you really want global mutable state, try using static mut or a global UnsafeCell.
 
 const_eval_unallowed_op_in_const_context =
     {$msg}

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -666,6 +666,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     }
                 }
 
+                // This can be called on stable via the `vec!` macro.
                 if tcx.is_lang_item(callee, LangItem::ExchangeMalloc) {
                     self.check_op(ops::HeapAllocation);
                     return;

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -402,7 +402,7 @@ impl<'tcx> NonConstOp<'tcx> for EscapingCellBorrow {
         DiagImportance::Secondary
     }
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        ccx.dcx().create_err(errors::InteriorMutableDataRefer {
+        ccx.dcx().create_err(errors::InteriorMutableRefEscaping {
             span,
             opt_help: matches!(ccx.const_kind(), hir::ConstContext::Static(_)),
             kind: ccx.const_kind(),
@@ -430,12 +430,12 @@ impl<'tcx> NonConstOp<'tcx> for EscapingMutBorrow {
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
         match self.0 {
-            hir::BorrowKind::Raw => ccx.tcx.dcx().create_err(errors::UnallowedMutableRaw {
+            hir::BorrowKind::Raw => ccx.tcx.dcx().create_err(errors::MutableRawEscaping {
                 span,
                 kind: ccx.const_kind(),
                 teach: ccx.tcx.sess.teach(E0764),
             }),
-            hir::BorrowKind::Ref => ccx.dcx().create_err(errors::UnallowedMutableRefs {
+            hir::BorrowKind::Ref => ccx.dcx().create_err(errors::MutableRefEscaping {
                 span,
                 kind: ccx.const_kind(),
                 teach: ccx.tcx.sess.teach(E0764),

compiler/rustc_const_eval/src/errors.rs

@@ -118,8 +118,8 @@ pub(crate) struct UnstableConstFn {
 }
 
 #[derive(Diagnostic)]
-#[diag(const_eval_unallowed_mutable_refs, code = E0764)]
-pub(crate) struct UnallowedMutableRefs {
+#[diag(const_eval_mutable_ref_escaping, code = E0764)]
+pub(crate) struct MutableRefEscaping {
     #[primary_span]
     pub span: Span,
     pub kind: ConstContext,
@@ -128,8 +128,8 @@ pub(crate) struct UnallowedMutableRefs {
 }
 
 #[derive(Diagnostic)]
-#[diag(const_eval_unallowed_mutable_raw, code = E0764)]
-pub(crate) struct UnallowedMutableRaw {
+#[diag(const_eval_mutable_raw_escaping, code = E0764)]
+pub(crate) struct MutableRawEscaping {
     #[primary_span]
     pub span: Span,
     pub kind: ConstContext,
@@ -181,8 +181,8 @@ pub(crate) struct UnallowedInlineAsm {
 }
 
 #[derive(Diagnostic)]
-#[diag(const_eval_interior_mutable_data_refer, code = E0492)]
-pub(crate) struct InteriorMutableDataRefer {
+#[diag(const_eval_interior_mutable_ref_escaping, code = E0492)]
+pub(crate) struct InteriorMutableRefEscaping {
     #[primary_span]
     #[label]
     pub span: Span,

tests/ui/error-codes/E0010-teach.stderr

@@ -4,7 +4,7 @@ error[E0010]: allocations are not allowed in constants
 LL | const CON: Vec<i32> = vec![1, 2, 3];
    |                       ^^^^^^^^^^^^^ allocation not allowed in constants
    |
-   = note: The value of statics and constants must be known at compile time, and they live for the entire lifetime of a program. Creating a boxed value allocates memory on the heap at runtime, and therefore cannot be done at compile time.
+   = note: The runtime heap is not yet available at compile-time, so no runtime heap allocations can be created.
    = note: this error originates in the macro `vec` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0015]: cannot call non-const fn `slice::<impl [i32]>::into_vec::<std::alloc::Global>` in constants