Rollup merge of #121893 - RalfJung:const-interior-mut-tests, r=oli-obk

Add tests (and a bit of cleanup) for interior mut handling in promotion and const-checking

Basically these are the parts of #121786 that can be salvaged.

r? ``@oli-obk``

Author: workingjubilee

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -412,10 +412,10 @@ pub fn const_validate_mplace<'mir, 'tcx>(
             _ if cid.promoted.is_some() => CtfeValidationMode::Promoted,
             Some(mutbl) => CtfeValidationMode::Static { mutbl }, // a `static`
             None => {
-                // In normal `const` (not promoted), the outermost allocation is always only copied,
-                // so having `UnsafeCell` in there is okay despite them being in immutable memory.
-                let allow_immutable_unsafe_cell = cid.promoted.is_none() && !inner;
-                CtfeValidationMode::Const { allow_immutable_unsafe_cell }
+                // This is a normal `const` (not promoted).
+                // The outermost allocation is always only copied, so having `UnsafeCell` in there
+                // is okay despite them being in immutable memory.
+                CtfeValidationMode::Const { allow_immutable_unsafe_cell: !inner }
             }
         };
         ecx.const_validate_operand(&mplace.into(), path, &mut ref_tracking, mode)?;

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -30,7 +30,7 @@ type QualifResults<'mir, 'tcx, Q> =
     rustc_mir_dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'mir, 'mir, 'tcx, Q>>;
 
 #[derive(Default)]
-pub struct Qualifs<'mir, 'tcx> {
+pub(crate) struct Qualifs<'mir, 'tcx> {
     has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
     needs_drop: Option<QualifResults<'mir, 'tcx, NeedsDrop>>,
     needs_non_const_drop: Option<QualifResults<'mir, 'tcx, NeedsNonConstDrop>>,

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -284,6 +284,7 @@ where
                 if Q::in_adt_inherently(cx, def, args) {
                     return true;
                 }
+                // Don't do any value-based reasoning for unions.
                 if def.is_union() && Q::in_any_value_of_ty(cx, rvalue.ty(cx.body, cx.tcx)) {
                     return true;
                 }

tests/ui/consts/enclosing-scope-rule.rs

@@ -0,0 +1,16 @@
+//@build-pass
+// Some code that looks like it might be relying on promotion, but actually this is using the
+// enclosing-scope rule, meaning the reference is "extended" to outlive its block and live as long
+// as the surrounding block (which in this case is the entire program). There are multiple
+// allocations being interned at once.
+
+struct Gen<T>(T);
+impl<'a, T> Gen<&'a T> {
+    // Can't be promoted because `T` might not be `'static`.
+    const C: &'a [T] = &[];
+}
+
+// Can't be promoted because of `Drop`.
+const V: &Vec<i32> = &Vec::new();
+
+fn main() {}

tests/ui/consts/promote-not.rs

@@ -3,6 +3,7 @@
 #![allow(unconditional_panic)]
 
 use std::cell::Cell;
+use std::mem::ManuallyDrop;
 
 // We do not promote mutable references.
 static mut TEST1: Option<&mut [i32]> = Some(&mut [1, 2, 3]); //~ ERROR temporary value dropped while borrowed
@@ -39,7 +40,16 @@ const TEST_INTERIOR_MUT: () = {
     let _val: &'static _ = &(Cell::new(1), 2).1; //~ ERROR temporary value dropped while borrowed
 };
 
-const TEST_DROP: String = String::new();
+// This gets accepted by the "outer scope" rule, not promotion.
+const TEST_DROP_OUTER_SCOPE: &String = &String::new();
+// To demonstrate that, we can rewrite it as follows. If this was promotion it would still work.
+const TEST_DROP_NOT_PROMOTE: &String = {
+    let x = &String::new(); //~ ERROR destructor of `String` cannot be evaluated at compile-time
+    // The "dropped while borrowed" error seems to be suppressed, but the key point is that this
+    // fails to compile.
+    x
+};
+
 
 fn main() {
     // We must not promote things with interior mutability. Not even if we "project it away".
@@ -58,6 +68,7 @@ fn main() {
     let _val: &'static _ = &([1,2,3][4]+1); //~ ERROR temporary value dropped while borrowed
 
     // No promotion of temporaries that need to be dropped.
+    const TEST_DROP: String = String::new();
     let _val: &'static _ = &TEST_DROP;
     //~^ ERROR temporary value dropped while borrowed
     let _val: &'static _ = &&TEST_DROP;
@@ -69,4 +80,12 @@ fn main() {
     let _val: &'static _ = &[&TEST_DROP; 1];
     //~^ ERROR temporary value dropped while borrowed
     //~| ERROR temporary value dropped while borrowed
+
+    // Make sure there is no value-based reasoning for unions.
+    union UnionWithCell {
+        f1: i32,
+        f2: ManuallyDrop<Cell<i32>>,
+    }
+    let x: &'static _ = &UnionWithCell { f1: 0 };
+    //~^ ERROR temporary value dropped while borrowed
 }

tests/ui/consts/promote-not.stderr

@@ -1,5 +1,5 @@
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:8:50
+  --> $DIR/promote-not.rs:9:50
    |
 LL | static mut TEST1: Option<&mut [i32]> = Some(&mut [1, 2, 3]);
    |                                        ----------^^^^^^^^^-
@@ -9,7 +9,7 @@ LL | static mut TEST1: Option<&mut [i32]> = Some(&mut [1, 2, 3]);
    |                                        using this value as a static requires that borrow lasts for `'static`
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:11:18
+  --> $DIR/promote-not.rs:12:18
    |
 LL |     let x = &mut [1,2,3];
    |                  ^^^^^^^ creates a temporary value which is freed while still in use
@@ -19,7 +19,7 @@ LL | };
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:33:29
+  --> $DIR/promote-not.rs:34:29
    |
 LL |     let _x: &'static i32 = &unsafe { U { x: 0 }.x };
    |             ------------    ^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -29,7 +29,7 @@ LL | };
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:39:29
+  --> $DIR/promote-not.rs:40:29
    |
 LL |     let _val: &'static _ = &(Cell::new(1), 2).1;
    |               ----------    ^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -38,8 +38,17 @@ LL |     let _val: &'static _ = &(Cell::new(1), 2).1;
 LL | };
    | - temporary value is freed at the end of this statement
 
+error[E0493]: destructor of `String` cannot be evaluated at compile-time
+  --> $DIR/promote-not.rs:47:14
+   |
+LL |     let x = &String::new();
+   |              ^^^^^^^^^^^^^ the destructor for this type cannot be evaluated in constants
+...
+LL | };
+   | - value is dropped here
+
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:20:32
+  --> $DIR/promote-not.rs:21:32
    |
 LL |         let _x: &'static () = &foo();
    |                 -----------    ^^^^^ creates a temporary value which is freed while still in use
@@ -49,7 +58,7 @@ LL |     }
    |     - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:28:29
+  --> $DIR/promote-not.rs:29:29
    |
 LL |     let _x: &'static i32 = &unsafe { U { x: 0 }.x };
    |             ------------    ^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -59,7 +68,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:46:29
+  --> $DIR/promote-not.rs:56:29
    |
 LL |     let _val: &'static _ = &(Cell::new(1), 2).0;
    |               ----------    ^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -70,7 +79,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:47:29
+  --> $DIR/promote-not.rs:57:29
    |
 LL |     let _val: &'static _ = &(Cell::new(1), 2).1;
    |               ----------    ^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -81,7 +90,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:50:29
+  --> $DIR/promote-not.rs:60:29
    |
 LL |     let _val: &'static _ = &(1/0);
    |               ----------    ^^^^^ creates a temporary value which is freed while still in use
@@ -92,7 +101,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:51:29
+  --> $DIR/promote-not.rs:61:29
    |
 LL |     let _val: &'static _ = &(1/(1-1));
    |               ----------    ^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -103,7 +112,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:52:29
+  --> $DIR/promote-not.rs:62:29
    |
 LL |     let _val: &'static _ = &((1+1)/(1-1));
    |               ----------    ^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -114,7 +123,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:53:29
+  --> $DIR/promote-not.rs:63:29
    |
 LL |     let _val: &'static _ = &(i32::MIN/-1);
    |               ----------    ^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -125,7 +134,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:54:29
+  --> $DIR/promote-not.rs:64:29
    |
 LL |     let _val: &'static _ = &(i32::MIN/(0-1));
    |               ----------    ^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -136,7 +145,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:55:29
+  --> $DIR/promote-not.rs:65:29
    |
 LL |     let _val: &'static _ = &(-128i8/-1);
    |               ----------    ^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -147,7 +156,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:56:29
+  --> $DIR/promote-not.rs:66:29
    |
 LL |     let _val: &'static _ = &(1%0);
    |               ----------    ^^^^^ creates a temporary value which is freed while still in use
@@ -158,7 +167,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:57:29
+  --> $DIR/promote-not.rs:67:29
    |
 LL |     let _val: &'static _ = &(1%(1-1));
    |               ----------    ^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -169,7 +178,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:58:29
+  --> $DIR/promote-not.rs:68:29
    |
 LL |     let _val: &'static _ = &([1,2,3][4]+1);
    |               ----------    ^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -180,7 +189,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:61:29
+  --> $DIR/promote-not.rs:72:29
    |
 LL |     let _val: &'static _ = &TEST_DROP;
    |               ----------    ^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -191,7 +200,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:63:29
+  --> $DIR/promote-not.rs:74:29
    |
 LL |     let _val: &'static _ = &&TEST_DROP;
    |               ----------    ^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -202,7 +211,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:63:30
+  --> $DIR/promote-not.rs:74:30
    |
 LL |     let _val: &'static _ = &&TEST_DROP;
    |               ----------     ^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -213,7 +222,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:66:29
+  --> $DIR/promote-not.rs:77:29
    |
 LL |     let _val: &'static _ = &(&TEST_DROP,);
    |               ----------    ^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -224,7 +233,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:66:31
+  --> $DIR/promote-not.rs:77:31
    |
 LL |     let _val: &'static _ = &(&TEST_DROP,);
    |               ----------      ^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -235,7 +244,7 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:69:29
+  --> $DIR/promote-not.rs:80:29
    |
 LL |     let _val: &'static _ = &[&TEST_DROP; 1];
    |               ----------    ^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
@@ -246,14 +255,26 @@ LL | }
    | - temporary value is freed at the end of this statement
 
 error[E0716]: temporary value dropped while borrowed
-  --> $DIR/promote-not.rs:69:31
+  --> $DIR/promote-not.rs:80:31
    |
 LL |     let _val: &'static _ = &[&TEST_DROP; 1];
    |               ----------      ^^^^^^^^^    - temporary value is freed at the end of this statement
    |               |               |
    |               |               creates a temporary value which is freed while still in use
    |               type annotation requires that borrow lasts for `'static`
 
-error: aborting due to 24 previous errors
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/promote-not.rs:89:26
+   |
+LL |     let x: &'static _ = &UnionWithCell { f1: 0 };
+   |            ----------    ^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary value which is freed while still in use
+   |            |
+   |            type annotation requires that borrow lasts for `'static`
+LL |
+LL | }
+   | - temporary value is freed at the end of this statement
+
+error: aborting due to 26 previous errors
 
-For more information about this error, try `rustc --explain E0716`.
+Some errors have detailed explanations: E0493, E0716.
+For more information about an error, try `rustc --explain E0493`.

tests/ui/consts/refs-to-cell-in-final.rs

@@ -15,4 +15,27 @@ static RAW_SYNC_S: SyncPtr<Cell<i32>> = SyncPtr { x: &Cell::new(42) };
 const RAW_SYNC_C: SyncPtr<Cell<i32>> = SyncPtr { x: &Cell::new(42) };
 //~^ ERROR: cannot refer to interior mutable data
 
+// This one does not get promoted because of `Drop`, and then enters interesting codepaths because
+// as a value it has no interior mutability, but as a type it does. See
+// <https://github.com/rust-lang/rust/issues/121610>. Value-based reasoning for interior mutability
+// is questionable (https://github.com/rust-lang/unsafe-code-guidelines/issues/493) so for now we
+// reject this, though not with a great error message.
+pub enum JsValue {
+    Undefined,
+    Object(Cell<bool>),
+}
+impl Drop for JsValue {
+    fn drop(&mut self) {}
+}
+const UNDEFINED: &JsValue = &JsValue::Undefined;
+//~^ERROR: mutable pointer in final value of constant
+
+// In contrast, this one works since it is being promoted.
+const NONE: &'static Option<Cell<i32>> = &None;
+// Making it clear that this is promotion, not "outer scope".
+const NONE_EXPLICIT_PROMOTED: &'static Option<Cell<i32>> = {
+    let x = &None;
+    x
+};
+
 fn main() {}

tests/ui/consts/refs-to-cell-in-final.stderr

@@ -12,6 +12,12 @@ error[E0492]: constants cannot refer to interior mutable data
 LL | const RAW_SYNC_C: SyncPtr<Cell<i32>> = SyncPtr { x: &Cell::new(42) };
    |                                                     ^^^^^^^^^^^^^^ this borrow of an interior mutable value may end up in the final value
 
-error: aborting due to 2 previous errors
+error: encountered mutable pointer in final value of constant
+  --> $DIR/refs-to-cell-in-final.rs:30:1
+   |
+LL | const UNDEFINED: &JsValue = &JsValue::Undefined;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0492`.