Rollup merge of #72934 - christianpoveda:mut-borrows-in-consts, r=oli-obk

forbid mutable references in all constant contexts except for const-fns

PR to address #71212

cc: @ecstatic-morse

Author: Manishearth

src/librustc_error_codes/error_codes.rs

@@ -444,6 +444,7 @@ E0760: include_str!("./error_codes/E0760.md"),
 E0761: include_str!("./error_codes/E0761.md"),
 E0762: include_str!("./error_codes/E0762.md"),
 E0763: include_str!("./error_codes/E0763.md"),
+E0764: include_str!("./error_codes/E0764.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

src/librustc_error_codes/error_codes/E0764.md

@@ -0,0 +1,39 @@
+Mutable references (`&mut`) can only be used in constant functions, not statics
+or constants. This limitation exists to prevent the creation of constants that
+have a mutable reference in their final value. If you had a constant of `&mut
+i32` type, you could modify the value through that reference, making the
+constant essentially mutable. While there could be a more fine-grained scheme
+in the future that allows mutable references if they are not "leaked" to the
+final value, a more conservative approach was chosen for now. `const fn` do not
+have this problem, as the borrow checker will prevent the `const fn` from
+returning new mutable references.
+
+Erroneous code example:
+
+```compile_fail,E0764
+#![feature(const_fn)]
+#![feature(const_mut_refs)]
+
+fn main() {
+    const OH_NO: &'static mut usize = &mut 1; // error!
+}
+```
+
+Remember: you cannot use a function call inside a constant or static. However,
+you can totally use it in constant functions:
+
+```
+#![feature(const_fn)]
+#![feature(const_mut_refs)]
+
+const fn foo(x: usize) -> usize {
+    let mut y = 1;
+    let z = &mut y;
+    *z += x;
+    y
+}
+
+fn main() {
+    const FOO: usize = foo(10); // ok!
+}
+```

src/librustc_mir/transform/check_consts/ops.rs

@@ -205,22 +205,34 @@ impl NonConstOp for CellBorrow {
 #[derive(Debug)]
 pub struct MutBorrow;
 impl NonConstOp for MutBorrow {
+    fn is_allowed_in_item(&self, ccx: &ConstCx<'_, '_>) -> bool {
+        // Forbid everywhere except in const fn
+        ccx.const_kind() == hir::ConstContext::ConstFn
+            && ccx.tcx.features().enabled(Self::feature_gate().unwrap())
+    }
+
     fn feature_gate() -> Option<Symbol> {
         Some(sym::const_mut_refs)
     }
 
     fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
-        let mut err = feature_err(
-            &ccx.tcx.sess.parse_sess,
-            sym::const_mut_refs,
-            span,
-            &format!(
-                "references in {}s may only refer \
-                      to immutable values",
-                ccx.const_kind()
-            ),
-        );
-        err.span_label(span, format!("{}s require immutable values", ccx.const_kind()));
+        let mut err = if ccx.const_kind() == hir::ConstContext::ConstFn {
+            feature_err(
+                &ccx.tcx.sess.parse_sess,
+                sym::const_mut_refs,
+                span,
+                &format!("mutable references are not allowed in {}s", ccx.const_kind()),
+            )
+        } else {
+            struct_span_err!(
+                ccx.tcx.sess,
+                span,
+                E0764,
+                "mutable references are not allowed in {}s",
+                ccx.const_kind(),
+            )
+        };
+        err.span_label(span, "`&mut` is only allowed in `const fn`".to_string());
         if ccx.tcx.sess.teach(&err.get_code().unwrap()) {
             err.note(
                 "References in statics and constants may only refer \

src/test/compile-fail/issue-52443.rs

@@ -9,7 +9,7 @@ fn main() {
     [(); { for _ in 0usize.. {}; 0}];
     //~^ ERROR `for` is not allowed in a `const`
     //~| ERROR calls in constants are limited to constant functions
-    //~| ERROR references in constants may only refer to immutable values
+    //~| ERROR mutable references are not allowed in constants
     //~| ERROR calls in constants are limited to constant functions
     //~| ERROR evaluation of constant value failed
 }

src/test/ui/check-static-immutable-mut-slices.rs

@@ -1,6 +1,6 @@
 // Checks that immutable static items can't have mutable slices
 
 static TEST: &'static mut [isize] = &mut [];
-//~^ ERROR references in statics may only refer to immutable values
+//~^ ERROR mutable references are not allowed in statics
 
 pub fn main() { }

src/test/ui/check-static-immutable-mut-slices.stderr

@@ -1,12 +1,9 @@
-error[E0658]: references in statics may only refer to immutable values
+error[E0764]: mutable references are not allowed in statics
   --> $DIR/check-static-immutable-mut-slices.rs:3:37
    |
 LL | static TEST: &'static mut [isize] = &mut [];
-   |                                     ^^^^^^^ statics require immutable values
-   |
-   = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
-   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
+   |                                     ^^^^^^^ `&mut` is only allowed in `const fn`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0658`.
+For more information about this error, try `rustc --explain E0764`.

src/test/ui/consts/const-eval/issue-65394.rs

@@ -5,7 +5,7 @@
 
 const _: Vec<i32> = {
     let mut x = Vec::<i32>::new(); //~ ERROR destructors cannot be evaluated at compile-time
-    let r = &mut x; //~ ERROR references in constants may only refer to immutable values
+    let r = &mut x; //~ ERROR mutable references are not allowed in constants
     let y = x;
     y
 };

src/test/ui/consts/const-eval/issue-65394.stderr

@@ -1,11 +1,8 @@
-error[E0658]: references in constants may only refer to immutable values
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/issue-65394.rs:8:13
    |
 LL |     let r = &mut x;
-   |             ^^^^^^ constants require immutable values
-   |
-   = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
-   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
+   |             ^^^^^^ `&mut` is only allowed in `const fn`
 
 error[E0493]: destructors cannot be evaluated at compile-time
   --> $DIR/issue-65394.rs:7:9
@@ -15,5 +12,5 @@ LL |     let mut x = Vec::<i32>::new();
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0493, E0658.
+Some errors have detailed explanations: E0493, E0764.
 For more information about an error, try `rustc --explain E0493`.

src/test/ui/consts/const-multi-ref.rs

@@ -3,7 +3,7 @@
 
 const _: i32 = {
     let mut a = 5;
-    let p = &mut a; //~ ERROR references in constants may only refer to immutable values
+    let p = &mut a; //~ ERROR mutable references are not allowed in constants
 
     let reborrow = {p};
     let pp = &reborrow;

src/test/ui/consts/const-multi-ref.stderr

@@ -1,11 +1,8 @@
-error[E0658]: references in constants may only refer to immutable values
+error[E0764]: mutable references are not allowed in constants
   --> $DIR/const-multi-ref.rs:6:13
    |
 LL |     let p = &mut a;
-   |             ^^^^^^ constants require immutable values
-   |
-   = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
-   = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
+   |             ^^^^^^ `&mut` is only allowed in `const fn`
 
 error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead
   --> $DIR/const-multi-ref.rs:16:13
@@ -15,5 +12,5 @@ LL |     let p = &a;
 
 error: aborting due to 2 previous errors
 
-Some errors have detailed explanations: E0492, E0658.
+Some errors have detailed explanations: E0492, E0764.
 For more information about an error, try `rustc --explain E0492`.