experiment: never promote fn calls

Author: RalfJung

compiler/rustc_mir_transform/src/promote_consts.rs

@@ -11,6 +11,7 @@
 //! This pass assumes that every use is dominated by an
 //! initialization and can otherwise silence errors, if
 //! move analysis runs after promotion on broken MIR.
+#![allow(unused)]
 
 use either::{Left, Right};
 use rustc_data_structures::fx::FxHashSet;
@@ -657,33 +658,8 @@ impl<'tcx> Validator<'_, 'tcx> {
             }
         }
 
-        // Ideally, we'd stop here and reject the rest.
-        // But for backward compatibility, we have to accept some promotion in const/static
-        // initializers. Inline consts are explicitly excluded, they are more recent so we have no
-        // backwards compatibility reason to allow more promotion inside of them.
-        let promote_all_fn = matches!(
-            self.const_kind,
-            Some(hir::ConstContext::Static(_) | hir::ConstContext::Const { inline: false })
-        );
-        if !promote_all_fn {
-            return Err(Unpromotable);
-        }
-        // Make sure the callee is a `const fn`.
-        let is_const_fn = match *fn_ty.kind() {
-            ty::FnDef(def_id, _) => self.tcx.is_const_fn_raw(def_id),
-            _ => false,
-        };
-        if !is_const_fn {
-            return Err(Unpromotable);
-        }
-        // The problem is, this may promote calls to functions that panic.
-        // We don't want to introduce compilation errors if there's a panic in a call in dead code.
-        // So we ensure that this is not dead code.
-        if !self.is_promotion_safe_block(block) {
-            return Err(Unpromotable);
-        }
-        // This passed all checks, so let's accept.
-        Ok(())
+        // Ideally, we'd stop here and reject the rest. So let's do that.
+        return Err(Unpromotable);
     }
 }
 

tests/ui/consts/const-eval/raw-bytes.64bit.stderr

@@ -337,50 +337,32 @@ LL | const SLICE_CONTENT_INVALID: &[bool] = &[unsafe { mem::transmute(3u8) }];
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .<deref>[0]: encountered 0x03, but expected a boolean
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
-   = note: the raw bytes of the constant (size: 8, align: 8) {
-               ╾ALLOC_ID╼                         │ ╾──────╼
+   = note: the raw bytes of the constant (size: 16, align: 8) {
+               ╾ALLOC_ID╼ 01 00 00 00 00 00 00 00 │ ╾──────╼........
            }
 
-note: erroneous constant encountered
-  --> $DIR/raw-bytes.rs:162:40
-   |
-LL | const SLICE_CONTENT_INVALID: &[bool] = &[unsafe { mem::transmute(3u8) }];
-   |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/raw-bytes.rs:168:1
    |
 LL | const MYSLICE_PREFIX_BAD: &MySliceBool = &MySlice(unsafe { mem::transmute(3u8) }, [false]);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .<deref>.0: encountered 0x03, but expected a boolean
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
-   = note: the raw bytes of the constant (size: 8, align: 8) {
-               ╾ALLOC_ID╼                         │ ╾──────╼
+   = note: the raw bytes of the constant (size: 16, align: 8) {
+               ╾ALLOC_ID╼ 01 00 00 00 00 00 00 00 │ ╾──────╼........
            }
 
-note: erroneous constant encountered
-  --> $DIR/raw-bytes.rs:168:42
-   |
-LL | const MYSLICE_PREFIX_BAD: &MySliceBool = &MySlice(unsafe { mem::transmute(3u8) }, [false]);
-   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/raw-bytes.rs:172:1
    |
 LL | const MYSLICE_SUFFIX_BAD: &MySliceBool = &MySlice(true, [unsafe { mem::transmute(3u8) }]);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .<deref>.1[0]: encountered 0x03, but expected a boolean
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
-   = note: the raw bytes of the constant (size: 8, align: 8) {
-               ╾ALLOC_ID╼                         │ ╾──────╼
+   = note: the raw bytes of the constant (size: 16, align: 8) {
+               ╾ALLOC_ID╼ 01 00 00 00 00 00 00 00 │ ╾──────╼........
            }
 
-note: erroneous constant encountered
-  --> $DIR/raw-bytes.rs:172:42
-   |
-LL | const MYSLICE_SUFFIX_BAD: &MySliceBool = &MySlice(true, [unsafe { mem::transmute(3u8) }]);
-   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/raw-bytes.rs:177:1
    |

tests/ui/consts/const-eval/ub-ref-ptr.stderr

@@ -60,12 +60,6 @@ LL | const REF_AS_USIZE_SLICE: &[usize] = &[unsafe { mem::transmute(&0) }];
    = help: this code performed an operation that depends on the underlying bytes representing a pointer
    = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
 
-note: erroneous constant encountered
-  --> $DIR/ub-ref-ptr.rs:36:38
-   |
-LL | const REF_AS_USIZE_SLICE: &[usize] = &[unsafe { mem::transmute(&0) }];
-   |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error[E0080]: evaluation of constant value failed
   --> $DIR/ub-ref-ptr.rs:39:86
    |
@@ -75,12 +69,6 @@ LL | const REF_AS_USIZE_BOX_SLICE: Box<[usize]> = unsafe { mem::transmute::<&[us
    = help: this code performed an operation that depends on the underlying bytes representing a pointer
    = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
 
-note: erroneous constant encountered
-  --> $DIR/ub-ref-ptr.rs:39:85
-   |
-LL | const REF_AS_USIZE_BOX_SLICE: Box<[usize]> = unsafe { mem::transmute::<&[usize], _>(&[mem::transmute(&0)]) };
-   |                                                                                     ^^^^^^^^^^^^^^^^^^^^^
-
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-ref-ptr.rs:42:1
    |

tests/ui/consts/const-eval/ub-wide-ptr.stderr

@@ -139,12 +139,6 @@ LL | const SLICE_CONTENT_INVALID: &[bool] = &[unsafe { mem::transmute(3u8) }];
                HEX_DUMP
            }
 
-note: erroneous constant encountered
-  --> $DIR/ub-wide-ptr.rs:86:40
-   |
-LL | const SLICE_CONTENT_INVALID: &[bool] = &[unsafe { mem::transmute(3u8) }];
-   |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-wide-ptr.rs:93:1
    |
@@ -156,12 +150,6 @@ LL | const MYSLICE_PREFIX_BAD: &MySliceBool = &MySlice(unsafe { mem::transmute(3
                HEX_DUMP
            }
 
-note: erroneous constant encountered
-  --> $DIR/ub-wide-ptr.rs:93:42
-   |
-LL | const MYSLICE_PREFIX_BAD: &MySliceBool = &MySlice(unsafe { mem::transmute(3u8) }, [false]);
-   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-wide-ptr.rs:97:1
    |
@@ -173,12 +161,6 @@ LL | const MYSLICE_SUFFIX_BAD: &MySliceBool = &MySlice(true, [unsafe { mem::tran
                HEX_DUMP
            }
 
-note: erroneous constant encountered
-  --> $DIR/ub-wide-ptr.rs:97:42
-   |
-LL | const MYSLICE_SUFFIX_BAD: &MySliceBool = &MySlice(true, [unsafe { mem::transmute(3u8) }]);
-   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error[E0080]: evaluation of constant value failed
   --> $DIR/ub-wide-ptr.rs:105:1
    |

tests/ui/consts/required-consts/interpret-in-promoted.noopt.stderr

@@ -1,27 +1,13 @@
-error[E0080]: evaluation of constant value failed
-  --> $SRC_DIR/core/src/hint.rs:LL:COL
-   |
-   = note: entering unreachable code
-   |
-note: inside `unreachable_unchecked`
-  --> $SRC_DIR/core/src/hint.rs:LL:COL
-note: inside `ub`
-  --> $DIR/interpret-in-promoted.rs:6:5
-   |
-LL |     std::hint::unreachable_unchecked();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-note: inside `FOO`
+error[E0716]: temporary value dropped while borrowed
   --> $DIR/interpret-in-promoted.rs:12:28
    |
 LL |     let _x: &'static () = &ub();
-   |                            ^^^^
-
-note: erroneous constant encountered
-  --> $DIR/interpret-in-promoted.rs:12:27
-   |
-LL |     let _x: &'static () = &ub();
-   |                           ^^^^^
+   |             -----------    ^^^^ creates a temporary value which is freed while still in use
+   |             |
+   |             type annotation requires that borrow lasts for `'static`
+LL | };
+   | - temporary value is freed at the end of this statement
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0080`.
+For more information about this error, try `rustc --explain E0716`.

tests/ui/consts/required-consts/interpret-in-promoted.opt.stderr

@@ -1,27 +1,13 @@
-error[E0080]: evaluation of constant value failed
-  --> $SRC_DIR/core/src/hint.rs:LL:COL
-   |
-   = note: entering unreachable code
-   |
-note: inside `unreachable_unchecked`
-  --> $SRC_DIR/core/src/hint.rs:LL:COL
-note: inside `ub`
-  --> $DIR/interpret-in-promoted.rs:6:5
-   |
-LL |     std::hint::unreachable_unchecked();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-note: inside `FOO`
+error[E0716]: temporary value dropped while borrowed
   --> $DIR/interpret-in-promoted.rs:12:28
    |
 LL |     let _x: &'static () = &ub();
-   |                            ^^^^
-
-note: erroneous constant encountered
-  --> $DIR/interpret-in-promoted.rs:12:27
-   |
-LL |     let _x: &'static () = &ub();
-   |                           ^^^^^
+   |             -----------    ^^^^ creates a temporary value which is freed while still in use
+   |             |
+   |             type annotation requires that borrow lasts for `'static`
+LL | };
+   | - temporary value is freed at the end of this statement
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0080`.
+For more information about this error, try `rustc --explain E0716`.