Use #![feature(const_panic)] to produce better assertion errors

Currently, `core_arch` uses `1 / (bool_expr as usize)` to generate a
post-monomorphization error when a condition is false. However, this
approach generates opaque error messages
('evaluation of constant value failed'), which could easily be mistaken
for an internal compiler error by users.

Using the `const_panic` feature, we can `panic!()` with a more
descriptive error message. Unfortunately, we cannot include any of the
actual values in our message, since `const` traits do not yet exist.

Author: Aaron1011

crates/core_arch/src/lib.rs

@@ -7,6 +7,7 @@
     asm,
     const_fn_union,
     const_fn_transmute,
+    const_panic,
     custom_inner_attributes,
     link_llvm_intrinsics,
     platform_intrinsics,

crates/core_arch/src/macros.rs

@@ -5,7 +5,9 @@
 pub(crate) struct ValidateConstImm<const IMM: i32, const MIN: i32, const MAX: i32>;
 impl<const IMM: i32, const MIN: i32, const MAX: i32> ValidateConstImm<IMM, MIN, MAX> {
     pub(crate) const VALID: () = {
-        let _ = 1 / ((IMM >= MIN && IMM <= MAX) as usize);
+        if !(IMM >= MIN && IMM <= MAX) {
+            panic!("IMM value not in expected range");
+        }
     };
 }
 
@@ -71,7 +73,9 @@ macro_rules! static_assert {
         struct Validate<const $imm: $ty>();
         impl<const $imm: $ty> Validate<$imm> {
             const VALID: () = {
-                let _ = 1 / ($e as usize);
+                if !$e {
+                    panic!("Assertion failed");
+                }
             };
         }
         let _ = Validate::<$imm>::VALID;

crates/core_arch/src/x86/macros.rs

@@ -5,7 +5,9 @@
 pub(crate) struct ValidateConstRound<const IMM: i32>;
 impl<const IMM: i32> ValidateConstRound<IMM> {
     pub(crate) const VALID: () = {
-        let _ = 1 / ((IMM == 4 || IMM == 8 || IMM == 9 || IMM == 10 || IMM == 11) as usize);
+        if !(IMM == 4 || IMM == 8 || IMM == 9 || IMM == 10 || IMM == 11) {
+            panic!("Invalid IMM value");
+        }
     };
 }
 
@@ -21,7 +23,9 @@ macro_rules! static_assert_rounding {
 pub(crate) struct ValidateConstSae<const IMM: i32>;
 impl<const IMM: i32> ValidateConstSae<IMM> {
     pub(crate) const VALID: () = {
-        let _ = 1 / ((IMM == 4 || IMM == 8) as usize);
+        if !(IMM == 4 || IMM == 8) {
+            panic!("Invalid IMM value");
+        }
     };
 }
 
@@ -37,7 +41,9 @@ macro_rules! static_assert_sae {
 pub(crate) struct ValidateConstMantissasSae<const IMM: i32>;
 impl<const IMM: i32> ValidateConstMantissasSae<IMM> {
     pub(crate) const VALID: () = {
-        let _ = 1 / ((IMM == 4 || IMM == 8 || IMM == 12) as usize);
+        if !(IMM == 4 || IMM == 8 || IMM == 12) {
+            panic!("Invalid IMM value");
+        }
     };
 }
 
@@ -53,7 +59,9 @@ macro_rules! static_assert_mantissas_sae {
 pub(crate) struct ValidateConstImmU32<const IMM: u32, const MIN: u32, const MAX: u32>;
 impl<const IMM: u32, const MIN: u32, const MAX: u32> ValidateConstImmU32<IMM, MIN, MAX> {
     pub(crate) const VALID: () = {
-        let _ = 1 / ((IMM >= MIN && IMM <= MAX) as usize);
+        if !(IMM >= MIN && IMM <= MAX) {
+            panic!("IMM value not in expected range");
+        }
     };
 }
 
@@ -70,7 +78,9 @@ macro_rules! static_assert_imm_u8 {
 pub(crate) struct ValidateConstGatherScale<const SCALE: i32>;
 impl<const SCALE: i32> ValidateConstGatherScale<SCALE> {
     pub(crate) const VALID: () = {
-        let _ = 1 / ((SCALE == 1 || SCALE == 2 || SCALE == 4 || SCALE == 8) as usize);
+        if !(SCALE == 1 || SCALE == 2 || SCALE == 4 || SCALE == 8) {
+            panic!("Invalid SCALE value");
+        }
     };
 }
 

crates/core_arch/src/x86_64/macros.rs

@@ -5,7 +5,9 @@
 pub(crate) struct ValidateConstRound<const IMM: i32>;
 impl<const IMM: i32> ValidateConstRound<IMM> {
     pub(crate) const VALID: () = {
-        let _ = 1 / ((IMM == 4 || IMM == 8 || IMM == 9 || IMM == 10 || IMM == 11) as usize);
+        if !(IMM == 4 || IMM == 8 || IMM == 9 || IMM == 10 || IMM == 11) {
+            panic!("Invalid IMM value");
+        }
     };
 }
 
@@ -21,7 +23,9 @@ macro_rules! static_assert_rounding {
 pub(crate) struct ValidateConstSae<const IMM: i32>;
 impl<const IMM: i32> ValidateConstSae<IMM> {
     pub(crate) const VALID: () = {
-        let _ = 1 / ((IMM == 4 || IMM == 8) as usize);
+        if !(IMM == 4 || IMM == 8) {
+            panic!("Invalid IMM value");
+        }
     };
 }