Add feature gate

compiler/rustc_feature/src/active.rs

@@ -516,6 +516,8 @@ declare_features! (
     /// Allows dyn upcasting trait objects via supertraits.
     /// Dyn upcasting is casting, e.g., `dyn Foo -> dyn Bar` where `Foo: Bar`.
     (active, trait_upcasting, "1.56.0", Some(65991), None),
+    /// Allows for transmuting between arrays with sizes that contain generic consts.
+    (active, transmute_generic_consts, "CURRENT_RUSTC_VERSION", Some(109929), None),
     /// Allows #[repr(transparent)] on unions (RFC 2645).
     (active, transparent_unions, "1.37.0", Some(60405), None),
     /// Allows inconsistent bounds in where clauses.

compiler/rustc_hir_typeck/src/intrinsicck.rs

@@ -88,7 +88,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 if let Some(size) = size.try_eval_target_usize(tcx, self.param_env) {
                     format!("{size} bytes")
                 } else {
-                    format!("generic size")
+                    format!("generic size {size}")
                 }
             }
             Err(LayoutError::Unknown(bad)) => {

compiler/rustc_middle/src/ty/layout.rs

@@ -332,7 +332,9 @@ impl<'tcx> SizeSkeleton<'tcx> {
                     ),
                 }
             }
-            ty::Array(inner, len) if len.ty() == tcx.types.usize => {
+            ty::Array(inner, len)
+                if len.ty() == tcx.types.usize && tcx.features().transmute_generic_consts =>
+            {
                 match SizeSkeleton::compute(inner, tcx, param_env)? {
                     // This may succeed because the multiplication of two types may overflow
                     // but a single size of a nested array will not.
@@ -483,17 +485,17 @@ fn mul_sorted_consts<'tcx>(
     }
     let mut k = 1;
     let mut overflow = false;
-    for _ in done.drain_filter(|c| {
+    done.retain(|c| {
         let Some(c) = c.try_eval_target_usize(tcx, param_env) else {
-            return false;
+            return true;
         };
         let Some(next) = c.checked_mul(k) else {
             overflow = true;
-            return true;
+            return false;
         };
-        k *= next;
-        true
-    }) {}
+        k = next;
+        false
+    });
     if overflow {
         return None;
     }

compiler/rustc_span/src/symbol.rs

@@ -1493,6 +1493,7 @@ symbols! {
         trait_alias,
         trait_upcasting,
         transmute,
+        transmute_generic_consts,
         transmute_opts,
         transmute_trait,
         transparent,

tests/ui/const-generics/transmute-fail.rs

@@ -1,3 +1,4 @@
+#![feature(transmute_generic_consts)]
 #![feature(generic_const_exprs)]
 #![allow(incomplete_features)]
 

tests/ui/const-generics/transmute-fail.stderr

@@ -1,14 +1,14 @@
 error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
-  --> $DIR/transmute-fail.rs:6:5
+  --> $DIR/transmute-fail.rs:7:5
    |
 LL |     std::mem::transmute(v)
    |     ^^^^^^^^^^^^^^^^^^^
    |
-   = note: source type: `[[u32; H+1]; W]` (generic size)
-   = note: target type: `[[u32; W+1]; H]` (generic size)
+   = note: source type: `[[u32; H+1]; W]` (generic size [const expr])
+   = note: target type: `[[u32; W+1]; H]` (generic size [const expr])
 
 error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
-  --> $DIR/transmute-fail.rs:15:5
+  --> $DIR/transmute-fail.rs:16:5
    |
 LL |     std::mem::transmute(v)
    |     ^^^^^^^^^^^^^^^^^^^
@@ -17,28 +17,28 @@ LL |     std::mem::transmute(v)
    = note: target type: `[[u32; W]; H]` (size can vary because of [u32; W])
 
 error[E0308]: mismatched types
-  --> $DIR/transmute-fail.rs:11:53
+  --> $DIR/transmute-fail.rs:12:53
    |
 LL | fn bar<const W: bool, const H: usize>(v: [[u32; H]; W]) -> [[u32; W]; H] {
    |                                                     ^ expected `usize`, found `bool`
 
 error[E0308]: mismatched types
-  --> $DIR/transmute-fail.rs:11:67
+  --> $DIR/transmute-fail.rs:12:67
    |
 LL | fn bar<const W: bool, const H: usize>(v: [[u32; H]; W]) -> [[u32; W]; H] {
    |                                                                   ^ expected `usize`, found `bool`
 
 error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
-  --> $DIR/transmute-fail.rs:22:5
+  --> $DIR/transmute-fail.rs:23:5
    |
 LL |     std::mem::transmute(v)
    |     ^^^^^^^^^^^^^^^^^^^
    |
-   = note: source type: `[[u32; H]; W]` (generic size)
-   = note: target type: `[u32; W * H * H]` (generic size)
+   = note: source type: `[[u32; H]; W]` (generic size [const expr])
+   = note: target type: `[u32; W * H * H]` (generic size [const expr])
 
 error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
-  --> $DIR/transmute-fail.rs:29:5
+  --> $DIR/transmute-fail.rs:30:5
    |
 LL |     std::mem::transmute(v)
    |     ^^^^^^^^^^^^^^^^^^^

tests/ui/const-generics/transmute.rs

@@ -1,5 +1,6 @@
 // run-pass
 #![feature(generic_const_exprs)]
+#![feature(transmute_generic_consts)]
 #![allow(incomplete_features)]
 
 fn transpose<const W: usize, const H: usize>(v: [[u32;H]; W]) -> [[u32; W]; H] {

tests/ui/const-generics/transmute_no_gate.rs

@@ -0,0 +1,91 @@
+// gate-test-transmute_generic_consts
+#![feature(generic_const_exprs)]
+#![allow(incomplete_features)]
+
+fn transpose<const W: usize, const H: usize>(v: [[u32;H]; W]) -> [[u32; W]; H] {
+  unsafe {
+    std::mem::transmute(v)
+    //~^ ERROR cannot transmute
+  }
+}
+
+fn ident<const W: usize, const H: usize>(v: [[u32; H]; W]) -> [[u32; H]; W] {
+  unsafe {
+    std::mem::transmute(v)
+  }
+}
+
+fn flatten<const W: usize, const H: usize>(v: [[u32; H]; W]) -> [u32; W * H] {
+  unsafe {
+    std::mem::transmute(v)
+    //~^ ERROR cannot transmute
+  }
+}
+
+fn coagulate<const W: usize, const H: usize>(v: [u32; H*W]) -> [[u32; W];H] {
+  unsafe {
+    std::mem::transmute(v)
+    //~^ ERROR cannot transmute
+  }
+}
+
+fn flatten_3d<const W: usize, const H: usize, const D: usize>(
+  v: [[[u32; D]; H]; W]
+) -> [u32; D * W * H] {
+  unsafe {
+    std::mem::transmute(v)
+    //~^ ERROR cannot transmute
+  }
+}
+
+fn flatten_somewhat<const W: usize, const H: usize, const D: usize>(
+  v: [[[u32; D]; H]; W]
+) -> [[u32; D * W]; H] {
+  unsafe {
+    std::mem::transmute(v)
+    //~^ ERROR cannot transmute
+  }
+}
+
+fn known_size<const L: usize>(v: [u16; L]) -> [u8; L * 2] {
+  unsafe {
+    std::mem::transmute(v)
+    //~^ ERROR cannot transmute
+  }
+}
+
+fn condense_bytes<const L: usize>(v: [u8; L * 2]) -> [u16; L] {
+  unsafe {
+    std::mem::transmute(v)
+    //~^ ERROR cannot transmute
+  }
+}
+
+fn singleton_each<const L: usize>(v: [u8; L]) -> [[u8;1]; L] {
+  unsafe {
+    std::mem::transmute(v)
+    //~^ ERROR cannot transmute
+  }
+}
+
+fn transpose_with_const<const W: usize, const H: usize>(
+  v: [[u32; 2 * H]; W + W]
+) -> [[u32; W + W]; 2 * H] {
+  unsafe {
+    std::mem::transmute(v)
+    //~^ ERROR cannot transmute
+  }
+}
+
+fn main() {
+  let _ = transpose([[0; 8]; 16]);
+  let _ = transpose_with_const::<8,4>([[0; 8]; 16]);
+  let _ = ident([[0; 8]; 16]);
+  let _ = flatten([[0; 13]; 5]);
+  let _: [[_; 5]; 13] = coagulate([0; 65]);
+  let _ = flatten_3d([[[0; 3]; 13]; 5]);
+  let _ = flatten_somewhat([[[0; 3]; 13]; 5]);
+  let _ = known_size([16; 13]);
+  let _: [u16; 5] = condense_bytes([16u8; 10]);
+  let _ = singleton_each([16; 10]);
+}

tests/ui/const-generics/transmute_no_gate.stderr

@@ -0,0 +1,84 @@
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute_no_gate.rs:7:5
+   |
+LL |     std::mem::transmute(v)
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `[[u32; H]; W]` (this type does not have a fixed size)
+   = note: target type: `[[u32; W]; H]` (this type does not have a fixed size)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute_no_gate.rs:20:5
+   |
+LL |     std::mem::transmute(v)
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `[[u32; H]; W]` (this type does not have a fixed size)
+   = note: target type: `[u32; W * H]` (this type does not have a fixed size)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute_no_gate.rs:27:5
+   |
+LL |     std::mem::transmute(v)
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `[u32; H*W]` (this type does not have a fixed size)
+   = note: target type: `[[u32; W]; H]` (this type does not have a fixed size)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute_no_gate.rs:36:5
+   |
+LL |     std::mem::transmute(v)
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `[[[u32; D]; H]; W]` (this type does not have a fixed size)
+   = note: target type: `[u32; D * W * H]` (this type does not have a fixed size)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute_no_gate.rs:45:5
+   |
+LL |     std::mem::transmute(v)
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `[[[u32; D]; H]; W]` (this type does not have a fixed size)
+   = note: target type: `[[u32; D * W]; H]` (this type does not have a fixed size)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute_no_gate.rs:52:5
+   |
+LL |     std::mem::transmute(v)
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `[u16; L]` (this type does not have a fixed size)
+   = note: target type: `[u8; L * 2]` (this type does not have a fixed size)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute_no_gate.rs:59:5
+   |
+LL |     std::mem::transmute(v)
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `[u8; L * 2]` (this type does not have a fixed size)
+   = note: target type: `[u16; L]` (this type does not have a fixed size)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute_no_gate.rs:66:5
+   |
+LL |     std::mem::transmute(v)
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `[u8; L]` (this type does not have a fixed size)
+   = note: target type: `[[u8; 1]; L]` (this type does not have a fixed size)
+
+error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
+  --> $DIR/transmute_no_gate.rs:75:5
+   |
+LL |     std::mem::transmute(v)
+   |     ^^^^^^^^^^^^^^^^^^^
+   |
+   = note: source type: `[[u32; 2 * H]; W + W]` (this type does not have a fixed size)
+   = note: target type: `[[u32; W + W]; 2 * H]` (this type does not have a fixed size)
+
+error: aborting due to 9 previous errors
+
+For more information about this error, try `rustc --explain E0512`.