librustc: Forbid `transmute` from being called on types whose size is

only known post-monomorphization, and report `transmute` errors before
the code is generated for that `transmute`.

This can break code that looked like:

    unsafe fn f<T>(x: T) {
        let y: int = transmute(x);
    }

Change such code to take a type parameter that has the same size as the
type being transmuted to.

Closes #12898.

[breaking-change]

1 files changed, 4 insertions, 2 deletions

diff --git a/src/librustc/plugin/load.rs b/src/librustc/plugin/load.rs
index 97ffcf279ae..7d74b8c7296 100644
--- a/src/librustc/plugin/load.rs
+++ b/src/librustc/plugin/load.rs
@@ -126,9 +126,11 @@ impl<'a> PluginLoader<'a> {
         };
 
         unsafe {
-            let registrar: PluginRegistrarFun =
+            let registrar =
                 match lib.symbol(symbol.as_slice()) {
-                    Ok(registrar) => registrar,
+                    Ok(registrar) => {
+                        mem::transmute::<*u8,PluginRegistrarFun>(registrar)
+                    }
                     // again fatal if we can't register macros
                     Err(err) => self.sess.span_fatal(vi.span, err.as_slice())
                 };