Auto merge of #116281 - Nadrieril:eager-const-eval, r=<try>

Cleanup number handling in match exhaustiveness

Doing a little bit of cleanup; handling number constants was somewhat messy. In particular, this:

- evals float consts once instead of repetitively
- reduces `Constructor` from 88 bytes to 56 (`mir::Const` is big!)

The `fast_try_eval_bits` function was mostly constructed from inlining existing code but I don't fully understand it; I don't follow how consts work and are evaluated very well.

Author: bors

compiler/rustc_middle/src/mir/consts.rs

@@ -293,7 +293,20 @@ impl<'tcx> Const<'tcx> {
 
     #[inline]
     pub fn try_eval_bits(&self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Option<u128> {
-        let int = self.try_eval_scalar_int(tcx, param_env)?;
+        debug_assert!(matches!(
+            self.ty().kind(),
+            ty::Bool | ty::Int(_) | ty::Uint(_) | ty::Float(_) | ty::Char
+        ));
+        let int = match self {
+            // If the constant is already evaluated, we shortcut here.
+            Const::Ty(c) if let ty::ConstKind::Value(valtree) = c.kind() => {
+                valtree.unwrap_leaf()
+            },
+            // This is a more general form of the previous case.
+            _ => {
+                self.try_eval_scalar_int(tcx, param_env)?
+            },
+        };
         let size =
             tcx.layout_of(param_env.with_reveal_all_normalized(tcx).and(self.ty())).ok()?.size;
         int.to_bits(size).ok()