Auto merge of #116012 - cjgillot:gvn-const, r=oli-obk

Implement constant propagation on top of MIR SSA analysis

This implements the idea I proposed in #110719 (comment)

Based on #109597

The value numbering "GVN" pass formulates each rvalue that appears in MIR with an abstract form (the `Value` enum), and assigns an integer `VnIndex` to each. This abstract form can be used to deduplicate values, reusing an earlier local that holds the same value instead of recomputing. This part is proposed in #109597.

From this abstract representation, we can perform more involved simplifications, for example in #111344.

With the abstract representation `Value`, we can also attempt to evaluate each to a constant using the interpreter. This builds a `VnIndex -> OpTy` map. From this map, we can opportunistically replace an operand or a rvalue with a constant if their value has an associated `OpTy`.

The most relevant commit is [Evaluated computed values to constants.](2767c49)"

r? `@oli-obk`

compiler/rustc_infer/src/infer/relate/generalize.rs

@@ -1,6 +1,7 @@
 use std::mem;
 
 use rustc_data_structures::sso::SsoHashMap;
+use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::def_id::DefId;
 use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
 use rustc_middle::ty::error::TypeError;
@@ -215,7 +216,9 @@ where
         let old_ambient_variance = self.ambient_variance;
         self.ambient_variance = self.ambient_variance.xform(variance);
         debug!(?self.ambient_variance, "new ambient variance");
-        let r = self.relate(a, b)?;
+        // Recursive calls to `relate` can overflow the stack. For example a deeper version of
+        // `ui/associated-consts/issue-93775.rs`.
+        let r = ensure_sufficient_stack(|| self.relate(a, b))?;
         self.ambient_variance = old_ambient_variance;
         Ok(r)
     }