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

Implement constant propagation on top of MIR SSA analysis

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

Based on rust-lang#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 rust-lang#109597.

From this abstract representation, we can perform more involved simplifications, for example in rust-lang#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.](rust-lang@2767c49)"

r? `@oli-obk`

Author: bors

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -2,6 +2,7 @@ use std::mem;
 
 use either::{Left, Right};
 
+use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::def::DefKind;
 use rustc_middle::mir::interpret::{AllocId, ErrorHandled, InterpErrorInfo};
 use rustc_middle::mir::pretty::write_allocation_bytes;
@@ -305,10 +306,13 @@ pub fn eval_to_allocation_raw_provider<'tcx>(
         // they do not have to behave "as if" they were evaluated at runtime.
         CompileTimeInterpreter::new(CanAccessStatics::from(is_static), CheckAlignment::Error),
     );
-    eval_in_interpreter(ecx, cid, is_static)
+    // Evaluating this MIR in the interpreter may in turn require evaluating constants to
+    // allocations. As the recursion depends on what the user wrote, we need to protect ourselves
+    // from stack overflow.
+    ensure_sufficient_stack(|| eval_in_interpreter(ecx, cid, is_static))
 }
 
-pub fn eval_in_interpreter<'mir, 'tcx>(
+fn eval_in_interpreter<'mir, 'tcx>(
     mut ecx: InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>>,
     cid: GlobalId<'tcx>,
     is_static: bool,