Auto merge of #101692 - cjgillot:generator-lazy-witness, r=oli-obk

Compute generator saved locals on MIR

Generators are currently type-checked by introducing a `witness` type variable, which is unified with a `GeneratorWitness(captured types)` whose purpose is to ensure that the auto traits correctly migrate from the captured types to the `witness` type.  This requires computing the captured types on HIR during type-checking, only to re-do it on MIR later.

This PR proposes to drop the HIR-based computation, and only keep the MIR one.  This is done in 3 steps.
1. During type-checking, the `witness` type variable is never unified.  This allows to stall all the obligations that depend on it until the end of type-checking.  Then, the stalled obligations are marked as successful, and saved into the typeck results for later verification.
2. At type-checking writeback, `witness` is replaced by `GeneratorWitnessMIR(def_id, substs)`.  From this point on, all trait selection involving `GeneratorWitnessMIR` will fetch the MIR-computed locals, similar to what opaque types do.  There is no lifetime to be preserved here: we consider all the lifetimes appearing in this witness type to be higher-ranked.
3. After borrowck, the stashed obligations are verified against the actually computed types, in the `check_generator_obligations` query.  If any obligation was wrongly marked as fulfilled in step 1, it should be reported here.

There are still many issues:
- ~I am not too happy having to filter out some locals from the checked bounds, I think this is MIR building that introduces raw pointers polluting the analysis;~ solved by a check specific to static variables.
- the diagnostics for captured types don't show where they are used/dropped;
- I do not attempt to support chalk.

cc `@eholk` `@jyn514` for the drop-tracking work
r? `@oli-obk` as you warned me of potential unsoundness

Author: bors

compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs

@@ -414,6 +414,7 @@ fn push_debuginfo_type_name<'tcx>(
         | ty::Placeholder(..)
         | ty::Alias(..)
         | ty::Bound(..)
+        | ty::GeneratorWitnessMIR(..)
         | ty::GeneratorWitness(..) => {
             bug!(
                 "debuginfo: Trying to create type name for \

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -151,7 +151,7 @@ pub(crate) fn const_to_valtree_inner<'tcx>(
         // FIXME(oli-obk): we can probably encode closures just like structs
         | ty::Closure(..)
         | ty::Generator(..)
-        | ty::GeneratorWitness(..) => Err(ValTreeCreationError::NonSupportedType),
+        | ty::GeneratorWitness(..) |ty::GeneratorWitnessMIR(..)=> Err(ValTreeCreationError::NonSupportedType),
     }
 }
 
@@ -314,6 +314,7 @@ pub fn valtree_to_const_value<'tcx>(
         | ty::Closure(..)
         | ty::Generator(..)
         | ty::GeneratorWitness(..)
+        | ty::GeneratorWitnessMIR(..)
         | ty::FnPtr(_)
         | ty::RawPtr(_)
         | ty::Str