Auto merge of #74113 - lcnr:type-dependent-consts-2, r=eddyb

Support const args in type dependent paths (Take 2)

once more, except it is sound this time :smiling_face_with_three_hearts: previously #71154

-----
```rust
#![feature(const_generics)]

struct A;
impl A {
    fn foo<const N: usize>(&self) -> usize { N }
}
struct B;
impl B {
    fn foo<const N: usize>(&self) -> usize { 42 }
}

fn main() {
    let a = A;
    a.foo::<7>();
}
```
When calling `type_of` for generic const arguments, we now use the `TypeckTables` of the surrounding body to get the expected type.

This alone causes cycle errors though, as we now have `typeck_tables_of(main)` -> `...` ->
`type_of(main_ANON0 := 7)` -> `typeck_tables_of(main)` :zap: (see https://github.com/rust-lang/rust/issues/68400#issuecomment-611760290)

To prevent this we must not call `type_of(const_arg)` during `typeck_tables_of`. This is achieved by
calling `type_of(param_def_id)` instead.

We have to somehow remember the `DefId` of the param through all of typeck, which is done using the
struct `ty::WithOptConstParam<DefId>`, which replaces `DefId` where needed and contains an `Option<DefId>` to
be able to store the const parameter in case it exists.

Queries which are currently cached on disk are split into two variants: `query_name`(cached) and `query_name_(of|for)_const_arg`(not cached), with `query_name_of_const_arg` taking a pair `(did, param_did): (LocalDefId, DefId)`.

For some queries a method `query_name_of_opt_const_arg` is added to `TyCtxt` which takes a `ty::WithOptConstParam` and either calls `query_name` or `query_name_of_const_arg` depending on the value of `const_param_did`.

r? @eddyb @varkor

2 files changed, 4 insertions, 4 deletions

diff --git a/src/librustc_codegen_ssa/back/symbol_export.rs b/src/librustc_codegen_ssa/back/symbol_export.rs
index e0910c693b1..3287e7b856a 100644
--- a/src/librustc_codegen_ssa/back/symbol_export.rs
+++ b/src/librustc_codegen_ssa/back/symbol_export.rs
@@ -248,9 +248,9 @@ fn exported_symbols_provider_local(
             }
 
             match *mono_item {
-                MonoItem::Fn(Instance { def: InstanceDef::Item(def_id), substs }) => {
+                MonoItem::Fn(Instance { def: InstanceDef::Item(def), substs }) => {
                     if substs.non_erasable_generics().next().is_some() {
-                        let symbol = ExportedSymbol::Generic(def_id, substs);
+                        let symbol = ExportedSymbol::Generic(def.did, substs);
                         symbols.push((symbol, SymbolExportLevel::Rust));
                     }
                 }
diff --git a/src/librustc_codegen_ssa/mir/constant.rs b/src/librustc_codegen_ssa/mir/constant.rs
index 11ec62f96ed..4943e279c7e 100644
--- a/src/librustc_codegen_ssa/mir/constant.rs
+++ b/src/librustc_codegen_ssa/mir/constant.rs
@@ -25,10 +25,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         constant: &mir::Constant<'tcx>,
     ) -> Result<ConstValue<'tcx>, ErrorHandled> {
         match self.monomorphize(&constant.literal).val {
-            ty::ConstKind::Unevaluated(def_id, substs, promoted) => self
+            ty::ConstKind::Unevaluated(def, substs, promoted) => self
                 .cx
                 .tcx()
-                .const_eval_resolve(ty::ParamEnv::reveal_all(), def_id, substs, promoted, None)
+                .const_eval_resolve(ty::ParamEnv::reveal_all(), def, substs, promoted, None)
                 .map_err(|err| {
                     if promoted.is_none() {
                         self.cx