This is useful for debugging drop-tracking; previously, you had to recompile
rustc from source and manually add a call to `write_graph_to_file`. This
makes the option more discoverable and configurable at runtime.

I also took the liberty of making the labels for the CFG nodes much easier to read:
previously, they looked like `id(2), local_id: 48`, now they look like
```
expr from_config (hir_id=HirId { owner: DefId(0:10 ~ default_struct_update[79f9]::foo), local_id: 2})
```

Because the `infcx` isn't needed. This removes one lifetime from
`Search`.

Currently, `search_for_structural_match_violation` constructs an `infcx`
from a `tcx` and then only uses the `tcx` within the `infcx`. This is
wasteful because `infcx` is a big type.

This commit changes it to use the `tcx` directly. When compiling
`pest-2.1.3`, this changes the memcpy stats reported by DHAT for a `check full`
build from this:
```
433,008,916 bytes (100%, 99,787.93/Minstr) in 2,148,668 blocks (100%, 495.17/Minstr), avg size 201.52 bytes
```
to this:
```
101,422,347 bytes (99.98%, 25,243.59/Minstr) in 1,318,407 blocks (99.96%, 328.15/Minstr), avg size 76.93 bytes
```
This translates to a 4.3% reduction in instruction counts.

Some of these are a little questionable because the output is so much longer, but I would really love to keep the bit that adds the pretty-printed expression to the generated CFG .dot file.

Before:
```
DEBUG rustc_typeck::check::generator_interior::drop_ranges::record_consumed_borrow consume PlaceWithHirId { hir_id: HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 15 }, place: Place { base_ty: impl std::future::Future<Output = ()>, base: Rvalue, projections: [] } }; diag_expr_id=HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 15 }, using parent expr HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 49 }
```

After:
```
DEBUG rustc_typeck::check::generator_interior::drop_ranges::record_consumed_borrow consume PlaceWithHirId { hir_id: HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 15 }, place: Place { base_ty: impl std::future::Future<Output = ()>, base: Rvalue, projections: [] } }; diag_expr_id=expr from_config(Config {
        nickname: None,
        ..Default::default()
    }) (hir_id=HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 15 }), using parent expr
    .await (hir_id=HirId { owner: DefId(0:7 ~ default_struct_update[79f9]::foo), local_id: 49 })
```

…holk

Add a `-Zdump-drop-tracking-cfg` debugging flag

This is useful for debugging drop-tracking; previously, you had to recompile
rustc from source and manually add a call to `write_graph_to_file`. This
makes the option more discoverable and configurable at runtime.

I also took the liberty of making the labels for the CFG nodes much easier to read:
previously, they looked like `id(2), local_id: 48`, now they look like
```
expr from_config (hir_id=HirId { owner: DefId(0:10 ~ default_struct_update[79f9]::foo), local_id: 2})
```

r? ``@eholk``

An optimization for `pest-2.1.3`

An easy win I found while looking at a profile of `pest-2.1.3`. It's also a small code cleanup.

r? `@pnkfelix`

…ssion-diagnostic-1, r=davidtwco

Migrate some diagnostics from `rustc_const_eval` to `SessionDiagnostic`

I'm still trying to get the hang of this, so it doesn't migrate _all_ of `rustc_const_eval`. Working on that later.

r? `@davidtwco`

…nv, r=michaelwoerister

Highlight conflicting param-env candidates

This could probably be further improved by noting _why_ equivalent param-env candidates (modulo regions) leads to ambiguity.

Fixes rust-lang#98786