Auto merge of #46882 - oli-obk:miri3, r=eddyb

Replace all const evaluation with miri

* error reporting in constants prints a stacktrace through all called const fns
* Trivial constant propagation and folding in MIR (always active, irrelevant of the optimization level)
* can now use floating constants in patterns (previously only floating point literals were allowed)
    * the future compat lint is still produced for both cases
* can index into constant arrays during const eval (previously feature gated)
* can create a constant union value with field `a` and read from field `b`
* can dereference references into constants
* can create references inside constants (`const X: &u32 = &22`)
* Tuple struct constructors can be used in constants
* regression in const eval errors spans (some of these need improvements in mir debug info)
* can cast floats to ints and vice versa (in constants, and even nan/inf constants)
* Mir dump prints false/true instead of 0u8/1u8
* `1i8 >> [8][0]` does not lint about exceeding bitshifts anymore.
    * Needs const propagation across projections
* `foo[I]` produces a const eval lint if `foo: [T; N]` and `N < I`
    * Essentially all builtin panics produce lints if they can be statically proven to trigger at runtime. This is on a best effort basis, so there might be some complex cases that don't trigger. (The runtime panic stays there, irrelevant of whether the lint is produced or not)
* can use `union`s to implement `transmute` for `Copy` types in constants without a feature gate. With all the greatness and nasal demons that come with this.
* can convert integers to `&'static T` in constants (useful for embedded)

fixes #34997 (stack overflow with many constants)
fixes #25574 (deref byte strings in patterns)
fixes #27918 (broken mir ICE)
fixes #46114 (ICE on struct constructors in patterns)
fixes #37448 (`SomeStruct { foo } as SomeStruct`)
fixes #43754 (`return` in const fn)
fixes #41898 (tuple struct constructors)
fixes #31364 (infinite recursion with const fn, fixed by miri's recursion limit)
closes #29947 (const indexing stabilization)
fixes #45044 (pattern matching repeat expressions)
fixes #47971 (ICE on const fn + references)
fixes #48081 (ICE on cyclic assoc const error)
fixes #48746 (nonhelpful error message with unions)

r? @eddyb

even though 1k loc are added in tests, this PR reduces the loc in this repository by 700

Author: bors

src/Cargo.lock

@@ -64,7 +64,6 @@ for details on how to format and write long error codes.
   [librustc](https://github.com/rust-lang/rust/blob/master/src/librustc/diagnostics.rs),
   [libsyntax](https://github.com/rust-lang/rust/blob/master/src/libsyntax/diagnostics.rs),
   [librustc_borrowck](https://github.com/rust-lang/rust/blob/master/src/librustc_borrowck/diagnostics.rs),
-  [librustc_const_eval](https://github.com/rust-lang/rust/blob/master/src/librustc_const_eval/diagnostics.rs),
   [librustc_metadata](https://github.com/rust-lang/rust/blob/master/src/librustc_metadata/diagnostics.rs),
   [librustc_mir](https://github.com/rust-lang/rust/blob/master/src/librustc_mir/diagnostics.rs),
   [librustc_passes](https://github.com/rust-lang/rust/blob/master/src/librustc_passes/diagnostics.rs),