PR 107844 made DropAndReplace unused, let's remove it completely
from the codebase.

Retry `pred_known_to_hold_modulo_regions` with fulfillment if ambiguous

Fixes rust-lang#108721

The problem here is that when we're checking `is_sized_raw` during codegen on some type that has a lot of opaques in it, something emits several nested obligations that are individually ambiguous, but when processed together in a loop then apply modulo regions. Since the `evaluate_predicates_recursively` inner loop doesn't process predicates until they stop changing, we return `EvaluatedToAmbig`, which makes the sized check return false incorrectly. See:

https://github.com/rust-lang/rust/blob/f15f0ea73972786e426732c5b92ba9a904b866c4/compiler/rustc_trait_selection/src/traits/select/mod.rs#L596-L606

... Compared to the analogous loop in the new solver:

https://github.com/rust-lang/rust/blob/f15f0ea73972786e426732c5b92ba9a904b866c4/compiler/rustc_trait_selection/src/solve/mod.rs#L481-L512

To fix this, if we get ambiguous during `pred_known_to_hold_modulo_regions`, just retry the obligation in a fulfillment context.

--

Unfortunately... I don't have a test for this. I've only tested this locally. Pending minimization :/

r? types

…t-release-notes-for-1.41.1-apple32, r=Mark-Simulacrum

1.41.1 supported 32-bit Apple targets

This question was raised by rust-lang#108556.

…oot-var, r=lcnr

Canonicalize root var when making response from new solver

During trait solving, if we equate two inference variables `?0` and `?1` but don't equate them with any rigid types, then `InferCtxt::probe_ty_var` will return `Err` for both of these. The canonicalizer code will then canonicalize the variables independently(!), and the response will not reflect the fact that these two variables have been made equal.

This hinders inference and I also don't think it's sound? I haven't thought too much about it past that, so let's talk about it.

r? ``@lcnr``

…asko

Remove DropAndReplace terminator

rust-lang#107844 made DropAndReplace unused, let's remove it completely from the codebase.

Tweak E0740

Also drive-by suppress E0740 if it's an unresolved type.

…k-Simulacrum

Set `LIBC_CHECK_CFG=1` when building Rust code in bootstrap

Downstream forks of the Rust compiler might want to use a custom `libc` to add support for targets that are not yet available upstream. Adding a patch to replace `libc` with a custom one would cause compilation errors though, because Cargo would interpret the custom `libc` as part of the workspace, and apply the check-cfg lints on it.

Since rust-lang/libc#3037, the `libc` build script emits check-cfg flags only when the `LIBC_CHECK_CFG` environment variable is set, so this PR allows the use of custom `libc`s.

…-tester-code, r=notriddle

Improve rustdoc-gui/tester.js code a bit

Just a small clean-up.

r? `@notriddle`

…er-errors

Remove an unused return value in `rustc_hir_typeck`

Unused since rust-lang#89580. Just something I noticed a while ago.

`@rustbot` label C-cleanup