Adresses rust-lang#70814

The unsafe code can be justified within range_search, as it makes sure to not
overlap the returned references, but from the callers perspective it's an
entirely safe algorithm and there's no need for the caller to know about the
duplication.

This reverts commit 99492e4.

… r=cramertj

Ignore -Zprofile when building compiler_builtins

rust-lang#70846 made the `compiler_builtins` crate ignore the default codegen-units setting and instead always split each function into a different codegen unit.

This unfortunately breaks `-Zprofile` which requires a single codegen unit per crate (see rust-lang#71283). You can notice this when building with `cargo -Zbuild-std` and `RUSTFLAGS` containing `-Zprofile`.

This PR works around this issue by just ignoring `-Zprofile` for the `compiler-builtins` crate.

…nkov

Fix span of while (let) expressions after lowering

Credit goes to @alex-700 who found this while trying to fix a suggestion in Clippy.

While `if`, `try`, `for` and `await` expressions get the span of the original expression when desugared, `while` loops got the span of the scrutinee, which lead to weird code, when building the suggestion, that randomly worked: https://github.com/rust-lang/rust-clippy/pull/5511/files#diff-df4e9d2bf840a5f2e3b580bef73da3bcR106-R108

I'm wondering, if `DesugaringKind` should get a variant `WhileLoop` and instead of using the span of the `ast::ExprKind::While` expr directly, a new span with `self.mark_span_with_reason` should be used, like it is done with `for` loops.

There was some fallout, but I think that is acceptable. If not, I need some help to find out where this can be fixed.

 Quick and dirty fix of the unused_braces lint

cc @lcnr

Adresses rust-lang#70814

This at least prevents lint output, if no span is available. Even though this also prevents the `unused_parens` lint from emitting, when the `DUMMY_SP` is used there, but I think that should be ok, since error messages without a span are quite useless anyway.

Clippy CI is currently blocked on this bug. If this quick and dirty fix should be rejected, I could try to work around this in Clippy.

r? @shepmaster

…=Amanieu

Take a single root node in range_search

The unsafe code can be justified within range_search, as it makes sure to not
overlap the returned references, but from the callers perspective it's an
entirely safe algorithm and there's no need for the caller to know about the
duplication.

cc @ssomers
r? @Amanieu

…idation-fix, r=Dylan-DPC

Revert PR 70566 for const validation fix

This is a port of PR rust-lang#71441 but ported to the master branch, as discussed in [yesterday's T-compiler meeting](https://zulip-archive.rust-lang.org/131828tcompiler/88751weeklymeeting2020042354818.html#195065903)