promotion: don't promote extern statics

[sjwang05]

Even though we're allowed to reference extern statics, it's impossible to know the value of an extern static during CTFE when it's used by-value, so we shouldn't try promoting them.

fixes #143174

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

r? @oli-obk

rustbot has assigned @oli-obk.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 73 candidates
• Random selection from 20 candidates

[oli-obk]

having to take this apart during promotion is kind of annoying. I wonder if we have enough information during MIR building to just reject creating reads from foreign statics in const contexts entirely.

View changes since the review

[oli-obk]

cc @RalfJung

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_mir_transform/src/promote_consts.rs:360:
         match operand {
             Operand::Copy(place) | Operand::Move(place) => {
                 self.validate_place(place.as_ref())?;
-                if let Some((place_base, ProjectionElem::Deref)) =
-                    place.as_ref().last_projection()
+                if let Some((place_base, ProjectionElem::Deref)) = place.as_ref().last_projection()
                     && let Some(local) = place_base.as_local()
                     && let TempState::Defined { location: loc, .. } = self.temps[local]
                     && let Left(stmt) = self.body.stmt_at(loc)
Diff in /checkout/compiler/rustc_mir_transform/src/promote_consts.rs:368:
-                    && let Some((_, Rvalue::Use(Operand::Constant(c), _))) =
-                        stmt.kind.as_assign()
+                    && let Some((_, Rvalue::Use(Operand::Constant(c), _))) = stmt.kind.as_assign()
                     && let Some(did) = c.check_static_ptr(self.tcx)
                     && self.tcx.is_foreign_item(did)
                 {
Diff in /checkout/compiler/rustc_mir_transform/src/promote_consts.rs:404:
                     if is_thread_local {
                         return Err(Unpromotable);
                     }