Always use a DropTree to build drops

[cjgillot]

MIR building currently has 2 ways to build drops.

A normal exit from a scope uses leave_top_scopes, which calls build_scope_drops. That method generates a compact sequence of StorageDead statements and Drop terminators.

For anormal exits (else, break, continue, return, unwind and coroutine drop), we use a DropTree which is a much more flexible structure in which we record blocks as entry points.

In order to work correctly, build_scope_drops assumes a lot about the structure of the DropTree. This forbids us from unscheduling drops for moved-from locals. This PR removes it altogether, to always use the more flexible DropTree. The generated MIR should be equivalent, but more verbose.

r? @ghost for perf, and because of a borrowck corner case

[cjgillot]

@bors try @rust-timer queue

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

---- [ui] tests/ui/mir/issue-80949.rs stdout ----

error: test compilation failed although it shouldn't!
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/mir/issue-80949.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/mir/issue-80949" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0597]: `storage` does not live long enough
##[error]  --> /checkout/tests/ui/mir/issue-80949.rs:32:23
   |
---
LL |     diff(dyn_trait, storage_ref);
LL | }
   | -
   | |
   | `storage` dropped here while still borrowed
   | borrow might be used here, when `dyn_trait` is dropped and runs the destructor for type `Box<dyn Trait<Item = &()>>`
   |
   = note: values in a scope are dropped in the opposite order they are defined

error: aborting due to 1 previous error

[rust-bors]

☀️ Try build successful (CI)
Build commit: 21c512d (21c512d73ff422b9805f6729b4b9b80672506a07)
Base parent: eff8269 (eff8269f797067c30555e77f160ec84c0ed15cd9)

[rust-timer]

Finished benchmarking commit (21c512d): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.1%, 1.4%]	45
Regressions ❌ (secondary)	1.0%	[0.1%, 3.1%]	53
Improvements ✅ (primary)	-0.4%	[-0.7%, -0.2%]	17
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.2%]	10
All ❌✅ (primary)	0.3%	[-0.7%, 1.4%]	62

Max RSS (memory usage)

Results (primary -1.5%, secondary 2.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-1.5%	[-2.8%, -0.7%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.5%	[-2.8%, -0.7%]	3

Cycles

Results (secondary 4.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	4.2%	[2.2%, 6.9%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.3%, secondary -0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 0.7%]	3
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	3
Improvements ✅ (primary)	-0.4%	[-1.8%, -0.0%]	45
Improvements ✅ (secondary)	-0.2%	[-1.8%, -0.0%]	24
All ❌✅ (primary)	-0.3%	[-1.8%, 0.7%]	48

Bootstrap: 489.126s -> 486.369s (-0.56%)
Artifact size: 389.52 MiB -> 389.47 MiB (-0.02%)