Prefer regions with an external_name in approx_universal_upper_bound

[Aaron1011]

Fixes #75785

When displaying a MIR borrowcheck error, we may need to find an upper
bound for a region, which gives us a region to point to in the error
message. However, a region might outlive multiple distinct universal
regions, in which case the only upper bound is 'static

To try to display a meaningful error message, we compute an
'approximate' upper bound by picking one of the universal regions.
Currently, we pick the region with the lowest index - however, this
caused us to produce a suboptimal error message in issue #75785

This PR approx_universal_upper_bound to prefer regions with an
external_name. This causes us to prefer regions from function
arguments/upvars, which seems to lead to a nicer error message in some
cases.

[rust-highfive]

r? @estebank

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[Aaron1011]

@estebank Are there any changes that you'd like me to make?

[SNCPlay42]

Now that the highlight points at the argument instead of the return type, this fixes #74072.

This means this PR will change the output of the test I added in #76468, so it will need rebasing on top of that.

[Aaron1011]

ping @estebank for review or reassignment

[tmandry]

@bors r+

Thanks!

[bors]

📌 Commit 02320d4b4122d1c5f0ccd4ca7e140a79ec5d9695 has been approved by tmandry

[bors]

⌛ Testing commit 02320d4b4122d1c5f0ccd4ca7e140a79ec5d9695 with merge d594f0f4b3745bfd16a2c9a005059bdfc4c19e63...

[rust-log-analyzer]

The job x86_64-gnu-llvm-9 of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

diff of stderr:

2   --> $DIR/issue-74072-lifetime-name-annotations.rs:9:5
3    |
4 LL | pub async fn async_fn(x: &mut i32) -> &i32 {
-    |                                       - let's call the lifetime of this reference `'1`
+    |                          - let's call the lifetime of this reference `'1`
6 LL |     let y = &*x;
7    |             --- borrow of `*x` occurs here
8 LL |     *x += 1;

The actual stderr differed from the expected stderr.
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/async-await/issue-74072-lifetime-name-annotations/issue-74072-lifetime-name-annotations.stderr
Actual stderr saved to /checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/async-await/issue-74072-lifetime-name-annotations/issue-74072-lifetime-name-annotations.stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args async-await/issue-74072-lifetime-name-annotations.rs`
error: 1 errors occurred comparing output.
status: exit code: 1
status: exit code: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/ui/async-await/issue-74072-lifetime-name-annotations.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zemit-future-incompat-report" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/async-await/issue-74072-lifetime-name-annotations" "-A" "unused" "-Crpath" "-O" "-Cdebuginfo=0" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--edition=2018" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/async-await/issue-74072-lifetime-name-annotations/auxiliary"
------------------------------------------

------------------------------------------
stderr:
stderr:
------------------------------------------
error[E0506]: cannot assign to `*x` because it is borrowed
   |
   |
LL | pub async fn async_fn(x: &mut i32) -> &i32 {
   |                          - let's call the lifetime of this reference `'1`
LL |     let y = &*x;
   |             --- borrow of `*x` occurs here
LL |     *x += 1; //~ ERROR cannot assign to `*x` because it is borrowed
   |     ^^^^^^^ assignment to borrowed `*x` occurs here
LL |     y
   |     - returning this value requires that `*x` is borrowed for `'1`

error[E0506]: cannot assign to `*x` because it is borrowed
   |
   |
LL |         let y = &*x;
   |                 --- borrow of `*x` occurs here
LL |         *x += 1; //~ ERROR cannot assign to `*x` because it is borrowed
   |         ^^^^^^^ assignment to borrowed `*x` occurs here
LL |         y
   |         - returning this value requires that `*x` is borrowed for `'1`
LL |     })()
   |     - return type of async closure is &'1 i32

error[E0506]: cannot assign to `*x` because it is borrowed
   |
   |
LL |     (async move || -> &i32 {
   |                       - let's call the lifetime of this reference `'1`
LL |         let y = &*x;
   |                 --- borrow of `*x` occurs here
LL |         *x += 1; //~ ERROR cannot assign to `*x` because it is borrowed
   |         ^^^^^^^ assignment to borrowed `*x` occurs here
LL |         y
   |         - returning this value requires that `*x` is borrowed for `'1`

error[E0506]: cannot assign to `*x` because it is borrowed
   |
   |
LL |         let y = &*x;
   |                 --- borrow of `*x` occurs here
LL |         *x += 1; //~ ERROR cannot assign to `*x` because it is borrowed
   |         ^^^^^^^ assignment to borrowed `*x` occurs here
LL |         y
   |         - returning this value requires that `*x` is borrowed for `'1`
LL |     }
   |     - return type of async block is &'1 i32
error: aborting due to 4 previous errors

For more information about this error, try `rustc --explain E0506`.

---

Some tests failed in compiletest suite=ui mode=ui host=x86_64-unknown-linux-gnu target=x86_64-unknown-linux-gnu


command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/ui" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--suite" "ui" "--mode" "ui" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-9/bin/FileCheck" "--nodejs" "/usr/bin/node" "--host-rustcflags" "-Crpath -O -Cdebuginfo=0 -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--target-rustcflags" "-Crpath -O -Cdebuginfo=0 -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--docck-python" "/usr/bin/python2.7" "--lldb-python" "/usr/bin/python2.7" "--gdb" "/usr/bin/gdb" "--llvm-version" "9.0.0" "--llvm-components" "aarch64 aarch64asmparser aarch64codegen aarch64desc aarch64disassembler aarch64info aarch64utils aggressiveinstcombine all all-targets amdgpu amdgpuasmparser amdgpucodegen amdgpudesc amdgpudisassembler amdgpuinfo amdgpuutils analysis arm armasmparser armcodegen armdesc armdisassembler arminfo armutils asmparser asmprinter avr avrasmparser avrcodegen avrdesc avrdisassembler avrinfo binaryformat bitreader bitstreamreader bitwriter bpf bpfasmparser bpfcodegen bpfdesc bpfdisassembler bpfinfo codegen core coroutines coverage debuginfocodeview debuginfodwarf debuginfogsym debuginfomsf debuginfopdb demangle dlltooldriver engine executionengine fuzzmutate globalisel hexagon hexagonasmparser hexagoncodegen hexagondesc hexagondisassembler hexagoninfo instcombine instrumentation interpreter ipo irreader jitlink lanai lanaiasmparser lanaicodegen lanaidesc lanaidisassembler lanaiinfo libdriver lineeditor linker lto mc mca mcdisassembler mcjit mcparser mips mipsasmparser mipscodegen mipsdesc mipsdisassembler mipsinfo mirparser msp430 msp430asmparser msp430codegen msp430desc msp430disassembler msp430info native nativecodegen nvptx nvptxcodegen nvptxdesc nvptxinfo objcarcopts object objectyaml option orcjit passes perfjitevents powerpc powerpcasmparser powerpccodegen powerpcdesc powerpcdisassembler powerpcinfo profiledata remarks riscv riscvasmparser riscvcodegen riscvdesc riscvdisassembler riscvinfo riscvutils runtimedyld scalaropts selectiondag sparc sparcasmparser sparccodegen sparcdesc sparcdisassembler sparcinfo support symbolize systemz systemzasmparser systemzcodegen systemzdesc systemzdisassembler systemzinfo tablegen target textapi transformutils vectorize webassembly webassemblyasmparser webassemblycodegen webassemblydesc webassemblydisassembler webassemblyinfo windowsmanifest x86 x86asmparser x86codegen x86desc x86disassembler x86info x86utils xcore xcorecodegen xcoredesc xcoredisassembler xcoreinfo xray" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always"


failed to run: /checkout/obj/build/bootstrap/debug/bootstrap --stage 2 test --exclude src/tools/tidy
Build completed unsuccessfully in 0:16:44

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[bors]

💔 Test failed - checks-actions

[Aaron1011]

@bors r=tmandry

[bors]

📌 Commit 419d3ae has been approved by tmandry

[bors]

⌛ Testing commit 419d3ae with merge f3800db...