[Step 1] Implement "small substs optimization" for substs of length 1

[csmoe]

addresses part of #58310
r?@arielb1

[bors]

☔ The latest upstream changes (presumably #58389) made this pull request unmergeable. Please resolve the merge conflicts.

[oli-obk]

Do you want this merged as a first step in order to reduce the maintainance burden on the full fix?

[csmoe]

@oli-obk yes.(will rebase for merging soon)

[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed on Travis (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.

travis_time:end:1c256f9a:start=1550583781458961467,finish=1550583784336939552,duration=2877978085
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout
Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export IMAGE=x86_64-gnu-llvm-6.0
---
[00:06:24] Compiling rustc_errors v0.0.0 (/checkout/src/librustc_errors)
[00:07:33] Compiling syntax_ext v0.0.0 (/checkout/src/libsyntax_ext)
[00:13:07] Compiling rustc_mir v0.0.0 (/checkout/src/librustc_mir)
[00:13:07] Compiling rustc_typeck v0.0.0 (/checkout/src/librustc_typeck)
[00:13:13] error[E0599]: no method named `subst` found for type `rustc::ty::FnSig<'_>` in the current scope
[00:13:13] --> src/librustc_typeck/check/method/confirm.rs:398:37
[00:13:13] |
[00:13:13] 398 | let method_sig = method_sig.subst(self.tcx, all_substs);
[00:13:13] |
[00:13:13] = help: items from traits can only be used if the trait is in scope
[00:13:13] = note: the following trait is implemented but not in scope, perhaps add a `use` for it:
[00:13:13] `use rustc::ty::subst::Subst;`
---
[00:17:08] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-j" "4" "--release" "--locked" "--color" "always" "--features" "" "--manifest-path" "/checkout/src/rustc/Cargo.toml" "--message-format" "json"
[00:17:08] expected success, got: exit code: 101
[00:17:08] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap build
[00:17:08] Build completed unsuccessfully in 0:12:51
[00:17:08] make: *** [all] Error 1
[00:17:08] Makefile:18: recipe for target 'all' failed
69084 ./obj/build/x86_64-unknown-linux-gnu/stage0-std
60736 ./obj/build/x86_64-unknown-linux-gnu/stage0/bin
59032 ./.git/modules/src/tools
57404 ./src/llvm-project/llvm/test/MC
---
travis_time:end:0af6f738:start=1550584824802203491,finish=1550584824807255653,duration=5052162
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:030a0137
$ ln -s . checkout && for CORE in obj/cores/core.*; do EXE=$(echo $CORE | sed 's|obj/cores/core\.[0-9]*\.!checkout!\(.*\)|\1|;y|!|/|'); if [ -f "$EXE" ]; then printf travis_fold":start:crashlog\n\033[31;1m%s\033[0m\n" "$CORE"; gdb --batch -q -c "$CORE" "$EXE" -iex 'set auto-load off' -iex 'dir src/' -iex 'set sysroot .' -ex bt -ex q; echo travis_fold":"end:crashlog; fi; done || true
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:10752de0
travis_time:start:10752de0
$ cat ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
cat: ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clan

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[bors]

☔ The latest upstream changes (presumably #58592) made this pull request unmergeable. Please resolve the merge conflicts.

[Centril]

Ping from triage, @arielb1, aside from the conflict, can you take a look at this?

[oli-obk]

What do you think about #58310 (comment) (renameing SubstsRef to Substs and getting rid of the old Substs type alias)?

[arielb1]

@csmoe

Do you think this is ready to review for step 1? In that case, remove the WIP tag and change the commit message to mention this is only step 1.

[oli-obk]

Needs a rebase and PR topic adjustments: WIP removed and "Step 1" added

[oli-obk]

@bors r+ p=1 (prioritizing due to extreme bitrotty nature)

[bors]

📌 Commit cf11729 has been approved by oli-obk

[varkor]

Closes #58310

As this is no longer the complete solution, I assume this PR should no longer be marked as fixing #58310?

[oli-obk]

I edited the main description

[bors]

⌛ Testing commit cf11729 with merge f0be457...

[bors]

☀️ Test successful - checks-travis, status-appveyor
Approved by: oli-obk
Pushing f0be457 to master...

[rust-highfive]

📣 Toolstate changed by #58321!

Tested on commit f0be457.
Direct link to PR: #58321

💔 clippy-driver on windows: test-pass → build-fail (cc @Manishearth@llogiq@mcarton@oli-obk@phansch, @rust-lang/infra).
💔 clippy-driver on linux: test-pass → build-fail (cc @Manishearth@llogiq@mcarton@oli-obk@phansch, @rust-lang/infra).
💔 rls on windows: test-pass → build-fail (cc @nrc@Xanewok, @rust-lang/infra).
💔 rls on linux: test-pass → build-fail (cc @nrc@Xanewok, @rust-lang/infra).