Implement O(1)-time Iterator::nth for Range*, and slim the Step trait

[SimonSapin]

Fixes #43064.
Fixes part of #39975.
Fixes items 1 and 3 of #42168.
CC #27741.

I think #42310 and #43012 should not have landed without the nth part of this PR, but oh well.

[rust-highfive]

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[nox]

Fallible.

[SimonSapin]

Fixed.

[kennytm]

A test case needs to be updated.

[00:40:46] ---- [compile-fail] compile-fail/range-1.rs stdout ----
[00:40:46] 	
[00:40:46] error: /checkout/src/test/compile-fail/range-1.rs:19: expected error not found: `for<'a> &'a bool: std::ops::Add` is not satisfied

[SimonSapin]

Test updated, thanks for pointing this out.

[kennytm]

Need to update run-pass/impl-trait/example-calendar.rs too 😂

Error report

[00:52:28] ---- [run-pass] run-pass/impl-trait/example-calendar.rs stdout ----
[00:52:28] 	
[00:52:28] error: compilation failed!
[00:52:28] status: exit code: 101
[00:52:28] command: /checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc /checkout/src/test/run-pass/impl-trait/example-calendar.rs -L /checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass --target=x86_64-unknown-linux-gnu -L /checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass/impl-trait/example-calendar.stage2-x86_64-unknown-linux-gnu.run-pass.libaux -C prefer-dynamic -o /checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass/impl-trait/example-calendar.stage2-x86_64-unknown-linux-gnu -Crpath -O -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers
[00:52:28] stdout:
[00:52:28] ------------------------------------------
[00:52:28] 
[00:52:28] ------------------------------------------
[00:52:28] stderr:
[00:52:28] ------------------------------------------
[00:52:28] error[E0407]: method `step` is not a member of trait `std::iter::Step`
[00:52:28]    --> /checkout/src/test/run-pass/impl-trait/example-calendar.rs:165:5
[00:52:28]     |
[00:52:28] 165 | /     fn step(&self, by: &Self) -> Option<Self> {
[00:52:28] 166 | |         Some(self + by)
[00:52:28] 167 | |     }
[00:52:28]     | |_____^ not a member of trait `std::iter::Step`
[00:52:28] 
[00:52:28] error[E0407]: method `steps_between_by_one` is not a member of trait `std::iter::Step`
[00:52:28]    --> /checkout/src/test/run-pass/impl-trait/example-calendar.rs:173:5
[00:52:28]     |
[00:52:28] 173 | /     fn steps_between_by_one(_: &Self, _: &Self) -> Option<usize> {
[00:52:28] 174 | |         unimplemented!()
[00:52:28] 175 | |     }
[00:52:28]     | |_____^ not a member of trait `std::iter::Step`
[00:52:28] 
[00:52:28] error[E0407]: method `is_negative` is not a member of trait `std::iter::Step`
[00:52:28]    --> /checkout/src/test/run-pass/impl-trait/example-calendar.rs:177:5
[00:52:28]     |
[00:52:28] 177 | /     fn is_negative(&self) -> bool {
[00:52:28] 178 | |         false
[00:52:28] 179 | |     }
[00:52:28]     | |_____^ not a member of trait `std::iter::Step`
[00:52:28] 
[00:52:28] error[E0050]: method `steps_between` has 3 parameters but the declaration in trait `std::iter::Step::steps_between` has 2
[00:52:28]    --> /checkout/src/test/run-pass/impl-trait/example-calendar.rs:169:45
[00:52:28]     |
[00:52:28] 169 |     fn steps_between(_: &Self, _: &Self, _: &Self) -> Option<usize> {
[00:52:28]     |                                             ^^^^^ expected 2 parameters, found 3
[00:52:28]     |
[00:52:28]     = note: `steps_between` from trait: `fn(&Self, &Self) -> std::option::Option<usize>`
[00:52:28] 
[00:52:28] error[E0046]: not all trait items implemented, missing: `add_n`
[00:52:28]    --> /checkout/src/test/run-pass/impl-trait/example-calendar.rs:164:1
[00:52:28]     |
[00:52:28] 164 | / impl std::iter::Step for NaiveDate {
[00:52:28] 165 | |     fn step(&self, by: &Self) -> Option<Self> {
[00:52:28] 166 | |         Some(self + by)
[00:52:28] 167 | |     }
[00:52:28] ...   |
[00:52:28] 195 | |     }
[00:52:28] 196 | | }
[00:52:28]     | |_^ missing `add_n` in implementation
[00:52:28]     |
[00:52:28]     = note: `add_n` from trait: `fn(&Self, usize) -> std::option::Option<Self>`
[00:52:28] 
[00:52:28] error: aborting due to 5 previous errors
[00:52:28] 
[00:52:28] 
[00:52:28] ------------------------------------------
[00:52:28] 
[00:52:28] thread '[run-pass] run-pass/impl-trait/example-calendar.rs' panicked at 'explicit panic', /checkout/src/tools/compiletest/src/runtest.rs:2473:8
[00:52:28] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[00:52:28] 
[00:52:28] 
[00:52:28] failures:
[00:52:28]     [run-pass] run-pass/impl-trait/example-calendar.rs

[SimonSapin]

Removed the i128/u128 changes as I suspect they were buggy. I’ll look into that separately in another PR.

[alexcrichton]

Thanks! Could you be sure to add some tests for the nth overrides here as well?

[SimonSapin]

Done, in a new commit.

[alexcrichton]

@bors: r+

cc @scottmcm

[bors]

📌 Commit 4e28949 has been approved by alexcrichton

[bors]

🔒 Merge conflict

[eddyb]

@bors r=alexcrichton

[bors]

📌 Commit e9a61ee has been approved by alexcrichton

[bors]

⌛ Testing commit e9a61ee with merge 4d4d76c...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 4d4d76c to master...