Remove unnecessary SpecFromIter impls

[steffahn]

Unless I’m missing something, these SpecFromIter<&'a T, …> for Vec<T> implementations were completely unused.

[rust-highfive]

r? @Mark-Simulacrum

(rust-highfive has picked a reviewer for you, use r? to override)

[Mark-Simulacrum]

Unfortunate that these aren't statically detected but I guess that's true of all (trait) impls...

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit c902fdc with merge 81b35bb706dcef83a467dfb93f9415cea94a3b09...

[bors]

☀️ Try build successful - checks-actions
Build commit: 81b35bb706dcef83a467dfb93f9415cea94a3b09 (81b35bb706dcef83a467dfb93f9415cea94a3b09)

[rust-timer]

Queued 81b35bb706dcef83a467dfb93f9415cea94a3b09 with parent d9feaaa, future comparison URL.

[the8472]

How did you determine that they're unused? They're specialization implementations, so they should be used for particular instances of an iterator. It looks like the last one is supposed to turn a [T].iter().collect() into [T].to_vec().

[the8472]

The trait documentation would be outdated with this change

rust/library/alloc/src/vec/spec_from_iter.rs

Lines 16 to 25 in c902fdc

	/// +-+-------------------------------+ +---------------------+
	/// |SpecFromIter +---->+SpecFromIterNested |
	/// |where I: | | |where I: |
	/// | Iterator (default)----------+ | | Iterator (default) |
	/// | vec::IntoIter | | | TrustedLen |
	/// | SourceIterMarker---fallback-+ | | |
	/// | slice::Iter | | |
	/// | Iterator<Item = &Clone> | +---------------------+
	/// +---------------------------------+
	/// ```

[steffahn]

How did you determine that they're unused?

There are impls of the form SpecFromIter<T, …> for Vec<T> and of the form SpecFromIter<&'a T, …> for Vec<T>. None of the first type of impl can ever overlap with any impl of the second type. I removed all the impls of the second form and std still compiles.

~/forks/rust/library @ master $ rg 'impl.*SpecFromIter<.*for' -A3
alloc/src/vec/source_iter_marker.rs
24:impl<T, I> SpecFromIter<T, I> for Vec<T>
25-where
26-    I: Iterator<Item = T> + SourceIterMarker,
27-{

alloc/src/vec/spec_from_iter.rs
31:impl<T, I> SpecFromIter<T, I> for Vec<T>
32-where
33-    I: Iterator<Item = T>,
34-{
--
40:impl<T> SpecFromIter<T, IntoIter<T>> for Vec<T> {
41-    fn from_iter(iterator: IntoIter<T>) -> Self {
42-        // A common case is passing a vector into a function which immediately
43-        // re-collects into a vector. We can short circuit this if the IntoIter
--
69:impl<'a, T: 'a, I> SpecFromIter<&'a T, I> for Vec<T>
70-where
71-    I: Iterator<Item = &'a T>,
72-    T: Clone,
--
83:impl<'a, T: 'a + Clone> SpecFromIter<&'a T, slice::Iter<'a, T>> for Vec<T> {
84-    #[cfg(not(test))]
85-    fn from_iter(iterator: slice::Iter<'a, T>) -> Self {
86-        iterator.as_slice().to_vec()

the last one is supposed to turn a [T].iter().collect() into […]

This doesn’t even compile.

[steffahn]

The trait documentation would be outdated with this change

Good catch. I’ve added another commit changing those docs.

[the8472]

Ah, I see. Looking through the history it looks this was originally used for the Extend<&'a T> implementation which shared some code with the FromIterator implementations which I then split into separate specializations but didn't clean up.
This would need an equivalent FromIterator<&'a T> implementation to be of any use and that doesn't exist.

👍

[rust-timer]

Finished benchmarking try commit (81b35bb706dcef83a467dfb93f9415cea94a3b09): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

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

[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 5ea3e73 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit 5ea3e73 with merge f1cee2c...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing f1cee2c to master...