[debuginfo] Make cpp-like debuginfo type names for slices and str consistent.

[michaelwoerister]

Before this PR, the compiler would emit the debuginfo name slice$<T> for all kinds of slices, regardless of whether they are behind a reference or not and regardless of the kind of reference. As a consequence, the types Foo<&[T]>, Foo<[T]>, and Foo<&mut [T]> would end up with the same type name Foo<slice$<T> > in debuginfo, making it impossible to disambiguate between them by name. Similarly, &str would get the name str in debuginfo, so the debuginfo name for Foo<str> and Foo<&str> would be the same. In contrast, *const [bool] and *mut [bool] would be ptr_const$<slice$<bool> > and ptr_mut$<slice$<bool> >, i.e. the encoding does not lose information about the type.

This PR removes all special handling for slices and str. The types &[bool], &mut [bool], and &str thus get the names ref$<slice2$<bool> >, ref_mut$<slice2$<bool> >, and ref$<str$> respectively -- as one would expect.

The new special name for slices is slice2$ to differentiate it from the previous name slice$, which has different semantics. The same is true for str and str$. This kind of versioning already has a precedent with the case of enum$ and enum2$ and hopefully will make it easier to transition existing consumers of these names.

cc @rust-lang/wg-debugging @vadimcn

r? @wesleywiser

UPDATE: Here is a table to clarify the changes

Rust type	DWARF name	C++-like name (before)	C++-like name (after)
[T]	[T]	slice$<T>	slice2$<T>
&[T]	&[T]	slice$<T>	ref$<slice2$<T> >
&mut [T]	&mut [T]	slice$<T>	ref_mut$<slice2$<T> >
str	str	str	str$
&str	&str	str	ref$<str$>
&mut str	&mut str	str	ref_mut$<str$>
*const [T]	*const [T]	ptr_const$<slice$<T> >	ptr_const$<slice2$<T> >
*mut [T]	*mut [T]	ptr_mut$<slice$<T> >	ptr_mut$<slice2$<T> >

As you can see, before the PR many types would end up with the same name, making it impossible to distinguish between them in NatVis or other places where types are matched or looked up by name. The DWARF version of names is not changed.

[wesleywiser]

Thanks for working on this @michaelwoerister!

[wesleywiser]

@bors r+

[bors]

📌 Commit 0cd2dd7 has been approved by wesleywiser

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0cd2dd7 with merge b0f3940...

[bors]

☀️ Test successful - checks-actions
Approved by: wesleywiser
Pushing b0f3940 to master...

[rust-timer]

Finished benchmarking commit (b0f3940): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.4%, 0.4%]	1

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.0%	[-1.0%, -1.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.2%	[-1.0%, 0.6%]	2

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.5%, 0.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.9%	[-0.9%, -0.9%]	1
Improvements ✅ (secondary)	-2.6%	[-3.0%, -2.1%]	4
All ❌✅ (primary)	-0.2%	[-0.9%, 0.5%]	2