reset anonymous-lifetime-mode as we enter () scopes

[nikomatsakis]

Background:

The anonymous lifetime mode is used to prohibit elided lifetimes where
they didn't used to be permitted, and instead require that '_ be
used. For example:

impl Trait for Ref<T> { .. }
//             ^^^^^^ ERROR: should be `Ref<'_, T>`

When we are parsing the parts of the impl header, we enter into an alternate mode called CreateParameter. In this mode, we give an error for things like Ref<T>, but for elided lifetimes in a reference type like &T we make the elided lifetime into an in-band lifetime:

rust/src/librustc/hir/lowering.rs

Lines 4017 to 4035 in 4f99f37

	/// Invoked to create the lifetime argument for a type `&T`
	/// with no explicit lifetime.
	fn elided_ref_lifetime(&mut self, span: Span) -> hir::Lifetime {
	match self.anonymous_lifetime_mode {
	// Intercept when we are in an impl header and introduce an in-band lifetime.
	// Hence `impl Foo for &u32` becomes `impl<'f> Foo for &'f u32` for some fresh
	// `'f`.
	AnonymousLifetimeMode::CreateParameter => {
	let fresh_name = self.collect_fresh_in_band_lifetime(span);
	hir::Lifetime {
	id: self.next_id().node_id,
	span,
	name: fresh_name,
	}
	}
	AnonymousLifetimeMode::PassThrough => self.new_implicit_lifetime(span),
	}
	}

This was not intended to change behavior because we only enter into that mode in contexts where elision was not historically permitted. However, the problem is that we fail to reset the mode when we enter into bounds like Fn(&u32), where elision was allowed -- the same occurs for fn types like fn(&u32). This PR restores the original mode in those contexts.

Fixes #51008

r? @cramertj

[cramertj]

@bors r+

[bors]

📌 Commit 85f5d46 has been approved by cramertj

[bors]

🔒 Merge conflict

[nikomatsakis]

@bors r=cramertj

[bors]

📌 Commit da69bbc has been approved by cramertj

[nikomatsakis]

@bors p=1 -- edition preview blocking bug

[bors]

⌛ Testing commit da69bbc with merge c1287c0...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: cramertj
Pushing c1287c0 to master...