rustdoc: intra-doc links on blanket impls cause extra allocations

[jsha]

In doing some perf debugging on something else, I added eprintln output here:

rust/src/librustdoc/clean/types.rs

Line 462 in 63fdb82

if let Ok((mut href, ..)) = href(*id, cx) {

And was surprised to find that for the stm32f4 crate, building with a single feature enabled, that line of code was hit 16,000,000 times even though only 4,000 documentation pages were being generated.

It turns out that the blanket impl for Into<U> for T where U: From<T> uses an intra-doc link:

rust/library/core/src/convert/mod.rs

Line 724 in 63fdb82

/// <code>[From]&lt;T&gt; for U</code> chooses to do.

So apparently that link gets inserted into the intra_doc_links: FxHashMap<ItemId, Vec<clean::ItemLink>> multiple times, once for each page (since every page includes this blanket impl); and then each time links() is called, it iterates through all 4,000 entries.