Remove dead diagnostic structs.

[nnethercote]

One of these has a "FIXME(autodiff): I should get used somewhere" comment, but I figure YAGNI applies and it's so small that reinstating it if necessary would be trivial.

r? @Kivooeo

[nnethercote]

#155301 inspired me to write the following script to find other dead diagnostics:

#! /bin/sh
#
# Find potentially dead error types in rustc.
#
# Searches compiler/**/errors.rs for structs and enums that are unused outside
# of the defining file.
#
# False positives can occur for types that are used meaningfully but only
# within the defining file. This is moderately common.
# 
# False negatives can occur if there are two or more types with the same name
# in different files. This is rarer?

# Find all `.rs` files containing `#[derive(Diagnostic)]`.
fs=`git grep -l '#\[derive(Diagnostic)\]' 'compiler/**/*.rs'`

for f in $fs; do
    echo "---------"
    echo $f

    # Find all structs and enums in these files.
    ts=`sed -n 's/^pub.*\(struct\|enum\) \([A-Za-z0-9_]*\).*/\2/p' $f`

    # For each struct/enum, search for a use outside the defining file. If
    # none, it might be dead.
    for t in $ts ; do
        #echo "*** $t"
        m=`git grep "\b$t\b" -- ":!$f"`
        #echo "^^^ $m"
        if [ -z "$m" ] ; then
            echo "DEAD? $t"
        fi
    done
done

Catches some false positives, but still very useful.

cc @mejrs

[Kivooeo]

@bors r+ rollup

[rust-bors]

📌 Commit f093767 has been approved by Kivooeo

It is now in the queue for this repository.