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`rustc_span::symbol` defines some things that are re-exported from
`rustc_span`, such as `Symbol` and `sym`. But it doesn't re-export some
closely related things such as `Ident` and `kw`. So you can do `use
rustc_span::{Symbol, sym}` but you have to do `use
rustc_span::symbol::{Ident, kw}`, which is inconsistent for no good
reason.
This commit re-exports `Ident`, `kw`, and `MacroRulesNormalizedIdent`,
and changes many `rustc_span::symbol::` qualifiers in `compiler/` to
`rustc_span::`. This is a 200+ net line of code reduction, mostly
because many files with two `use rustc_span` items can be reduced to
one.
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@lqd commented on https://github.com/rust-lang/rust/pull/114351 asking
if `sort_by_words(lookup)` is computed repeatedly. I was assuming that
rustc should have no difficulties to hoist it automatically outside of
the loop to avoid repeated pure computation, but according to
https://godbolt.org/z/frs8Kj1rq it seems like I was wrong:
original version seems to have 2 calls per loop iteration
```
.LBB16_3:
mov rbx, qword ptr [r13]
mov r14, qword ptr [r13 + 8]
lea rdi, [rsp + 40]
mov rsi, rbx
mov rdx, r14
call example::sort_by_words
lea rdi, [rsp + 64]
mov rsi, qword ptr [rsp + 8]
mov rdx, qword ptr [rsp + 16]
call example::sort_by_words
mov rdi, qword ptr [rsp + 40]
mov rdx, qword ptr [rsp + 56]
mov rsi, qword ptr [rsp + 64]
cmp rdx, qword ptr [rsp + 80]
mov qword ptr [rsp + 32], rdi
mov qword ptr [rsp + 24], rsi
jne .LBB16_5
call qword ptr [rip + bcmp@GOTPCREL]
test eax, eax
sete al
mov dword ptr [rsp + 4], eax
mov rsi, qword ptr [rsp + 72]
test rsi, rsi
jne .LBB16_8
jmp .LBB16_9
```
but the manually hoisted version just 1:
```
.LBB16_3:
mov r13, qword ptr [r15]
mov r14, qword ptr [r15 + 8]
lea rdi, [rsp + 64]
mov rsi, r13
mov rdx, r14
call example::sort_by_words
mov rdi, qword ptr [rsp + 64]
mov rdx, qword ptr [rsp + 16]
cmp qword ptr [rsp + 80], rdx
mov qword ptr [rsp + 32], rdi
jne .LBB16_5
mov rsi, qword ptr [rsp + 8]
call qword ptr [rip + bcmp@GOTPCREL]
test eax, eax
sete bpl
mov rsi, qword ptr [rsp + 72]
test rsi, rsi
jne .LBB16_8
jmp .LBB16_9
```
This code is probably not very hot, but there is no reason to leave
such a low hanging fruit.
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