inform compiler about local variable definedness

JET's new analysis pass now detects local variables that may be
undefined, which has revealed such issues in several functions within
Base (#58762).

This commit addresses local variables whose definedness the compiler
cannot properly determine, primarily in functions reachable from JET's
test suite. No functional changes are made.

Author: aviatesk

base/partr.jl

@@ -107,7 +107,6 @@ function multiq_sift_down(heap::taskheap, idx::Int32)
     end
 end
 
-
 function multiq_size(tpid::Int8)
     nt = UInt32(Threads._nthreads_in_pool(tpid))
     tp = tpid + 1
@@ -138,7 +137,6 @@ function multiq_size(tpid::Int8)
     return heap_p
 end
 
-
 function multiq_insert(task::Task, priority::UInt16)
     tpid = ccall(:jl_get_task_threadpoolid, Int8, (Any,), task)
     @assert tpid > -1
@@ -171,10 +169,8 @@ function multiq_insert(task::Task, priority::UInt16)
     return true
 end
 
-
 function multiq_deletemin()
-    local rn1, rn2
-    local prio1, prio2
+    local rn1::UInt32
 
     tid = Threads.threadid()
     tp = ccall(:jl_threadpoolid, Int8, (Int16,), tid-1) + 1
@@ -208,6 +204,8 @@ function multiq_deletemin()
         end
     end
 
+    @assert @isdefined(rn1) # tell the compiler about the defineness of this variable
+
     heap = tpheaps[rn1]
     task = heap.tasks[1]
     if ccall(:jl_set_task_tid, Cint, (Any, Cint), task, tid-1) == 0

base/ryu/shortest.jl

@@ -196,6 +196,7 @@ integer. If a `maxsignif` argument is provided, then `b < maxsignif`.
         e10 = 0
 
         if maxsignif !== nothing && b > maxsignif
+            roundup = false
             b_allzero = true
             # reduce to max significant digits
             while true