[checklocks] Use function line info for error messages when return instruction has no line numbers attached

Resolves line information issues described by #11203

For functions that don't include explicit return statements in the Go source code, the ssa package inserts return instructions without line information attached, leading to poor error messages.
Ex: -: return with unexpected locks held (locks: &({param:f}.mu) exclusively)

Quick fix is to instead use the line number information of the parent function instead.

FUTURE_COPYBARA_INTEGRATE_REVIEW=#12347 from uzairnawaz:return-parent-line-numbers 9a04865
PiperOrigin-RevId: 842286019

Author: Uzair Nawaz

tools/checklocks/analysis.go

@@ -731,17 +731,25 @@ func (pc *passContext) checkBasicBlock(fn *ssa.Function, block *ssa.BasicBlock,
 		rv, rls = pc.checkInstruction(inst, lff, ls)
 		if rls != nil {
 			failed := false
+
+			// if there is no line information attached to the return
+			// instruction, use the line info of the function itself
+			pos := rv.Pos()
+			if pos == token.NoPos {
+				pos = rv.Parent().Pos()
+			}
+
 			// Validate held locks.
 			for fieldName, fg := range lff.HeldOnExit {
 				r := fg.Resolver.resolveStatic(pc, ls, fn, rv)
 				if !r.valid() {
 					// This cannot be forced, since we have no reference.
-					pc.maybeFail(rv.Pos(), "lock %s cannot be resolved", fieldName)
+					pc.maybeFail(pos, "lock %s cannot be resolved", fieldName)
 					continue
 				}
 				if s, ok := rls.isHeld(r, fg.Exclusive); !ok {
 					if _, ok := pc.forced[pc.positionKey(rv.Pos())]; !ok && !lff.Ignore {
-						pc.maybeFail(rv.Pos(), "lock %s (%s) not held %s (locks: %s)", fieldName, s, exclusiveStr(fg.Exclusive), rls.String())
+						pc.maybeFail(pos, "lock %s (%s) not held %s (locks: %s)", fieldName, s, exclusiveStr(fg.Exclusive), rls.String())
 						failed = true
 					} else {
 						// Force the lock to be acquired.
@@ -751,7 +759,7 @@ func (pc *passContext) checkBasicBlock(fn *ssa.Function, block *ssa.BasicBlock,
 			}
 			// Check for other locks, but only if the above didn't trip.
 			if !failed && rls.count() != len(lff.HeldOnExit) && !lff.Ignore {
-				pc.maybeFail(rv.Pos(), "return with unexpected locks held (locks: %s)", rls.String())
+				pc.maybeFail(pos, "return with unexpected locks held (locks: %s)", rls.String())
 			}
 		}
 	}